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

From: "Guilherme G. Piccoli" <gpiccoli@canonical.com>
To: stable@vger.kernel.org
Cc: Song Liu <songliubraving@fb.com>,
	linux-raid@vger.kernel.org, axboe@kernel.dk,
	gregkh@linuxfoundation.org, sashal@kernel.org
Subject: Re: [PATCH 1/2] block: Fix a NULL pointer dereference in generic_make_request()
Date: Wed, 12 Jun 2019 13:36:37 -0300	[thread overview]
Message-ID: <d1448b8d-1534-3b7a-d8e1-4c4bf467cf54@canonical.com> (raw)
In-Reply-To: <20190523172345.1861077-1-songliubraving@fb.com>

+ Greg, Sasha


On 23/05/2019 14:23, Song Liu wrote:
> From: "Guilherme G. Piccoli" <gpiccoli@canonical.com>
> 
> 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>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> 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>
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
>  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 a55389ba8779..d24a29244cb8 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1074,10 +1074,8 @@ 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) {
> @@ -1108,6 +1106,7 @@ 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);
>