FAILED: patch "[PATCH] dm: correctly handle chained bios in dec_pending()" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: NeilBrown <[email protected]>
Date: Thu, 15 Feb 2018 20:00:15 +1100
Subject: [PATCH] dm: correctly handle chained bios in dec_pending()

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.

index d6de00f367ef..68136806d365 100644
@@ -903,7 +903,8 @@ static void dec_pending(struct dm_io *io, blk_status_t error)
 			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);
 		}
 	}

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.

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

index c5522551122f..2ffe7db75acb 100644
@@ -809,7 +809,8 @@ static void dec_pending(struct dm_io *io, int error)
 		} 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);
 		}
 	}

2.14.0.rc0.dirty