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

[Eric Ren <renzhengeek@gmail.com>]

Hello,

On Wed, 1 May 2019 at 06:38, 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 v5.1-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)
>
> 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: Bart Van Assche <bvanassche@acm.org>
> Cc: stable@vger.kernel.org # v4.17
> 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>

Thanks for the bugfix. I also had a panic having very similar
calltrace below as this one,
when using devicemapper in container scenario and deleting many thin
snapshots by dmsetup
remove_all -f, meanwhile executing lvm command like vgs.

After applied this one, my testing doesn't crash kernel any more for
one week.  Could the block
developers please give more feedback/priority on this one?

My panic trace:
```
50515.136279] BUG: unable to handle kernel NULL pointer dereference at
00000000000003b8
[50515.144704] PGD 0 P4D 0
[50515.147576] Oops: 0000 [#1] SMP PTI
[50515.151403] CPU: 24 PID: 45287 Comm: vgs Kdump: loaded Not tainted
4.19.24-9.x86_64#1
[50515.169295] RIP: 0010:generic_make_request+0x24/0x350
[50515.174684] Code: e9 e1 45 42 00 90 0f 1f 44 00 00 55 48 89 e5 41
55 41 54 53 48 89 fb 48 83 e4 f0 48 83 ec 20 48 8b 47 08 f6 47 15 08
8b 77 10 <4c> 8b a0 b8 03 00 00 0f 84 82 00 00 00 49 8b 84 24 50 07 00
00 a8
[50515.194303] RSP: 0018:ffffa90862373a10 EFLAGS: 00010246
[50515.199869] RAX: 0000000000000000 RBX: ffff99d7338b7800 RCX: 0000000000000000
[50515.207347] RDX: ffff99d51d89c380 RSI: 0000000000000000 RDI: ffff99d7338b7800
[50515.214828] RBP: ffffa90862373a48 R08: 0000000000000000 R09: ffff99a840007300
[50515.222305] R10: ffffa90862373b88 R11: 0000000000000000 R12: ffff99d833592200
[50515.229782] R13: ffffa90862373b58 R14: 0000000000000001 R15: 0000000000000000
[50515.237264] FS:  00007fc36b322880(0000) GS:ffff99d83f000000(0000)
knlGS:0000000000000000
[50515.245944] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[50515.252036] CR2: 00000000000003b8 CR3: 0000005c53ed0001 CR4: 00000000003626a0
[50515.259519] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[50515.266996] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[50515.274473] Call Trace:
[50515.277264]  ? bio_add_page+0x42/0x50
[50515.281267]  submit_bio+0x6e/0x130
[50515.285013]  new_read+0xa2/0xf0 [dm_bufio]
[50515.289454]  dm_bm_read_lock+0x21/0x70 [dm_persistent_data]
[50515.295369]  ro_step+0x31/0x60 [dm_persistent_data]
[50515.300589]  dm_btree_find_key+0xb0/0x180 [dm_persistent_data]
[50515.306765]  dm_thin_get_highest_mapped_block+0x75/0x90 [dm_thin_pool]
[50515.313639]  thin_status+0x145/0x290 [dm_thin_pool]
...
```
Regards,
Eric