dm cache: fix null-deref with concurrent writes in passthrough mode

In the Linux kernel, the following vulnerability has been resolved:

dm cache: fix null-deref with concurrent writes in passthrough mode

In passthrough mode, when dm-cache starts to invalidate a cache

entry and bio prison cell lock fails due to concurrent write to

the same cached block, mg->cell remains NULL. The error path in

invalidate_complete() attempts to unlock and free the cell

unconditionally, causing a NULL pointer dereference:

KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]

CPU: 0 UID: 0 PID: 134 Comm: fio Not tainted 6.19.0-rc7 #3 PREEMPT

RIP: 0010:dm_cell_unlock_v2+0x3f/0x210

<snip>

Call Trace:

invalidate_complete+0xef/0x430

map_bio+0x130f/0x1a10

cache_map+0x320/0x6b0

__map_bio+0x458/0x510

dm_submit_bio+0x40e/0x16d0

__submit_bio+0x419/0x870

<snip>

Reproduce steps:

1. Create a cache device

dmsetup create cmeta --table "0 8192 linear /dev/sdc 0"

dmsetup create cdata --table "0 131072 linear /dev/sdc 8192"

dmsetup create corig --table "0 262144 linear /dev/sdc 262144"

dd if=/dev/zero of=/dev/mapper/cmeta bs=4k count=1 oflag=direct

dmsetup create cache --table "0 262144 cache /dev/mapper/cmeta \

/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writethrough smq 0"

2. Promote the first data block into cache

fio --filename=/dev/mapper/cache --name=populate --rw=write --bs=4k \

--direct=1 --size=64k

3. Reload the cache into passthrough mode

dmsetup suspend cache

dmsetup reload cache --table "0 262144 cache /dev/mapper/cmeta \

/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 passthrough smq 0"

dmsetup resume cache

4. Write to the first cached block concurrently

fio --filename=/dev/mapper/cache --name test --rw=randwrite --bs=4k \

--randrepeat=0 --direct=1 --numjobs=2 --size 64k

Fix by checking if mg->cell is valid before attempting to unlock it.