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Date: Thu, 3 Mar 2011 21:42:17 -0500
From: Dan Rosenberg <>
Subject: Suid mount helpers fail to anticipate RLIMIT_FSIZE

Hi all,

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.


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