Dan Rosenberg reported a flaw with suid mount helpers handle access to /etc/mtab [1], which could allow an unprivileged user to corrupt /etc/mtab and possibly manipulate mountpoint options or unmount a filesystem. The original report follows. This was originally sent to the now-defunct vendor-sec mailing list. Seeing how it's a relatively low-severity issue and that we're currently lacking a mechanism for coordination among package maintainers and vendors, this list seems like a perfectly acceptable venue for discussing how to fix it. I discovered that essentially every suid mount helper that uses addmntent() (or invokes util-linux mount, which in turn calls addmntent()) to add entries to /etc/mtab fails to anticipate a low value for RLIMIT_FSIZE, allowing unprivileged users to corrupt /etc/mtab and possibly manipulate mountpoint options. Affected software includes at least: mount.cifs (samba) fusermount (FUSE) mount (util-linux) ncpmount (ncpfs) vmware-hgfsmounter (open-vm-tools) Also affected are all their unmount equivalents. This can be exploited by checking the current size of /etc/mtab, setting an RLIMIT_FSIZE of some small amount greater than that, and invoking a suid mount helper. The edits to /etc/mtab will be truncated to the ulimit and no newline will be appended, so multiple invocations allow near-arbitrary appending to /etc/mtab. addmntent() will octal-encode most special characters, which makes exploitation beyond simple corruption not quite as straightforward, but I'm confident that with some creativity it would be possible to perform unauthorized unmounting, for example. There are a few possible options We could patch glibc to try to raise the rlimit in addmntent(). Or we could fix every suid mount helper to raise the rlimit or have proper error handling for the case when addmntent() fails. This final option requires that mtab editing be done in a temporary file and aborted on failure, which isn't the case for all helpers. Of course, once we figure out how to fix this, we can talk about assigning CVEs, etc. And a followup regarding specific mount helpers: I did a survey of some suid helpers I'm aware of. Here's the existing behavior: util-linux mount ============= * Edits /etc/mtab.tmp with custom my_addmntent(), behaves identically to glibc addmntent() in terms of return code * Succeeds on partial writes, does not remove temp file on failure (could result in additional corruption of /etc/mtab through multiple invocations), does not remove lock file /etc/mtab~ on failure (also an issue) fusermount (FUSE) ================ * Does not edit mtab directly, calls into util-linux mount/umount, no changes needed mount.cifs (samba) ================ * mount.cifs edits /etc/mtab directly, no cleanup on addmntent() failure * umount.cifs edits /etc/mtab.tmp but does not check return code of addmntent() ncpmount (ncpfs) ============== * ncpmount edits /etc/mtab directly, no cleanup on failure, does not remove lock file /etc/mtab~ on failure * ncpumount edits /etc/mtab.tmp but does not check return code of addmntent() vmware-hgfsmounter (open-vm-tools) =============================== * edits /etc/mtab directly, no cleanup on failure Further discussion is ongoing via the oss-security mailing list. [1] http://thread.gmane.org/gmane.comp.security.oss.general/4374
glibc bug for the addmntent() issue that causes it to not report error when output written to the file is truncated when hitting file size limit: http://sourceware.org/bugzilla/show_bug.cgi?id=12625
Common Vulnerabilities and Exposures assigned an identifier CVE-2011-1089 to the following vulnerability: Name: CVE-2011-1089 URL: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-1089 Assigned: 20110224 Reference: http://openwall.com/lists/oss-security/2011/03/04/9 Reference: http://sourceware.org/bugzilla/show_bug.cgi?id=12625 Reference: https://bugzilla.redhat.com/show_bug.cgi?id=688980 Reference: http://www.securityfocus.com/bid/46740 The addmntent function in the GNU C Library (aka glibc or libc6) 2.13 and earlier does not report an error status for failed attempts to write to the /etc/mtab file, which makes it easier for local users to trigger corruption of this file, as demonstrated by writes from a process with a small RLIMIT_FSIZE value, a different vulnerability than CVE-2010-0296.
MITRE assigned a number of CVEs to these issues; I've filed separate bugs for each one: CVE-2011-1675 (bug #695916) mount fails to anticipate RLIMIT_FSIZE CVE-2011-1676 (bug #695921) mount does not remove /etc/mtab.tmp after failed mount entry addition CVE-2011-1677 (bug #695924) mount does not remove /etc/mtab~ lock file after failed mount entry addition CVE-2011-1678 (bug #695925) mount.cifs and umount.cifs fail to anticipate RLIMIT_FSIZE CVE-2011-1679 (bug #695927) ncpmount and ncpumount fail to anticipate RLIMIT_FSIZE CVE-2011-1680 (bug #695932) ncpmount does not remove /etc/mtab~ lock file after failed mount entry addition
(In reply to comment #1) > glibc bug for the addmntent() issue that causes it to not report error when > output written to the file is truncated when hitting file size limit: > http://sourceware.org/bugzilla/show_bug.cgi?id=12625 Fix committed upstream now as: http://sourceware.org/git/?p=glibc.git;a=commitdiff;h=e1fb097f447a89aa69a926e45e673a52d86a6c57
Acknowledgements: Red Hat would like to thank Dan Rosenberg for reporting this issue.
This issue has been addressed in following products: Red Hat Enterprise Linux 6 Via RHSA-2011:1526 https://rhn.redhat.com/errata/RHSA-2011-1526.html
This issue has been addressed in following products: Red Hat Enterprise Linux 5 Via RHSA-2012:0126 https://rhn.redhat.com/errata/RHSA-2012-0126.html
This issue has been addressed in following products: Red Hat Enterprise Linux 4 Via RHSA-2012:0125 https://rhn.redhat.com/errata/RHSA-2012-0125.html
Statement: (none)