linux-kernel - Re: [PATCH] xfs: prevent leaking uninitialized stack memory in FSGEOMETRY

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Message-ID: <20110214114629.GA13052@dastard>
Date:	Mon, 14 Feb 2011 22:46:29 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Dan Rosenberg <drosenberg@...curity.com>
Cc:	aelder@....com, xfs-masters@....sgi.com, xfs@....sgi.com,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	security@...nel.org
Subject: Re: [PATCH] xfs: prevent leaking uninitialized stack memory in
 FSGEOMETRY_V1

On Thu, Feb 10, 2011 at 09:25:04AM -0500, Dan Rosenberg wrote:
> The FSGEOMETRY_V1 ioctl (and its compat equivalent) calls out to
> xfs_fs_geometry() with a version number of 3.  This code path does not
> fill in the logsunit member of the passed xfs_fsop_geom_t, leading to
> the leaking of four bytes of uninitialized stack data to potentially
> unprivileged callers.  Since all other members are filled in all code
> paths and there are no padding bytes in this structure, it's safe to
> avoid an expensive memset() in favor of just clearing this one field.

If this really is a security problem, then it should use a memset.
This is not a performance critical path and there are differences in
the padding of the structure between 32 bit and 64 bit ioctl
variants (it has a compat ioctl handler) and that can only be
correctly handled by memset().

Also, using a memset means we won't have the problem of introducing
new uninitialised fields or padding if we ever rev the structure
again...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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