Re: general protection fault in loop_validate_file (2)

[Dongli Zhang]

On 3/19/19 5:41 PM, Jan Kara wrote:
> On Mon 18-03-19 20:27:02, Dongli Zhang wrote:
>> Hi Jan,
>>
>> Indeed there is another issue implicitly reported by below console output from
>> syzkaller:
>>
>> [  245.524424][ T9455] loop_reread_partitions: partition scan of loop0 () failed
>> (rc=-13)
>> [  245.563340][ T9499] kasan: CONFIG_KASAN_INLINE enabled
>> [  245.576412][ T9489] __loop_clr_fd: partition scan of loop0 failed (rc=-13)
>> [  245.581275][ T9499] kasan: GPF could be caused by NULL-ptr deref or user
>> memory access
>> [  245.602596][ T9499] general protection fault: 0000 [#1] PREEMPT SMP KASAN
>>
>> I think rc=-13 is because of below code at line 168:
>>
>> 162 int __blkdev_reread_part(struct block_device *bdev)
>> 163 {
>> 164         struct gendisk *disk = bdev->bd_disk;
>> 165
>> 166         if (!disk_part_scan_enabled(disk) || bdev != bdev->bd_contains)
>> 167                 return -EINVAL;
>> 168         if (!capable(CAP_SYS_ADMIN))
>> 169                 return -EACCES;
>> 170
>> 171         lockdep_assert_held(&bdev->bd_mutex);
>> 172
>> 173         return rescan_partitions(disk, bdev);
>> 174 }
>> 175 EXPORT_SYMBOL(__blkdev_reread_part);
>>
>> I can reproduce this by 'chown username /dev/loop0' on my test machine.
>>
>> Taking 'losetup -d /dev/loop0' as sample, as /dev/loop0 belongs to my username,
>> I am able to detach the loop without 'su'.
>>
>> However, because of above line 168, the partition scan would fail.
>>
>> Should we always assume the user should have admin privilege to detach
>> the loop and this is not a bug?
> 
> Firstly, __blkdev_reread_part() is used for all devices so we have to be
> *very* careful when we relax the permission checks there.
> 
> Secondly, I'm not convinced it is always safe to allow non-priviledged user
> to force repartitioning of a device. That exposes the whole partition table
> parsing code to non-priviledged user and thus increases possible attack
> surface.
> 
> But in this specific case we call __blkdev_reread_part() only to tear down
> partitions. So in this specific case calling it by unpriviledged user is
> fine plus leaving stale partitions behind is certainly not nice. What we
> could do is:
> 
> Export drop_partitions() functionality as a function like
> __blkdev_drop_part() that would call drop_partitions(bdev->bd_disk, bdev).
> It would expect (and assert) that &bdev->bd_mutex is already locked. It
> should probably also replicate the check:
> 
>         if (!disk_part_scan_enabled(disk) || bdev != bdev->bd_contains)
>                 return -EINVAL;
> 
> from __blkdev_reread_part(). And we can call this from __loop_clr_fd().
> 
> Then we can just unexport __blkdev_reread_part() (since only loop.c is
> using it) and fold it inside blkdev_reread_part().
> 
> What do you think?
> 
> 								Honza
> 

The idea is to duplicate a new caller of drop_partitions() which is specifically
used by loop. I think it works. It is safe as well as the functionality is
limited within loop.

However, perhaps we should not regard this as a bug? Below is the sample to
reproduce this issue:

$ dd if=/dev/zero of=tmp.raw bs=1M count=100 oflag=direct

$ parted tmp.raw --script mklabel msdos mkpart primary 0% 50% mkpart primary 50%
100%

$ sudo chown zhang /dev/loop0

$ losetup -P /dev/loop0 tmp.raw

$ ls -l /dev | grep loop0
brw-rw---- 1 zhang disk      7,   0 Mar  20 11:42 loop0   --> user (zhang)
brw-rw---- 1 root  disk    259,   0 Mar  20 11:42 loop0p1 --> root
brw-rw---- 1 root  disk    259,   1 Mar  20 11:42 loop0p2 --> root

$ losetup -d /dev/loop

>From above case, /dev/loop0 is owned by user (zhang), while the partitions are
owned by root.

Should the detach of loop owned by user also unmaps all related partitions owned
by root?

Perhaps we should assume the '-P' option is always used by root?


This is similar to the fact that the administrator should always use '-P' in
order to enforce partscan when the loop is detached. Otherwise, the partitions
belong to the loop would remain after 'losetup -d'.


Another example is from kpartx as below. If the partitions are mapped via
'kpartx -av', the user should always run 'kpartx -d' to remove all partition
mappings.

Otherwise, there would be dm-0 and dm-1 left.

$ sudo kpartx -av  tmp.raw
add map loop0p1 (253:0): 0 102400 linear 7:0 1
add map loop0p2 (253:1): 0 102399 linear 7:0 102401

$ ls -l /dev/mapper/
total 0
crw------- 1 root root 10, 236 3æ  20 11:16 control
lrwxrwxrwx 1 root root       7 3æ  20 11:21 loop0p1 -> ../dm-0
lrwxrwxrwx 1 root root       7 3æ  20 11:21 loop0p2 -> ../dm-1

$ sudo losetup -d /dev/loop0

$ ls -l /dev/mapper/
total 0
crw------- 1 root root 10, 236 3æ  20 11:16 control
lrwxrwxrwx 1 root root       7 3æ  20 11:21 loop0p1 -> ../dm-0
lrwxrwxrwx 1 root root       7 3æ  20 11:21 loop0p2 -> ../dm-1

Dongli Zhang