Re: [PATCH] btrfs: don't access possibly stale fs_info data in device_list_add

[David Sterba]

On Thu, Mar 03, 2022 at 10:40:27PM +0800, Dongliang Mu wrote:
> From: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
> 
> Syzbot reported a possible use-after-free in printing information
> in device_list_add.
> 
> Very similar with the bug fixed by commit 0697d9a61099 ("btrfs: don't
> access possibly stale fs_info data for printing duplicate device"),
> but this time the use occurs in btrfs_info_in_rcu.
> 
> ============================================================
> Call Trace:
>  kasan_report.cold+0x83/0xdf mm/kasan/report.c:459
>  btrfs_printk+0x395/0x425 fs/btrfs/super.c:244
>  device_list_add.cold+0xd7/0x2ed fs/btrfs/volumes.c:957
>  btrfs_scan_one_device+0x4c7/0x5c0 fs/btrfs/volumes.c:1387
>  btrfs_control_ioctl+0x12a/0x2d0 fs/btrfs/super.c:2409
>  vfs_ioctl fs/ioctl.c:51 [inline]
>  __do_sys_ioctl fs/ioctl.c:874 [inline]
>  __se_sys_ioctl fs/ioctl.c:860 [inline]
>  __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:860
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> ============================================================
> 
> Fix this by modifying device->fs_info to NULL too.
> 
> Reported-and-tested-by: syzbot+82650a4e0ed38f218363@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
> ---
>  fs/btrfs/volumes.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index b07d382d53a8..c1325bdae9a1 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -954,7 +954,7 @@ static noinline struct btrfs_device *device_list_add(const char *path,
>  						  task_pid_nr(current));
>  				return ERR_PTR(-EEXIST);
>  			}
> -			btrfs_info_in_rcu(device->fs_info,
> +			btrfs_info_in_rcu(NULL,

A few lines above this is also NULL and was fixed by 0697d9a61099
("btrfs: don't access possibly stale fs_info data for printing duplicate
device"), so yeah we probably need the same here.