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

[Bart Van Assche <bvanassche@acm.org>]

On Tue, 2019-04-30 at 19:37 -0300, Guilherme G. Piccoli 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 v5.1-rc7 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)

Reviewed-by: Bart Van Assche <bvanassche@acm.org>