Re: [PATCH V2 1/2] block: Fix a NULL pointer dereference in generic_make_request()

[Song Liu <liu.song.a23@gmail.com>]

On Mon, May 20, 2019 at 3:10 PM Guilherme G. Piccoli
<gpiccoli@canonical.com> wrote:
>
> 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: stable@vger.kernel.org # v4.17
> Reviewed-by: Bart Van Assche <bvanassche@acm.org>
> Reviewed-by: Ming Lei <ming.lei@redhat.com>
> Tested-by: Eric Ren <renzhengeek@gmail.com>
> Fixes: 37f9579f4c31 ("blk-mq: Avoid that submitting a bio concurrently with device removal triggers a crash")
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@canonical.com>

Applied both patches! Thanks for the fix!

> ---
>
> 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/20190515030310.20393-1-ming.lei@redhat.com/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
>