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

Summary

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.