Dec 3, 2007

Good Temporary Files ( or randomness )

This issue of correctly performing atomic operations particularly comes up when creating temporary files. Temporary files in Unix-like systems are traditionally created in the /tmp or /var/tmp directories, which are shared by all users. A common trick by attackers is to create symbolic links in the temporary directory to some other file (e.g., /etc/passwd) while your secure program is running. The attacker's goal is to create a situation where the secure program determines that a given filename doesn't exist, the attacker then creates the symbolic link to another file, and then the secure program performs some operation (but now it actually opened an unintended file). Often important files can be clobbered or modified this way. There are many variations to this attack, such as creating normal files, all based on the idea that the attacker can create (or sometimes otherwise access) file system objects in the same directory used by the secure program for temporary files. Michal Zalewski exposed in 2002 another serious problem with temporary directories involving automatic cleaning of temporary directories. For more information, see his posting to Bugtraq dated December 20, 2002, (subject "[RAZOR] Problems with mkstemp()"). Basically, Zalewski notes that it's a common practice to have a program automatically sweep temporary directories like /tmp and /var/tmp and remove "old" files that have not been accessed for a while (e.g., several days). Such programs are sometimes called "tmp cleaners" (pronounced "temp cleaners"). Possibly the most common tmp cleaner is "tmpwatch" by Erik Troan and Preston Brown of Red Hat Software; another common one is 'stmpclean' by Stanislav Shalunov; many administrators roll their own as well. Unfortunately, the existance of tmp cleaners creates an opportunity for new security-critical race conditions; an attacker may be able to arrange things so that the tmp cleaner interferes with the secure program. For example, an attacker could create an "old" file, arrange for the tmp cleaner to plan to delete the file, delete the file himself, and run a secure program that creates the same file - now the tmp cleaner will delete the secure program's file! Or, imagine that a secure program can have long delays after using the file (e.g., a setuid program stopped with SIGSTOP and resumed after many days with SIGCONT, or simply intentionally creating a lot of work). If the temporary file isn't used for long enough, its temporary files are likely to be removed by the tmp cleaner. The general problem when creating files in these shared directories is that you must guarantee that the filename you plan to use doesn't already exist at time of creation, and atomically create the file. Checking ``before'' you create the file doesn't work, because after the check occurs, but before creation, another process can create that file with that filename. Using an ``unpredictable'' or ``unique'' filename doesn't work in general, because another process can often repeatedly guess until it succeeds. Once you create the file atomically, you must alway use the returned file descriptor (or file stream, if created from the file descriptor using routines like fdopen()). You must never re-open the file, or use any operations that use the filename as a parameter - always use the file descriptor or associated stream. Otherwise, the tmpwatch race issues noted above will cause problems. You can't even create the file, close it, and re-open it, even if the permissions limit who can open it. Note that comparing the descriptor and a reopened file to verify inode numbers, creation times or file ownership is not sufficient - please refer to "Symlinks and Cryogenic Sleep" by Olaf Kirch. Fundamentally, to create a temporary file in a shared (sticky) directory, you must repetitively: (1) create a ``random'' filename, (2) open it using O_CREAT | O_EXCL and very narrow permissions (which atomically creates the file and fails if it's not created), and (3) stop repeating when the open succeeds. According to the 1997 ``Single Unix Specification'', the preferred method for creating an arbitrary temporary file (using the C interface) is tmpfile(3). The tmpfile(3) function creates a temporary file and opens a corresponding stream, returning that stream (or NULL if it didn't). Unfortunately, the specification doesn't make any guarantees that the file will be created securely. In earlier versions of this book, I stated that I was concerned because I could not assure myself that all implementations do this securely. I've since found that older System V systems have an insecure implementation of tmpfile(3) (as well as insecure implementations of tmpnam(3) and tempnam(3)), so on at least some systems it's absolutely useless. Library implementations of tmpfile(3) should securely create such files, of course, but users don't always realize that their system libraries have this security flaw, and sometimes they can't do anything about it. Kris Kennaway recommends using mkstemp(3) for making temporary files in general. His rationale is that you should use well-known library functions to perform this task instead of rolling your own functions, and that this function has well-known semantics. This is certainly a reasonable position. I would add that, if you use mkstemp(3), be sure to use umask(2) to limit the resulting temporary file permissions to only the owner. This is because some implementations of mkstemp(3) (basically older ones) make such files readable and writable by all, creating a condition in which an attacker can read or write private data in this directory. A minor nuisance is that mkstemp(3) doesn't directly support the environment variables TMP or TMPDIR (as discussed below), so if you want to support them you have to add code to do so. Here's a program in C that demonstrates how to use mkstemp(3) for this purpose, both directly and when adding support for TMP and TMPDIR: #include #include #include #include void failure(msg) { fprintf(stderr, "%s\n", msg); exit(1); } /* * Given a "pattern" for a temporary filename * (starting with the directory location and ending in XXXXXX), * create the file and return it. * This routines unlinks the file, so normally it won't appear in * a directory listing. * The pattern will be changed to show the final filename. */ FILE *create_tempfile(char *temp_filename_pattern) { int temp_fd; mode_t old_mode; FILE *temp_file; old_mode = umask(077); /* Create file with restrictive permissions */ temp_fd = mkstemp(temp_filename_pattern); (void) umask(old_mode); if (temp_fd == -1) { failure("Couldn't open temporary file"); } if (!(temp_file = fdopen(temp_fd, "w+b"))) { failure("Couldn't create temporary file's file descriptor"); } if (unlink(temp_filename_pattern) == -1) { failure("Couldn't unlink temporary file"); } return temp_file; } /* * Given a "tag" (a relative filename ending in XXXXXX), * create a temporary file using the tag. The file will be created * in the directory specified in the environment variables * TMPDIR or TMP, if defined and we aren't setuid/setgid, otherwise * it will be created in /tmp. Note that root (and su'd to root) * _will_ use TMPDIR or TMP, if defined. * */ FILE *smart_create_tempfile(char *tag) { char *tmpdir = NULL; char *pattern; FILE *result; if ((getuid()==geteuid()) && (getgid()==getegid())) { if (! ((tmpdir=getenv("TMPDIR")))) { tmpdir=getenv("TMP"); } } if (!tmpdir) {tmpdir = "/tmp";} pattern = malloc(strlen(tmpdir)+strlen(tag)+2); if (!pattern) { failure("Could not malloc tempfile pattern"); } strcpy(pattern, tmpdir); strcat(pattern, "/"); strcat(pattern, tag); result = create_tempfile(pattern); free(pattern); return result; } main() { int c; FILE *demo_temp_file1; FILE *demo_temp_file2; char demo_temp_filename1[] = "/tmp/demoXXXXXX"; char demo_temp_filename2[] = "second-demoXXXXXX"; demo_temp_file1 = create_tempfile(demo_temp_filename1); demo_temp_file2 = smart_create_tempfile(demo_temp_filename2); fprintf(demo_temp_file2, "This is a test.\n"); printf("Printing temporary file contents:\n"); rewind(demo_temp_file2); while ( (c=fgetc(demo_temp_file2)) != EOF) { putchar(c); } putchar('\n'); printf("Exiting; you'll notice that there are no temporary files on exit.\n"); } Kennaway states that if you can't use mkstemp(3), then make yourself a directory using mkdtemp(3), which is protected from the outside world. However, as Michal Zalewski notes, this is a bad idea if there are tmp cleaners in use; instead, use a directory inside the user's HOME. Finally, if you really have to use the insecure mktemp(3), use lots of X's - he suggests 10 (if your libc allows it) so that the filename can't easily be guessed (using only 6 X's means that 5 are taken up by the PID, leaving only one random character and allowing an attacker to mount an easy race condition). Note that this is fundamentally insecure, so you should normally not do this. I add that you should avoid tmpnam(3) as well - some of its uses aren't reliable when threads are present, and it doesn't guarantee that it will work correctly after TMP_MAX uses (yet most practical uses must be inside a loop). In general, you should avoid using the insecure functions such as mktemp(3) or tmpnam(3), unless you take specific measures to counter their insecurities or test for a secure library implementation as part of your installation routines. If you ever want to make a file in /tmp or a world-writable directory (or group-writable, if you don't trust the group) and don't want to use mk*temp() (e.g. you intend for the file to be predictably named), then always use the O_CREAT and O_EXCL flags to open() and check the return value. If you fail the open() call, then recover gracefully (e.g. exit). The GNOME programming guidelines recommend the following C code when creating filesystem objects in shared (temporary) directories to securely open temporary files [Quintero 2000]: char *filename; int fd; do { filename = tempnam (NULL, "foo"); fd = open (filename, O_CREAT | O_EXCL | O_TRUNC | O_RDWR, 0600); free (filename); } while (fd == -1); Note that, although the insecure function tempnam(3) is being used, it is wrapped inside a loop using O_CREAT and O_EXCL to counteract its security weaknesses, so this use is okay. Note that you need to free() the filename. You should close() and unlink() the file after you are done. If you want to use the Standard C I/O library, you can use fdopen() with mode "w+b" to transform the file descriptor into a FILE *. Note that this approach won't work over NFS version 2 (v2) systems, because older NFS doesn't correctly support O_EXCL. Note that one minor disadvantage to this approach is that, since tempnam can be used insecurely, various compilers and security scanners may give you spurious warnings about its use. This isn't a problem with mkstemp(3). If you need a temporary file in a shell script, you're probably best off using pipes, using a local directory (e.g., something inside the user's home directory), or in some cases using the current directory. That way, there's no sharing unless the user permits it. If you really want/need the temporary file to be in a shared directory like /tmp, do not use the traditional shell technique of using the process id in a template and just creating the file using normal operations like ">". Shell scripts can use "$$" to indicate the PID, but the PID can be easily determined or guessed by an attacker, who can then pre-create files or links with the same name. Thus the following "typical" shell script is unsafe: echo "This is a test" > /tmp/test$$ # DON'T DO THIS. If you need a temporary file or directory in a shell script, and you want it in /tmp, a solution sometimes suggested is to use mktemp(1), which is intended for use in shell scripts (note that mktemp(1) and mktemp(3) are different things). However, as Michal Zalewski notes, this is insecure in many environments that run tmp cleaners; the problem is that when a privileged program sweeps through a temporary directory, it will probably expose a race condition. Even if this weren't true, I do not recommend using shell scripts that create temporary files in shared directories; creating such files in private directories or using pipes instead is generally preferable, even if you're sure your tmpwatch program is okay (or that you have no local users). If you must use mktemp(1), note that mktemp(1) takes a template, then creates a file or directory using O_EXCL and returns the resulting name; thus, mktemp(1) won't work on NFS version 2 filesystems. Here are some examples of correct use of mktemp(1) in Bourne shell scripts; these examples are straight from the mktemp(1) man page: # Simple use of mktemp(1), where the script should quit # if it can't get a safe temporary file. # Note that this will be INSECURE on many systems, since they use # tmpwatch-like programs that will erase "old" files and expose race # conditions. TMPFILE=`mktemp /tmp/$0.XXXXXX` || exit 1 echo "program output" >> $TMPFILE # Simple example, if you want to catch the error: TMPFILE=`mktemp -q /tmp/$0.XXXXXX` if [ $? -ne 0 ]; then echo "$0: Can't create temp file, exiting..." exit 1 fi Perl programmers should use File::Temp, which tries to provide a cross-platform means of securely creating temporary files. However, read the documentation carefully on how to use it properly first; it includes interfaces to unsafe functions as well. I suggest explicitly setting its safe_level to HIGH; this will invoke additional security checks. The Perl 5.8 documentation of File::Temp is available on-line. Don't reuse a temporary filename (i.e. remove and recreate it), no matter how you obtained the ``secure'' temporary filename in the first place. An attacker can observe the original filename and hijack it before you recreate it the second time. And of course, always use appropriate file permissions. For example, only allow world/group access if you need the world or a group to access the file, otherwise keep it mode 0600 (i.e., only the owner can read or write it). Clean up after yourself, either by using an exit handler, or making use of UNIX filesystem semantics and unlink()ing the file immediately after creation so the directory entry goes away but the file itself remains accessible until the last file descriptor pointing to it is closed. You can then continue to access it within your program by passing around the file descriptor. Unlinking the file has a lot of advantages for code maintenance: the file is automatically deleted, no matter how your program crashes. It also decreases the likelihood that a maintainer will insecurely use the filename (they need to use the file descriptor instead). The one minor problem with immediate unlinking is that it makes it slightly harder for administrators to see how disk space is being used, since they can't simply look at the file system by name. You might consider ensuring that your code for Unix-like systems respects the environment variables TMP or TMPDIR if the provider of these variable values is trusted. By doing so, you make it possible for users to move their temporary files into an unshared directory (and eliminating the problems discussed here), such as a subdirectory inside their home directory. Recent versions of Bastille can set these variables to reduce the sharing between users. Unfortunately, many users set TMP or TMPDIR to a shared directory (say /tmp), so your secure program must still correctly create temporary files even if these environment variables are set. This is one advantage of the GNOME approach, since at least on some systems tempnam(3) automatically uses TMPDIR, while the mkstemp(3) approach requires more code to do this. Please don't create yet more environment variables for temporary directories (such as TEMP), and in particular don't create a different environment name for each application (e.g., don't use "MYAPP_TEMP"). Doing so greatly complicates managing systems, and users wanting a special temporary directory for a specific application can just set the environment variable specially when running that particular application. Of course, if these environment variables might have been set by an untrusted source, you should ignore them - which you'll do anyway if you follow the advice in Section 5.2.3. These techniques don't work if the temporary directory is remotely mounted using NFS version 2 (NFSv2), because NFSv2 doesn't properly support O_EXCL. See Section for more information. NFS version 3 and later properly support O_EXCL; the simple solution is to ensure that temporary directories are either local or, if mounted using NFS, mounted using NFS version 3 or later. There is a technique for safely creating temporary files on NFS v2, involving the use of link(2) and stat(2), but it's complex; see Section which has more information about this. As an aside, it's worth noting that FreeBSD has recently changed the mk*temp() family to get rid of the PID component of the filename and replace the entire thing with base-62 encoded randomness. This drastically raises the number of possible temporary files for the "default" usage of 6 X's, meaning that even mktemp(3) with 6 X's is reasonably (probabilistically) secure against guessing, except under very frequent usage. However, if you also follow the guidance here, you'll eliminate the problem they're addressing. Much of this information on temporary files was derived from Kris Kennaway's posting to Bugtraq about temporary files on December 15, 2000. I should note that the Openwall Linux patch from includes an optional ``temporary file directory'' policy that counters many temporary file based attacks. The Linux Security Module (LSM) project includes an "owlsm" module that implements some of the OpenWall ideas, so Linux Kernels with LSM can quickly insert these rules into a running system. When enabled, it has two protections: * Hard links: Processes may not make hard links to files in certain cases. The OpenWall documentation states that "Processes may not make hard links to files they do not have write access to." In the LSM version, the rules are as follows: if both the process' uid and fsuid (usually the same as the euid) is is different from the linked-to-file's uid, the process uid is not root, and the process lacks the FOWNER capability, then the hard link is forbidden. The check against the process uid may be dropped someday (they are work-arounds for the atd(8) program), at which point the rules would be: if both the process' fsuid (usually the same as the euid) is is different from the linked-to-file's uid and and the process lacks the FOWNER capability, then the hard link is forbidden. In other words, you can only create hard links to files you own, unless you have the FOWNER capability. * Symbolic links (symlinks): Certain symlinks are not followed. The original OpenWall documentation states that "root processes may not follow symlinks that are not owned by root", but the actual rules (from looking at the code) are more complicated. In the LSM version, if the directory is sticky ("+t" mode, used in shared directories like /tmp), symlinks are not followed if the symlink was created by anyone other than either the owner of the directory or the current process' fsuid (which is usually the effective uid). Many systems do not implement this openwall policy, so you can't depend on this in general protecting your system. However, I encourage using this policy on your own system, and please make sure that your application will work when this policy is in place.

No comments: