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

[Jaegeuk Kim]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/linux-f2fs-devel/0000000000005c7ccb061e032b9b@xxxxxxxxxx
> Fixes: 7950e9ac638e ("f2fs: stop gc/discard thread after fs shutdown")
> Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
> ---
>  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()?

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