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

[Anand Jain <anand.jain@oracle.com>]

On 04/03/2022 02:24, David Sterba wrote:
> On Thu, Mar 03, 2022 at 10:40:27PM +0800, Dongliang Mu wrote:
>> From: Dongliang Mu <mudongliangabcd@gmail.com>
>>
>> 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@syzkaller.appspotmail.com
>> Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
>> ---
>>   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.

So it appears that device->fs_info was garbage instead of NULL OR
fs_info->sb was NULL?
Because we always had a check if fs_info is null in btrfs_printk()
further the commit a0f6d924cada ("btrfs: remove stub device info from
messages when we have no fs_info") made it better.

Thanks, Anand