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

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Date:   Thu, 14 Mar 2019 16:02:02 +0800
From:   Qu Wenruo <quwenruo.btrfs@....com>
To:     Nikolay Borisov <nborisov@...e.com>, Kangjie Lu <kjlu@....edu>
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 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.

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,
>>