Commit 37f9579f4c31 ("blk-mq: Avoid that submitting a bio concurrently
with device removal triggers a crash") introduced a NULL pointer
dereference in generic_make_request(). The patch sets q to NULL and
enter_succeeded to false; right after, there's an 'if (enter_succeeded)'
which is not taken, and then the 'else' will dereference q in
blk_queue_dying(q).This patch just moves the 'q = NULL' to a point in which it won't trigger the oops, although the semantics of this NULLification remains untouched. A simple test case/reproducer is as follows: a) Build kernel v5.2-rc1 with CONFIG_BLK_CGROUP=n. b) Create a raid0 md array with 2 NVMe devices as members, and mount it with an ext4 filesystem. c) Run the following oneliner (supposing the raid0 is mounted in /mnt): (dd of=/mnt/tmp if=/dev/zero bs=1M count=999 &); sleep 0.3; echo 1 > /sys/block/nvme0n1/device/device/remove (whereas nvme0n1 is the 2nd array member) This will trigger the following oops: BUG: unable to handle kernel NULL pointer dereference at 0000000000000078 PGD 0 P4D 0 Oops: 0000 [#1] SMP PTI RIP: 0010:generic_make_request+0x32b/0x400 Call Trace: submit_bio+0x73/0x140 ext4_io_submit+0x4d/0x60 ext4_writepages+0x626/0xe90 do_writepages+0x4b/0xe0 [...] This patch has no functional changes and preserves the md/raid0 behavior when a member is removed before kernel v4.17. Cc: [email protected] # v4.17 Reviewed-by: Bart Van Assche <[email protected]> Reviewed-by: Ming Lei <[email protected]> Tested-by: Eric Ren <[email protected]> Fixes: 37f9579f4c31 ("blk-mq: Avoid that submitting a bio concurrently with device removal triggers a crash") Signed-off-by: Guilherme G. Piccoli <[email protected]> --- Changes V1->V2: * Implemented Ming's suggestion (drop {} from if) - thanks Ming! * Rebased to v5.2-rc1 * Added Reviewed-by/Tested-by tags Also, Ming mentioned a new patch series[0] that will refactor legacy IO path so probably the bug won't happen anymore. Even in this case, I consider this patch important specially aiming the stable releases, in which backporting small bugfixes is much simpler than more complex patch sets. [0] https://lore.kernel.org/linux-block/[email protected]/T/#t block/blk-core.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/block/blk-core.c b/block/blk-core.c index 419d600e6637..e887915c7804 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -1054,10 +1054,8 @@ blk_qc_t generic_make_request(struct bio *bio) flags = 0; if (bio->bi_opf & REQ_NOWAIT) flags = BLK_MQ_REQ_NOWAIT; - if (blk_queue_enter(q, flags) < 0) { + if (blk_queue_enter(q, flags) < 0) enter_succeeded = false; - q = NULL; - } } if (enter_succeeded) { @@ -1088,6 +1086,7 @@ blk_qc_t generic_make_request(struct bio *bio) bio_wouldblock_error(bio); else bio_io_error(bio); + q = NULL; } bio = bio_list_pop(&bio_list_on_stack[0]); } while (bio); -- 2.21.0
