Re: [PATCH 1/1] ext4: fix potential negative array index in do_split()

[Eric Sandeen <sandeen@sandeen.net>]

On 6/19/20 2:08 AM, Lukas Czerner wrote:
> On Fri, Jun 19, 2020 at 08:41:22AM +0200, Lukas Czerner wrote:
>> On Wed, Jun 17, 2020 at 02:19:04PM -0500, Eric Sandeen wrote:
>>> If for any reason a directory passed to do_split() does not have enough
>>> active entries to exceed half the size of the block, we can end up
>>> iterating over all "count" entries without finding a split point.
>>>
>>> In this case, count == move, and split will be zero, and we will
>>> attempt a negative index into map[].
>>>
>>> Guard against this by detecting this case, and falling back to
>>> split-to-half-of-count instead; in this case we will still have
>>> plenty of space (> half blocksize) in each split block.
>>>
>>> Fixes: ef2b02d3e617 ("ext34: ensure do_split leaves enough free space in both blocks")
>>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>>> ---
>>>
>>> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
>>> index a8aca4772aaa..8b60881f07ee 100644
>>> --- a/fs/ext4/namei.c
>>> +++ b/fs/ext4/namei.c
>>> @@ -1858,7 +1858,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
>>>  			     blocksize, hinfo, map);
>>>  	map -= count;
>>>  	dx_sort_map(map, count);
>>> -	/* Split the existing block in the middle, size-wise */
>>> +	/* Ensure that neither split block is over half full */
>>>  	size = 0;
>>>  	move = 0;
>>>  	for (i = count-1; i >= 0; i--) {
>>> @@ -1868,8 +1868,18 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
>>>  		size += map[i].size;
>>>  		move++;
>>>  	}
>>> -	/* map index at which we will split */
>>> -	split = count - move;
>>> +	/*
>>> +	 * map index at which we will split
>>> +	 *
>>> +	 * If the sum of active entries didn't exceed half the block size, just
>>> +	 * split it in half by count; each resulting block will have at least
>>> +	 * half the space free.
>>> +	 */
>>> +	if (i > 0)
>>> +		split = count - move;
>>> +	else
>>> +		split = count/2;
>>
>> Won't we have exactly the same problem as we did before your commit
>> ef2b02d3e617cb0400eedf2668f86215e1b0e6af ? Since we do not know how much
>> space we actually moved we might have not made enough space for the new
>> entry ?
>>
>> Also since we have the move == count when the problem appears then it's
>> clear that we never hit the condition
>>
>> 1865 →       →       /* is more than half of this entry in 2nd half of the block? */
>> 1866 →       →       if (size + map[i].size/2 > blocksize/2)
>> 1867 →       →       →       break;
>>
>> in the loop. This is surprising but it means the the entries must have
>> gaps between them that are small enough that we can't fit the entry
>> right in ? Should not we try to compact it before splitting, or is it
>> the case that this should have been done somewhere else ?
> 
> The other possibility is that map[i].size is not right and indeed there
> seems to be a bug in dx_make_map()
> 
> map_tail->size = le16_to_cpu(de->rec_len);
> 
> should be
> 
> map_tail->size = ext4_rec_len_from_disk(de->rec_len, blocksize));
> 
> right ? Otherwise with large enough records the size will be smaller
> than it really is.

well, those are the same thing unless (PAGE_SIZE >= 65536) so I don't
think that's the issue here.

static inline unsigned int
ext4_rec_len_from_disk(__le16 dlen, unsigned blocksize)
{
        unsigned len = le16_to_cpu(dlen);

#if (PAGE_SIZE >= 65536)
...
#else
        return len;
#endif
}

Should be fixed for consistency, but seems to not be a root cause here.

> A quick look at fs/ext4/namei.c reveals couple of places there rec_len
> is used without the conversion and we should check whether it needs
> fixing.

...