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

Overview

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.

Affected

• Linux >= b29d4986d0da1a27cd35917cdb433672f5c95d7f < 01264a6a3a3ad7ac1d73443299cd5a9568002454
• Linux >= b29d4986d0da1a27cd35917cdb433672f5c95d7f < ee38fb00e1a80f46a4990e38f25ecb04ae7b7417
• Linux >= b29d4986d0da1a27cd35917cdb433672f5c95d7f < c7fb6bc864c4910b344dafa36dd5028e9b980768
• Linux >= b29d4986d0da1a27cd35917cdb433672f5c95d7f < 0aa745fea1f8dc81bcdd0a45e215b6706727b482
• Linux >= b29d4986d0da1a27cd35917cdb433672f5c95d7f < a2635d541a93fd111e743cf14b6275dc81be2abc
• Linux >= b29d4986d0da1a27cd35917cdb433672f5c95d7f < 25dcc1989c194ba2b5fb6d03cbb9b83814ac0d15
• Linux >= b29d4986d0da1a27cd35917cdb433672f5c95d7f < df3b8ef06cc62de4fca5d2108e285085b3cffd44
• Linux >= b29d4986d0da1a27cd35917cdb433672f5c95d7f < 7d1f98d668ee34c1d15bdc0420fdd062f24a27c0
• Linux 4.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.