Re: [PATCH] btrfs: Fix UAF

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

On 01/29/2018 03:01 PM, Nikolay Borisov wrote:
> 
> 
> On 29.01.2018 04:38, Anand Jain wrote:
>>
>>
>> On 01/26/2018 09:20 PM, Nikolay Borisov wrote:
>>> Commit 4fde46f0cc71 ("Btrfs: free the stale device") introduced
>>> btrfs_free_stale_device which iterates the device lists for all
>>> registered btrfs filesystems and deletes those devices which aren't
>>> mounted. In a btrfs_devices structure has only 1 device attached to it
>>> and it is unused then btrfs_free_stale_devices will proceed to also
>>> free the btrfs_fs_devices struct itself. Currently this leads to a UAF
>>> since list_for_each_entry will try to perform a check on the already-
>>> freed memory to see if it has to terminated the loop.
>>>
>>> The fix is to use 'break' when we know we are freeing the current
>>> fs_devs.
>>
>>   No break is needed as we need to iterate all stale devices and delete
>>   the found stale entry, so commit [1] used list_for_each_entry_safe()
>>   and removed the break,
> 
> We only do the break if we know we have a single device in the current
> fs_devs struct. And executing free_fs_devices would have already freed
> the device + fs_devs.
> 
>>
>>   [1]
>>    commit 38cf665d338fca33af4b16f9ec7cad6637fc0fec
>>    Author: Anand Jain <Anand.Jain@oracle.com>
>>      btrfs: make btrfs_free_stale_device() to iterate all stales
>>
>>
>>   I am guessing UAF might be in[2], instead ?
>>
>>   [2]
>>      free_fs_devices(fs_devs)
>> ::
>>          while (!list_empty(&fs_devices->devices)) {
>>                  device = list_entry(fs_devices->devices.next,
>>                                      struct btrfs_device, dev_list);
> 
> It's not that, I thought so at first. But here using list_empty you are
> awlays accessing the head the of the list, which is guaranteed to be
> valid since you do the freeing of fs_devices outside of the while loop.

break when num_devices == 1 is fine;

   Reviewed-by: Anand Jain <anand.jain@oracle.com>

Thanks, Anand

>>
>> Thanks, Anand
>>
>>> Fixes: 4fde46f0cc71 ("Btrfs: free the stale device")
>>> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
>>>
>>> ---
>>>    fs/btrfs/volumes.c | 1 +
>>>    1 file changed, 1 insertion(+)
>>>
>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>>> index f7147740b68e..c3ab55336ee0 100644
>>> --- a/fs/btrfs/volumes.c
>>> +++ b/fs/btrfs/volumes.c
>>> @@ -645,6 +645,7 @@ static void btrfs_free_stale_devices(const char
>>> *path,
>>>                    btrfs_sysfs_remove_fsid(fs_devs);
>>>                    list_del(&fs_devs->list);
>>>                    free_fs_devices(fs_devs);
>>> +                break;
>>>                } else {
>>>                    fs_devs->num_devices--;
>>>                    list_del(&dev->dev_list);
>>>
>>
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