Re: [PATCH -next] md: fix regression for null-ptr-deference in __md_stop()

[Song Liu]

On Tue, Mar 28, 2023 at 2:44 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Commit 3e453522593d ("md: Free resources in __md_stop") tried to fix
> null-ptr-deference for 'active_io' by moving percpu_ref_exit() to
> __md_stop(), however, the commit also moving 'writes_pending' to
> __md_stop(), and this will cause mdadm tests broken:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000038
> Oops: 0000 [#1] PREEMPT SMP
> CPU: 15 PID: 17830 Comm: mdadm Not tainted 6.3.0-rc3-next-20230324-00009-g520d37
> RIP: 0010:free_percpu+0x465/0x670
> Call Trace:
>  <TASK>
>  __percpu_ref_exit+0x48/0x70
>  percpu_ref_exit+0x1a/0x90
>  __md_stop+0xe9/0x170
>  do_md_stop+0x1e1/0x7b0
>  md_ioctl+0x90c/0x1aa0
>  blkdev_ioctl+0x19b/0x400
>  vfs_ioctl+0x20/0x50
>  __x64_sys_ioctl+0xba/0xe0
>  do_syscall_64+0x6c/0xe0
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> And the problem can be reporduced 100% by following test:
>
> mdadm -CR /dev/md0 -l1 -n1 /dev/sda --force
> echo inactive > /sys/block/md0/md/array_state
> echo read-auto  > /sys/block/md0/md/array_state
> echo inactive > /sys/block/md0/md/array_state
>
> Root cause:
>
> // start raid
> raid1_run
>  mddev_init_writes_pending
>   percpu_ref_init
>
> // inactive raid
> array_state_store
>  do_md_stop
>   __md_stop
>    percpu_ref_exit
>
> // start raid again
> array_state_store
>  do_md_run
>   raid1_run
>    mddev_init_writes_pending
>     if (mddev->writes_pending.percpu_count_ptr)
>     // won't reinit
>
> // inactive raid again
> ...
> percpu_ref_exit
> -> null-ptr-deference
>
> Before the commit, 'writes_pending' is exited when mddev is freed, and
> it's safe to restart raid because mddev_init_writes_pending() already make
> sure that 'writes_pending' will only be initialized once.
>
> Fix the prblem by moving 'writes_pending' back, it's a litter hard to find
> the relationship between alloc memory and free memory, however, code
> changes is much less and we lived with this for a long time already.
>
> Fixes: 3e453522593d ("md: Free resources in __md_stop")
>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>

Applied to md-fixes.

Thanks!
Song