[PATCH] erofs: fix pcluster use-after-free on UP platforms

[Chao Yu]

On 2022/9/2 12:57, Gao Xiang wrote:
> During stress testing with CONFIG_SMP disabled, KASAN reports as below:
> 
> ==================================================================
> BUG: KASAN: use-after-free in __mutex_lock+0xe5/0xc30
> Read of size 8 at addr ffff8881094223f8 by task stress/7789
> [ 3482.258885]
> CPU: 0 PID: 7789 Comm: stress Not tainted 6.0.0-rc1-00002-g0d53d2e882f9 #3
> Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
> Call Trace:
>   <TASK>
> ..
>   __mutex_lock+0xe5/0xc30
> ..
>   z_erofs_do_read_page+0x8ce/0x1560
> ..
>   z_erofs_readahead+0x31c/0x580
> ..
> Freed by task 7787
>   kasan_save_stack+0x1e/0x40
>   kasan_set_track+0x20/0x30
>   kasan_set_free_info+0x20/0x40
>   __kasan_slab_free+0x10c/0x190
>   kmem_cache_free+0xed/0x380
>   rcu_core+0x3d5/0xc90
>   __do_softirq+0x12d/0x389
> [ 3482.295630]
> Last potentially related work creation:
>   kasan_save_stack+0x1e/0x40
>   __kasan_record_aux_stack+0x97/0xb0
>   call_rcu+0x3d/0x3f0
>   erofs_shrink_workstation+0x11f/0x210
>   erofs_shrink_scan+0xdc/0x170
>   shrink_slab.constprop.0+0x296/0x530
>   drop_slab+0x1c/0x70
>   drop_caches_sysctl_handler+0x70/0x80
>   proc_sys_call_handler+0x20a/0x2f0
>   vfs_write+0x555/0x6c0
>   ksys_write+0xbe/0x160
>   do_syscall_64+0x3b/0x90
> 
> The root cause is that erofs_workgroup_unfreeze() doesn't reset
> to orig_val thus it causes a race that the pcluster reuses unexpectedly
> before freeing.
> 
> Since UP platforms are quite rare now, such path becomes unnecessary.
> Let's drop such specific-designed path directly instead.
> 
> Fixes: 73f5c66df3e2 ("staging: erofs: fix `erofs_workgroup_{try_to_freeze, unfreeze}'")
> Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>

Reviewed-by: Chao Yu <chao at kernel.org>

Thanks,