linux-kernel - Re: [RESEND] [PATCH] VFS: make file->f

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Message-ID: <20081009130131.GV25780@parisc-linux.org>
Date:	Thu, 9 Oct 2008 07:01:32 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Pavel Machek <pavel@...e.cz>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	torvalds@...ux-foundation.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch

On Thu, Oct 09, 2008 at 02:23:19PM +0200, Pavel Machek wrote:
> On Tue 2008-10-07 20:52:09, Matthew Wilcox wrote:
> > And it's worth saying that letter-of-the-standard arguments aren't
> > necessarily enough.  Linux does not honour the POSIX guarantee that
> > writes are atomic (if they cross page boundaries, it's not certain).
> > This seems like even more of a corner case to me.
> 
> We have append-only files, and normal users should not be able to work
> around that restriction.

Is it possible to work around this restriction by exploiting this?

IS_APPEND() forces the user to have O_APPEND in their flags.
O_APPEND is only checked in generic_write_checks() where it sets '*pos'
to i_size.

For the majority of filesystems, generic_write_checks() is called from 
__generic_file_aio_write_nolock.  __generic_file_aio_write_nolock is
only called from generic_file_aio_write_nolock (which passes the address
of a kiocb->ki_pos) and generic_file_aio_write (same).

The filesystems that call generic_write_checks() directly are:
XFS (xfs_write): Passes the address of a local variable
OCFS2 (ocfs2_file_aio_write): Passes the address of a ki_pos
CIFS (cifs_user_write): Not sure.
NFS (nfs_file_direct_write): "Note that O_APPEND is not supported".
NTFS (ntfs_file_aio_write_nolock): Address of a local variable
FUSE (fuse_file_aio_write): Address of a local variable
FUSE (fuse_direct_write): Not sure.

So the only two that might be affected are CIFS and FUSE (O_DIRECT?!) as
far as I can tell.  I'm having a hard time believing this is a security
problem.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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