Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch

Matthew Wilcox
2008-10-09T13:01:46Z

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."