Re: [PATCH] nilfs2: Fix nilfs_sufile_mark_dirty() not set segment usage as dirty

[Chen Zhongjin <chenzhongjin@huawei.com>]

On 2022/11/19 6:11, Andrew Morton wrote:
> On Fri, 18 Nov 2022 14:33:04 +0800 Chen Zhongjin <chenzhongjin@huawei.com> wrote:
>
>> In nilfs_sufile_mark_dirty(), the buffer and inode are set dirty, but
>> nilfs_segment_usage is not set dirty, which makes it can be found by
>> nilfs_sufile_alloc() because it checks nilfs_segment_usage_clean(su).
>>
>> This will cause the problem reported by syzkaller:
>> https://syzkaller.appspot.com/bug?id=c7c4748e11ffcc367cef04f76e02e931833cbd24
>>
>> It's because the case starts with segbuf1.segnum = 3, nextnum = 4, and
>> nilfs_sufile_alloc() not called to allocate a new segment.
>>
>> The first time nilfs_segctor_extend_segments() allocated segment
>> segbuf2.segnum = segbuf1.nextnum = 4, then nilfs_sufile_alloc() found
>> nextnextnum = 4 segment because its su is not set dirty.
>> So segbuf2.nextnum = 4, which causes next segbuf3.segnum = 4.
>>
>> sb_getblk() will get same bh for segbuf2 and segbuf3, and this bh is
>> added to both buffer lists of two segbuf.
>> It makes the list head of second list linked to the first one. When
>> iterating the first one, it will access and deref the head of second,
>> which causes NULL pointer dereference.
>>
>> Fixes: 9ff05123e3bf ("nilfs2: segment constructor")
> Merged in 2009!

Yes, seems it is introduced at the beginning of this file and the 
function called nilfs_touch_segusage().

Wired that this problem is not discovered utill now. So I'm wondering 
that whether this is a real-world
problem or just a use case constructed maliciously by syzkaller. But 
according to the result of syzkaller bisection,
this problem should have a history.
>> --- a/fs/nilfs2/sufile.c
>> +++ b/fs/nilfs2/sufile.c
>> @@ -495,12 +495,18 @@ void nilfs_sufile_do_free(struct inode *sufile, __u64 segnum,
>>   int nilfs_sufile_mark_dirty(struct inode *sufile, __u64 segnum)
>>   {
>>   	struct buffer_head *bh;
>> +	void *kaddr;
>> +	struct nilfs_segment_usage *su;
>>   	int ret;
>>   
>>   	ret = nilfs_sufile_get_segment_usage_block(sufile, segnum, 0, &bh);
>>   	if (!ret) {
>>   		mark_buffer_dirty(bh);
>>   		nilfs_mdt_mark_dirty(sufile);
>> +		kaddr = kmap_atomic(bh->b_page);
>> +		su = nilfs_sufile_block_get_segment_usage(sufile, segnum, bh, kaddr);
>> +		nilfs_segment_usage_set_dirty(su);
>> +		kunmap_atomic(kaddr);
>>   		brelse(bh);
>>   	}
>>   	return ret;
> Do we feel that this fix should be backported into -stable kernels?
Sorry that I'm not familiar with the specific use scenarios of nilfs2. 
So I can't offer a better advice. I think if it
is a problem that not happen easily in normal situations there's no 
necessary to backport it to stable.