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    Subject[PATCH 5.1 086/155] nfsd: allow fh_want_write to be called twice
    Date
    [ Upstream commit 0b8f62625dc309651d0efcb6a6247c933acd8b45 ]

    A fuzzer recently triggered lockdep warnings about potential sb_writers
    deadlocks caused by fh_want_write().

    Looks like we aren't careful to pair each fh_want_write() with an
    fh_drop_write().

    It's not normally a problem since fh_put() will call fh_drop_write() for
    us. And was OK for NFSv3 where we'd do one operation that might call
    fh_want_write(), and then put the filehandle.

    But an NFSv4 protocol fuzzer can do weird things like call unlink twice
    in a compound, and then we get into trouble.

    I'm a little worried about this approach of just leaving everything to
    fh_put(). But I think there are probably a lot of
    fh_want_write()/fh_drop_write() imbalances so for now I think we need it
    to be more forgiving.

    Signed-off-by: J. Bruce Fields <bfields@redhat.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/nfsd/vfs.h | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    diff --git a/fs/nfsd/vfs.h b/fs/nfsd/vfs.h
    index a7e107309f76..db351247892d 100644
    --- a/fs/nfsd/vfs.h
    +++ b/fs/nfsd/vfs.h
    @@ -120,8 +120,11 @@ void nfsd_put_raparams(struct file *file, struct raparms *ra);

    static inline int fh_want_write(struct svc_fh *fh)
    {
    - int ret = mnt_want_write(fh->fh_export->ex_path.mnt);
    + int ret;

    + if (fh->fh_want_write)
    + return 0;
    + ret = mnt_want_write(fh->fh_export->ex_path.mnt);
    if (!ret)
    fh->fh_want_write = true;
    return ret;
    --
    2.20.1


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