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    SubjectRe: [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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