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Subject: Re: [PATCH] btrfs: fix a NULL pointer dereference
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>, Kangjie Lu <kjlu@umn.edu>
Cc:     pakki001@umn.edu, Chris Mason <clm@fb.com>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20190314075041.28966-1-kjlu@umn.edu>
 <b0b494f4-8686-8e50-faca-625d6f89bbe3@suse.com>
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From:   Nikolay Borisov <nborisov@suse.com>
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Message-ID: <fcdf722f-d128-03c0-99fb-a220f680193c@suse.com>
Date:   Thu, 14 Mar 2019 10:03:21 +0200
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On 14.03.19 г. 10:02 ч., Qu Wenruo wrote:
> 
> 
> On 2019/3/14 下午3:54, Nikolay Borisov wrote:
>>
>>
>> On 14.03.19 г. 9:50 ч., 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@umn.edu>
>>> ---
>>>  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,
>>>
>