Re: [PATCH] btrfs: fix a NULL pointer dereference

[Nikolay Borisov]

On 14.03.19 Ð. 10:02 Ñ., Qu Wenruo wrote:
> 
> 
> On 2019/3/14 äå3:54, Nikolay Borisov wrote:
>>
>>
>> On 14.03.19 Ð. 9:50 Ñ., Kangjie Lu wrote:
>>> btrfs_lookup_block_group may fail and return NULL. The fix goes
>>> to out when it fails to avoid NULL pointer dereference.
>>
>> Actually no, in this case btrfs_lookup_block_group must never fail
>> because if we have an allocated eb then it must have been allocated from
>> a bg.
> 
> Yep, that's the normal case.
> 
> However I'm wondering if it's possible to get a bad eb which is cached.
> 
> Then we could hit such situation.
> 
> So I still believe being safe here still makes sense, especially who
> knows future fuzzed image will be.

Then I'd rather have ASSERT(cache)

> 
> Thanks,
> Qu
> 
>>
>>>
>>> Signed-off-by: Kangjie Lu <kjlu@xxxxxxx>
>>> ---
>>>  fs/btrfs/extent-tree.c | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>>> index 994f0cc41799..b1e7985bcb9d 100644
>>> --- a/fs/btrfs/extent-tree.c
>>> +++ b/fs/btrfs/extent-tree.c
>>> @@ -7303,6 +7303,8 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
>>>  
>>>  		pin = 0;
>>>  		cache = btrfs_lookup_block_group(fs_info, buf->start);
>>> +		if (!cache)
>>> +			goto out;
>>>  
>>>  		if (btrfs_header_flag(buf, BTRFS_HEADER_FLAG_WRITTEN)) {
>>>  			pin_down_extent(fs_info, cache, buf->start,
>>>
>