linux-kernel - Re: [PATCH] btrfs: fix a NULL pointer dereference

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Message-ID: <9c9159c6-0807-65a5-5c26-30134dbb318e@umn.edu>
Date:   Thu, 14 Mar 2019 10:26:58 -0500
From:   Kangjie Lu <kjlu@....edu>
To:     Qu Wenruo <quwenruo.btrfs@....com>,
        Nikolay Borisov <nborisov@...e.com>
Cc:     pakki001@....edu, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: fix a NULL pointer dereference


On 3/14/19 4:15 AM, Qu Wenruo wrote:
>
> On 2019/3/14 下午4:03, Nikolay Borisov wrote:
>>
>> 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)
> Isn't assert() a bad idea for production build without assert() support?

I also agree with that, in general, assert should not be used in

production runs. The first patch might be better.


>
> Thanks,
> Qu
>
>>> Thanks,
>>> Qu
>>>
>>>>> Signed-off-by: Kangjie Lu <kjlu@....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,
>>>>>