oss-security February 2011 archive
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oss-security: [oss-security] Physical access vulnerabilities and

[oss-security] Physical access vulnerabilities and auto-mounting

From: Dan Rosenberg <dan.j.rosenberg_at_nospam>
Date: Wed Feb 23 2011 - 04:17:54 GMT
To: oss-security@lists.openwall.com

I originally started writing this as a response to the recent CVE
requests for issues in partition handling, but thought it might be a
useful discussion on its own. I was wondering if there are any
clear-cut policies on issues involving physical access, since these
can be very difficult in terms of assigning blame.

For example, many Linux distributions will auto-mount filesystems on
removable storage, often going so far as to load corresponding kernel
modules for filesystems that aren't compiled in or don't already have
an LKM loaded. Sometimes, this will happen even if the screen is
locked.

Incidentally, many Linux filesystem implementations don't have
especially robust error handling for failures during attempts to mount
corrupt filesystems. As an example, I have a deliberately corrupted
btrfs filesystem that triggers a BUG() if you attempt to mount it. I
formatted a USB stick with this filesystem, so now I have a USB stick
that will panic the kernels of distributions that support
auto-mounting, in some cases even when the screen is locked.

Should this be considered a vulnerability? Probably. But what should
be fixed? Should auto-mounting be disabled entirely? Is it no longer
a vulnerability if auto-mounting is disabled only when the screen is
locked? Should all filesystems have graceful error handling for every
possible edge case that can occur when dealing with corruption?

I'd be interested to hear opinions on this. And depending on how the
discussion goes, I'd be happy to provide more details on specific
cases, such as the btrfs example.

-Dan