[PATCH 4.14 133/167] dm: correctly handle chained bios in dec_pending()

Greg Kroah-Hartman
2018-02-21T12:49:04Z

From: NeilBrown <neilb@suse.com>

commit 8dd601fa8317243be887458c49f6c29c2f3d719f upstream.

dec_pending() is given an error status (possibly 0) to be recorded
against a bio. It can be called several times on the one 'struct
dm_io', and it is careful to only assign a non-zero error to
io->status. However when it then assigned io->status to bio->bi_status,
it is not careful and could overwrite a genuine error status with 0.

This can happen when chained bios are in use. If a bio is chained
beneath the bio that this dm_io is handling, the child bio might
complete and set bio->bi_status before the dm_io completes.

This has been possible since chained bios were introduced in 3.14, and
has become a lot easier to trigger with commit 18a25da84354 ("dm: ensure
bio submission follows a depth-first tree walk") as that commit caused
dm to start using chained bios itself.

A particular failure mode is that if a bio spans an 'error' target and a
working target, the 'error' fragment will complete instantly and set the
->bi_status, and the other fragment will normally complete a little
later, and will clear ->bi_status.

The fix is simply to only assign io_error to bio->bi_status when
io_error is not zero.

Reported-and-tested-by: Milan Broz <gmazyland@gmail.com>
Cc: stable@vger.kernel.org (v3.14+)
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/md/dm.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -815,7 +815,8 @@ static void dec_pending(struct dm_io *io
queue_io(md, bio);
} else {
/* done with normal IO or empty flush */
- bio->bi_status = io_error;
+ if (io_error)
+ bio->bi_status = io_error;
bio_endio(bio);
}
}