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@xxxxxxxxxx>
> 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.
>
> Thanks, Anand
>
>> Fixes: 4fde46f0cc71 ("Btrfs: free the stale device")
>> Signed-off-by: Nikolay Borisov <nborisov@xxxxxxxx>
>>
>> ---
>> 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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