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Date: Wed, 23 Feb 2011 13:11:56 +0800
From: Eugene Teo <>
CC: Dan Rosenberg <>
Subject: Re: Physical access vulnerabilities and auto-mounting

On 02/23/2011 12:17 PM, Dan Rosenberg wrote:
> 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.

 From the security response perspective, I will likely classify them as 
security bugs but with a /very/ low impact. The attacking party must 
already have some form of physical access to the affected system, or the 
attack must require some social engineering to trick the user to mount a 
corrupted file system using a portable media.

It will be hard to break existing user experience if we were to disable 
auto-mounting entirely, but it makes sense to disable it if the screen 
is locked. I'm not sure if this will affect how we classify such bugs. 
I'm happy to hear more thoughts on this.

Eugene Teo / Red Hat Security Response Team

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