Problem - Kevin Tafuro

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Solution Two basic types of locks exist: advisory and mandatory. Unix supports both advisory and, to an extremely limited extent, mandatory locks, while Windows supports only mandatory locks. Discussion In the following sections, we will look at the different issues for Unix and Windows. Locking files on Unix All modern Unix variants support advisory locks. An advisory lock is a lock in which the operating system does not enforce the lock. Instead, programs sharing the same file must cooperate with each other to ensure that locks are properly observed. From a security perspective, advisory locks are of little use because any program is free to perform any action on a file regardless of the state of any advisory locks that other programs may hold on the file. Support for mandatory locks varies greatly from one Unix variant to another. Both Linux and Solaris support mandatory locks, but Darwin, FreeBSD, NetBSD, and OpenBSD do not, even though they export the interface used by Linux and Solaris to support them. On such systems, this interface creates advisory locks. Support for mandatory locking does not extend to NFS. In other words, both Linux and Solaris are capable only of using mandatory locks on local filesystems. Further, Linux requires that filesystems be mounted with support for mandatory locking, which is disabled by default. In the end, Solaris is really the only Unix variant on which you can reasonably expect mandatory locking to work, and even then, relying on mandatory locks is like playing with fire. As if the story for mandatory locking on Unix were not bad enough already, it gets worse. To be able to use mandatory locks on a file, the file must have the setgid bit enabled and the group execute bit disabled in its permissions. Even if a process holds a mandatory lock on a file, another process may remove the setgid bit from the file’s permissions, which effectively turns the mandatory lock into an advisory lock! Essentially, there is no such thing as a mandatory lock on Unix. Just to add more fuel to the fire, neither Solaris nor Linux fully or properly implement the System V defined semantics for mandatory locks, and both systems differ in where they stray from the System V definitions. The details of the differences are not important here. We strongly recommend that you avoid the Unix mandatory lock debacle altogether. If you want to use advisory locking on Unix, then we recommend using a standalone lock file, as described in Recipe 2.9. 58 | Chapter 2: Access Control This is the Title of the Book, eMatter Edition Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
Locking files on Windows Where Unix falls flat on its face with respect to supporting file locking, Windows gets it right. Windows supports only mandatory file locks, and it fully enforces them. If a process has a lock on a file or a portion of a file, another process cannot mistakenly or maliciously steal that lock. Windows provides four functions for locking and unlocking files. Two functions, LockFile( ) and LockFileEx( ), are provided for engaging locks, and two functions, UnlockFile( ) and UnlockFileEx( ), are provided for removing them. Neither LockFile( ) nor UnlockFile( ) will return until the lock can be successfully obtained or released, respectively. LockFileEx( ) and UnlockFileEx( ), however, can be called in such a way that they will always return immediately, either returning failure or signalling an event object when the requested operation completes. Locks can be placed on a file in its entirety or on a portion of a file. A single file may have multiple locks owned by multiple processes so long as none of the locks overlap. When removing a lock, you must specify the exact portion of the file that was locked. For example, two locks covering contiguous portions of a file may not be removed with a single unlock operation that spans the two locks. When a lock is held on a file, closing the file does not necessarily remove the lock. The behavior is actually undefined and may vary across different filesystems and versions of Windows. Always make sure to remove any locks on a file before closing it. There are two types of locks on Windows: Shared lock This type of lock allows other processes to read from the locked portion of the file, while denying all processes—including the process that obtained the lock— permission to write to the locked portion of the file. Exclusive lock This type of lock denies other processes both read and write access to the locked portion of the file, while allowing the locking process to read or write to the locked portion of the file. Using LockFile( ) to obtain a lock always obtains an exclusive lock. However, LockFileEx( ) obtains a shared lock unless the flag LOCKFILE_EXCLUSIVE_LOCK is specified. Here are the signatures for LockFile and UnlockFile( ): BOOL LockFile(HANDLE hFile, DWORD dwFileOffsetLow, DWORD dwFileOffsetHigh, DWORD nNumberOfBytesToLockLow, DWORD nNumberOfBytesToLockHigh); This is the Title of the Book, eMatter Edition Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved. Locking Files | 59
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Secure Programming Cookbook ΤΜ fo
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Secure Programming CookbookΤΜ for
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Table of Contents Foreword . . . .
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5.8 Using a Generic OFB Mode Implem
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8.10 Performing Password-Based Auth
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12.10 Restructuring Arrays 672 12.1
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they are in the top 50% of safe dri
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think you are in, you will probably
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the C programming language, with so
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igger risks than anticipated. You s
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Recipe Compatibility Most of the re
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The O’Reilly web site for the boo
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Chapter 1 CHAPTER 1 Safe Initializa
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using the code in this recipe on Wi
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variables you preserve, be sure to
Page 35 and 36: ptr = (char *)new_environ + (arr_si
Page 37 and 38: an impersonation token for a thread
Page 39 and 40: Creating a new process with a restr
Page 41 and 42: array that is the Privileges field.
Page 43 and 44: eturn 0; } if (!LookupAccountName(0
Page 45 and 46: privileges. We strongly advise agai
Page 47 and 48: #include #include #include #incl
Page 49 and 50: sockets provide a means by which fi
Page 51 and 52: The privman library provides a numb
Page 53 and 54: The potential for security vulnerab
Page 55 and 56: It’s important to remember that t
Page 57 and 58: Discussion execve( ) is the system
Page 59 and 60: should call pclose( ) to clean up t
Page 61 and 62: dup2(stdout_pipe[1], 1); close(stdo
Page 63 and 64: lpCommandLine Any command-line argu
Page 65 and 66: #include #include #include void
Page 67 and 68: and a saved user ID. * The effectiv
Page 69 and 70: See Also Recipes 1.3, 1.4, 2.8 2.2
Page 71 and 72: of the object, regardless of what t
Page 73 and 74: There are many other situations whe
Page 75 and 76: if (!(fd = opendir("."))) break; if
Page 77 and 78: spc_file_wipe( ) A wrapper around t
Page 79 and 80: if (pattern_pass(fd, buf, sizeof(bu
Page 81 and 82: CryptReleaseContext(hProvider, 0);
Page 83 and 84: 2.7 Restricting Access Permissions
Page 85: eset the umask between your adjustm
Page 89 and 90: NFS older than Version 3 in particu
Page 91 and 92: a loop until all the data is read.
Page 93 and 94: if (!(hResourceLock = CreateMutex(0
Page 95 and 96: generator. The code presented here
Page 97 and 98: access to the filesystem, the poten
Page 99 and 100: Chapter 3 CHAPTER 3 Input Validatio
Page 101 and 102: components of the system are truste
Page 103 and 104: try to use them. For example, what
Page 105 and 106: To combat malicious uses of “%n
Page 107 and 108: Discussion Buffer overflows get a l
Page 109 and 110: Unfortunately, strlcpy( ) and strlc
Page 111 and 112: The second problem area occurs when
Page 113 and 114: (see Recipe 3.2). The format-string
Page 115 and 116: safestr_release( ) or safestr_free(
Page 117 and 118: Solution Unfortunately, integer coe
Page 119 and 120: which the records begin. Generally,
Page 121 and 122: pointer or a different value altoge
Page 123 and 124: if (!(new_environ = (char **)malloc
Page 125 and 126: if (spc_environ) free(environ); env
Page 127 and 128: GetFullPathName( ) requires the len
Page 129 and 130: if (*c != '%' || !isxdigit(c[1]) ||
Page 131 and 132: 3.10 Preventing Cross-Site Scriptin
Page 133 and 134: * These are HTML tags that do not t
Page 135 and 136: *ptr++ = *c; } while (*++c != ’>
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Finally, if you are using the LIKE
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Table 3-2. UTF-8 encoding byte sequ
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the connections to the server. The
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SPC_FD_SET read_mask; for (;;) { sp
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To ensure that you choose the right
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See Also Recipe 13.2 4.2 Generating
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This function takes the following a
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} break; default: if (isspace(p[0])
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*p = 0; return output; } } } The pu
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} return result; case -1: if (stric
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#define MAX_WORDLEN 4 /* len parame
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Discussion This function spc_words2
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4.9 Using Salts, Nonces, and Initia
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the lifetime of the key; the counte
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#include #include #include #incl
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#include #include /* This value n
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} CryptReleaseContext(hProvider, hK
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cryptographic one-way hash from a t
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HMAC_CTX c; unsigned long ctr = 0,
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database does not support binary st
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Remember to use a MAC anytime you e
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For many different reasons, it can
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A more portable but less accurate w
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Chapter 5 CHAPTER 5 Symmetric Encry
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Discussion Be sure to read this dis
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Pentium III, which should give a go
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locations where 40-bit keys or 56-b
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can arise. For general-purpose use,
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or similar mechanism is used. As wi
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shares many properties with CTR mod
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case, the faster of the two algorit
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parison to standard modes. As of th
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work, which severely limits portabi
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Table 5-4. Implementations for the
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Plaintext block 1 Plaintext block 2
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typedef struct { SPC_KEY_SCHED ks;
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If you are using padding and you kn
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This function has the following arg
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if (ol) *ol = out - start; return 1
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that the state function is always r
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Note that this code depends on the
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if (!il--) return 1; ctx->nonce[ctx
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one block at a time, by encrypting
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spc_memset(key,0, kl); memcpy(ctx->
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5.9 Using a Generic CTR Mode Implem
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These two functions also erase the
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we ignore that because it will alwa
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Once those bindings are made, the Z
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output Buffer into which the plaint
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eturn the number of valid bytes in
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In CBC, CFB, and OFB modes, encrypt
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void spc_pctr_do_odd(SPC_CTR2_CTX *
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5.15 Performing File or Disk Encryp
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Therefore, we’ll show you LION, b
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} RC4_set_key(&k, HASH_SZ, (char *)
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techniques from Chapter 8), the key
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communicate asynchronously (that is
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key Pointer to the encryption key t
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Table 5-6. Cipher instantiation ref
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While RC2, RC4, and RC5 support abs
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The function EVP_CIPHER_CTX_ctrl( )
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Discussion As a reminder, use a raw
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ol = 0; if (!(ret = (char *)malloc(
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The “official” RC4 key setup fu
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Discussion One-time pads are provab
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Once a provider context has been su
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Table 5-8. Symmetric ciphers suppor
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hKey Key to use for performing the
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if (!CryptAcquireContext(&hProvider
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exportable from key objects, but th
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Chapter 6 CHAPTER 6 Hashes and Mess
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Generally, you should prefer an enc
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See Also Recipes 6.7, 6.8, 6.12 6.2
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Noncorrelation It should also be co
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Let’s look briefly at the pros an
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Solution In most cases, instead of
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MAC (which we recommend), we strong
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Libraries with cryptographic hash f
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#include int main(int argc, char *
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See Also Implementations of SHA-256
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CryptAcquireContext(&hProvider, 0,
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To ensure the best security, we str
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if (!(vfy = (unsigned char *)malloc
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you might use a library such as Ope
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Otherwise, you can use the HMAC imp
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if (klen inner[i] ^= key[i]; ctx->o
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OMAC has been explicitly specified
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unsigned char c1[SPC_BLOCK_SZ]; /*
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ing the all-zero data block. Each p
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CMAC is the message-integrity compo
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To postprocess, we encrypt the hash
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6.15 Constructing a Hash Function f
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unsigned char b[SPC_KEY_SZ]; size_t
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Figure 6-2. The Mayas-Meyer-Oseas c
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c->ix = 0; c->tl = 0; } static void
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modes such as CWC and CCM are start
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authentication in a straightforward
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SPC_HMAC_Reset(&ctx); SPC_HMAC_Upda
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divide up the data stream between t
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Chapter 7 CHAPTER 7 Public Key Cryp
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The code presented in this chapter
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Because public key encryption is so
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Solution There’s some debate on t
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7.4 Manipulating Big Numbers Proble
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There’s a similar function for as
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Additionally, you can ask for a ran
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In addition, you might wish to test
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Table 7-2. Math operations supporte
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313, 317, 331, 337, 347, 349, 353,
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for (b = 0; !BN_is_odd(x); b++) BN_
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BIGNUM *dP, *dQ, *qInv; } RSA_PRIVA
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This would map to the hexadecimal v
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compute d. Without those two primes
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The constants that may be used to s
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When using OpenSSL, decryption can
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ecommend using it, unless you are c
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ture is valid. A successful check w
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et = RSA_verify(NID_sha1, hash, 20,
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#define MIN(x,y) ((x) > (y) ? (y) :
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memcpy(signedtext, decrypt, 16); if
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h_ret Optional argument that, if no
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siglen The number of bytes written
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data ready to go into the certifica
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ut it requires you to pass in the l
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LjKQ2r1Yt9foxbHdLKZeClqZuzN7PoEmy+b
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ing in data, pass in a pointer to a
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Table 7-6 lists the FILE object-bas
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Solution The correct method depends
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Use across applications For some pe
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key agreement protocol, where both
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consider slightly more sophisticate
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Kerberos does assume that the envir
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The function used to look up user i
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The group structure that is returne
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found, NULL will be returned, and G
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ReferencedDomainName = (LPTSTR)Loca
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if (errno != EINTR && errno != EAGA
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if (!(spc_host_rulecount % 256)) {
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} else { if (inet_addr(tmp) = = INA
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8.5 Generating Random Passwords and
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for this list is not insignificant.
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file, it may allocate a sizable amo
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On Windows, you can use the standar
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Once getpass( ) or readpassphrase(
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ters typed by the user. Instead, th
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compute the future time at which th
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decrypt the encrypted data. To make
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On systems that support MCF through
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for (i = 0; i < 1000; i++) { MD5_In
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CryptHashData(hHash1, lpszKey, dwKe
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Solution Use the PBKDF2 method of c
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Verifying a password encrypted usin
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c Pointer to an integer that will r
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each of these pieces of information
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server-side part of the authenticat
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8.14 Authenticating with HTTP Cooki
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char *spc_cookie_encode(char *cooki
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• Let the salt be public, in whic
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extra Additional application-specif
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xl Pointer into which the length of
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With a valid socket descriptor in h
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11. The client and the server compu
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Some people like to use a fixed mod
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Often, you’ll want to generate a
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Discussion Remember, authentication
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• Provide the user with some way
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if (!fgets(answer, sizeof(answer),
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uffer = (char *)realloc(buffer, buf
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compromise. If our system has such
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When using RSA, if you’re doing o
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see the confirmation request and th
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Upon receipt of a response to a con
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BOOL SpcConfirmationCreate(LPCTSTR
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Additionally, over time, SSPI-speci
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if (!spc_verify_cert_hostname(SSL_g
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The next step is to call spc_accept
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Discussion Session caching is norma
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if (BIO_do_connect(conn)
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The next step is to connect to the
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DWORD dwHeadersLength = 0; LPSTR lp
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attempt to develop and debug SSL-en
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API for encryption and decryption,
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memcpy(in_data.data, inbuf, inlen);
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*outlen = out_data.length - (blksz
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so. Likewise, any process with the
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struct sockaddr_un *addr_unix; *dom
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error_exit: if (sock) spc_socket_cl
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different ways. On FreeBSD systems,
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msg.msg_iov->iov_base = (void *)&sy
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it’s unique and unpredictable! Yo
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if (mysql_real_connect(mysql, host,
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maintenance. Adding or removing ser
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ut they shouldn’t tolerate reorde
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#define SPC_CLIENT_DISTINGUISHER 0x
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} static void spc_ssock_write( int
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while (mlen) { if ((r = read(fd, ms
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In such cases, you need to be able
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Discussion One of the big motivator
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A digital certificate contains info
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Certificate revocation What happens
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cate as long as it still has its pr
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Currently, OCSP is not nearly as wi
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ple is the permissible uses for a c
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automated email to the address you
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The type of code-signing certificat
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10.3 Using Root Certificates Proble
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Table 10-1. CA certificates, their
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ights of the entity tied to that ce
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10.5 Performing X.509 Certificate V
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sk_X509_free(spc_store->crls); sk_X
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X509_LOOKUP *lookup; store = X509_S
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CertGetIssuerCertificateFromStore(
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HCERTSTORE hCertStore; PCCERT_CONTE
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See Also Recipe 10.11 10.7 Verifyin
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SPC_X509STORE_SSL_VERIFY_NONE This
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verify_flags |= SSL_VERIFY_FAIL_IF_
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int spc_verify_cert_hostname(X509 *
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for (i = 0; !bResult && i < pNameIn
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only work you really need to do is
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digest = EVP_sha1( ); fingerprint_l
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if (cert && (uri = get_distribution
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"\xb3\x9c\x25\xb1\xc3\x2e\x32\x53\x
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headerlen -= (*datalen + 2); if (*d
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In this recipe, we have used a numb
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{ "\x8d\x26\xff\x2f\x31\x6d\x59x\29
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LocalFree(pvStructInfo); return 0;
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lpFullBuffer = lpNewBuffer; lpBuffe
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of tunable variables that affect th
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SPC_OCSPRESULT_CERTIFICATE_VALID =
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* All done. Set the return code bas
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Solution There are essentially thre
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the byte is even, he reduces the nu
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See Also Recipes 11.16, 11.18, 11.1
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11.3 Using the Standard Unix Random
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a pointer to data. Instead, they re
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if (errno = = EINTR) continue; perr
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Here we show how to use this functi
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attacks are a realistic threat, you
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This function never fails (save for
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Using a stream cipher as a generato
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One very safe way to use a cryptogr
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out Buffer into which the random da
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1. Figure out how big a seed you ne
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0x00 Query the amount of entropy be
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strncpy(a.sun_path, EGD_SOCKET_PATH
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} } while (nb = = -1); } } } See Al
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nbytes Number of bytes of entropy t
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See Also • EGADS by Secure Softwa
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If the generator is not seeded with
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Discussion In all cases, you will s
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Solution Because of the way that fl
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double spc_rand_cunifvariate(double
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See Also Recipe 11.11 11.16 Compres
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It would be nice if you did not hav
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For this reason, we think the full
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} int cur_val, i, j, runsz; unsigne
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See Also • NIST Cryptographic Mod
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ecause even if a source does fail c
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you use a seed file, you can just c
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• If you have to accept data from
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with a high-resolution clock (i.e.,
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Collecting entropy from the keyboar
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f[c + 2] = '>'; } else snprintf(bf,
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SpcGatherKeyboardEntropy( ) uses th
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} } if (hWndParent) EnableWindow(hW
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number of bits of entropy has been
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} return TRUE; pDlgData->cbRequeste
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See Also Recipes 11.19, 11.20 11.22
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Discussion We strongly recommend th
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Chapter 12 CHAPTER 12 Anti-Tamperin
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tecture-specific checks to determin
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ure or to examine a hardware valida
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• Detecting modification to a com
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for (j = 8; j > 0; j--) { if (crc &
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#include CRC_START_BLOCK(test) int
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Solution Obfuscating compiled code
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" movl 0f( , %%ebx ), %%eax \n\t" \
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Because library functions are loade
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encoded as a series of 32 Obcode bi
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12.5 Performing Constant Transforms
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12.7 Splitting Variables Problem La
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12.9 Using Function Pointers Proble
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arrays. This array type will hide t
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eak; case SPC_ARRAY_FOLD: index = (
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The following example demonstrates
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Discussion The techniques for hidin
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12.12 Detecting Debuggers Problem S
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Discussion The spc_trap_detect( ) f
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12.14 Detecting Windows Debuggers P
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The int3 interface can also be used
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To demonstrate the effect this macr
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08048388 83 db 83h ; â 08048389 7D
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ine the instruction making the refe
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mprotect(buf, buf_len, PROT_READ |
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eturn 4; } /* get entry point : her
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need to use one of the three ring3
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cates failure, it’s likely that n
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tion gets caught. The only ways to
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memory location will generally stil
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eturn dst; } volatile void *spc_mem
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The mlock( ) system call on Unix im
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Discussion Both C and C++ allow the
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Do not share signal handlers. Sever
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#endif } static int signal_was_caug
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paper reached the widest audience,
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The proper way to handle a program
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handles[0] = cond; handles[1] = mut
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#ifndef WIN32 pool->tids = (pthread
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#define SPC_ACQUIRE_MUTEX(mtx) pthr
Page 755 and 756:
if (socketpool_limit > 0 && socketp
Page 757 and 758:
Table 13-1. Resources that may be l
Page 759 and 760:
ability to run. The default limits
Page 761 and 762:
Note that the structures as present
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access to the machine. This makes i
Page 765:
ing log entries. It is possible, ho
Page 768 and 769:
attacks (continued) capture replay
Page 770 and 771:
CFB (Cipher Feedback) mode, 167, 18
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deriving symmetric keys from a pass
Page 774 and 775:
EVP_CIPHER_CTX_set_padding( ), 227
Page 776 and 777:
integrity checking cipher modes, 16
Page 778 and 779:
message digests (continued) desirab
Page 780 and 781:
parallelizing MACs, 304 parent and
Page 782 and 783:
RAND_bytes( ), 604 RAND_load_file(
Page 784 and 785:
session ID context, 461 session IDs
Page 786 and 787:
spc_gather_keyboard_entropy( ), 631
Page 788 and 789:
symmetric cryptography, 116-154 alg
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Windows (continued) Win 2000, restr
Page 792:
oads. They will often travel up to
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Problem - Kevin Tafuro

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