linux-kernel - Re: file offset corruption on 32-bit machines?

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Date:	Mon, 14 Apr 2008 21:03:09 +0200
From:	Jan Kara <jack@...e.cz>
To:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
Cc:	Bodo Eggert <7eggert@....de>, Diego Calleja <diegocg@...il.com>,
	Jiri Kosina <jkosina@...e.cz>, Michal Hocko <mhocko@...e.cz>,
	Meelis Roos <mroos@...ux.ee>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: file offset corruption on 32-bit machines?

On Mon 14-04-08 13:06:13, Lennart Sorensen wrote:
> On Mon, Apr 14, 2008 at 06:53:54PM +0200, Jan Kara wrote:
> >   Well, but imagine you have a file /proc/my_secret_file from which you
> > are able to read from position A:a and B:b but not from position
> > A:b. Concievably, checks for the file position could be bypassed because of
> > this race... I know this is kind of dumb example but I can imagine someone
> > can eventually find something like this. So I guess one spin lock/unlock
> > pair is a price worth paying in the callpath which is quite long anyway.
> 
> But only two threads within the process can read from the filehandle and
> hence the process would be doing locking.  And external attacker can't
  Why would it be doing locking? If some nasty user runs the process, he
*wants* his two threads to race as much as possible and trigger the race.
And then use corrupted f_pos.

> break the internal locking of the process between the threads, and even
> if you do open the file in /proc that the process is using, being and
> external process you would have your own file handle and hence your own
> file position since you aren't part of that process.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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