4.4-stable review patch. If anyone has any objections, please let me know.
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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 @@ -974,7 +974,8 @@ static void dec_pending(struct dm_io *io } else { /* done with normal IO or empty flush */ trace_block_bio_complete(md->queue, bio, io_error); - bio->bi_error = io_error; + if (io_error) + bio->bi_error = io_error; bio_endio(bio); } }