Re: [PATCH] f2fs: fix to avoid use-after-free in f2fs_stop_gc_thread()

[Chao Yu <chao@kernel.org>]

On 2024/7/27 23:27, Jaegeuk Kim wrote:
> On 07/27, Jaegeuk Kim wrote:
>> On 07/25, Chao Yu wrote:
>>> syzbot reports a f2fs bug as below:
>>>
>>>   __dump_stack lib/dump_stack.c:88 [inline]
>>>   dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
>>>   print_report+0xe8/0x550 mm/kasan/report.c:491
>>>   kasan_report+0x143/0x180 mm/kasan/report.c:601
>>>   kasan_check_range+0x282/0x290 mm/kasan/generic.c:189
>>>   instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
>>>   atomic_fetch_add_relaxed include/linux/atomic/atomic-instrumented.h:252 [inline]
>>>   __refcount_add include/linux/refcount.h:184 [inline]
>>>   __refcount_inc include/linux/refcount.h:241 [inline]
>>>   refcount_inc include/linux/refcount.h:258 [inline]
>>>   get_task_struct include/linux/sched/task.h:118 [inline]
>>>   kthread_stop+0xca/0x630 kernel/kthread.c:704
>>>   f2fs_stop_gc_thread+0x65/0xb0 fs/f2fs/gc.c:210
>>>   f2fs_do_shutdown+0x192/0x540 fs/f2fs/file.c:2283
>>>   f2fs_ioc_shutdown fs/f2fs/file.c:2325 [inline]
>>>   __f2fs_ioctl+0x443a/0xbe60 fs/f2fs/file.c:4325
>>>   vfs_ioctl fs/ioctl.c:51 [inline]
>>>   __do_sys_ioctl fs/ioctl.c:907 [inline]
>>>   __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:893
>>>   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>>>   do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
>>>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>
>>> The root cause is below race condition, it may cause use-after-free
>>> issue in sbi->gc_th pointer.
>>>
>>> - remount
>>>   - f2fs_remount
>>>    - f2fs_stop_gc_thread
>>>     - kfree(gc_th)
>>> 				- f2fs_ioc_shutdown
>>> 				 - f2fs_do_shutdown
>>> 				  - f2fs_stop_gc_thread
>>> 				   - kthread_stop(gc_th->f2fs_gc_task)
>>>
>>> We will call f2fs_do_shutdown() in two paths:
>>> - for f2fs_ioc_shutdown() path, we should grab sb->s_umount semaphore
>>> for fixing.
>>> - for f2fs_shutdown() path, it's safe since caller has already grabbed
>>> sb->s_umount semaphore.
>>>
>>> Reported-by: syzbot+1a8e2b31f2ac9bd3d148@syzkaller.appspotmail.com
>>> Closes: https://lore.kernel.org/linux-f2fs-devel/0000000000005c7ccb061e032b9b@google.com
>>> Fixes: 7950e9ac638e ("f2fs: stop gc/discard thread after fs shutdown")
>>> Signed-off-by: Chao Yu <chao@kernel.org>
>>> ---
>>>   fs/f2fs/file.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>> index 7a37f2b393b9..62d72da25754 100644
>>> --- a/fs/f2fs/file.c
>>> +++ b/fs/f2fs/file.c
>>> @@ -2388,7 +2388,10 @@ static int f2fs_ioc_shutdown(struct file *filp, unsigned long arg)
>>>   		}
>>>   	}
>>>   
>>> +	/* grab sb->s_umount to avoid racing w/ remount() */
>>> +	down_read(&sbi->sb->s_umount);
>>
>> Is this safe for freeze_bdev()?
> 
> bdev_freeze
>   -> fs_bdev_freeze
>    -> get_bdev_super
>     -> bdev_super_lock(bdev, true)
>      -> super_lock(sb, true)
>       -> __super_lock(sb, true)
>        -> down_write(s_umount)

Oops, let me check this.

Thanks,

> 
>>
>>>   	ret = f2fs_do_shutdown(sbi, in, readonly);
>>> +	up_read(&sbi->sb->s_umount);
>>>   
>>>   	if (need_drop)
>>>   		mnt_drop_write_file(filp);
>>> -- 
>>> 2.40.1