linux-ext4 - Re: [PATCH 1/4] ext4: fix double-free of blocks due to wrong extents moved

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Date: Tue, 19 Dec 2023 09:51:12 +0800
From: Baokun Li <libaokun1@...wei.com>
To: Jan Kara <jack@...e.cz>
CC: <linux-ext4@...r.kernel.org>, <tytso@....edu>, <adilger.kernel@...ger.ca>,
	<ritesh.list@...il.com>, <linux-kernel@...r.kernel.org>,
	<yi.zhang@...wei.com>, <yangerkun@...wei.com>, <yukuai3@...wei.com>, Wei Chen
	<harperchen1110@...il.com>, xingwei lee <xrivendell7@...il.com>,
	<stable@...r.kernel.org>, Baokun Li <libaokun1@...wei.com>
Subject: Re: [PATCH 1/4] ext4: fix double-free of blocks due to wrong extents
 moved_len

On 2023/12/18 23:32, Jan Kara wrote:
> On Mon 18-12-23 22:18:11, Baokun Li wrote:
>> In ext4_move_extents(), moved_len is only updated when all moves are
>> successfully executed, and only discards orig_inode and donor_inode
>> preallocations when moved_len is not zero. When the loop fails to exit
>> after successfully moving some extents, moved_len is not updated and
>> remains at 0, so it does not discard the preallocations.
>>
>> If the moved extents overlap with the preallocated extents, the
>> overlapped extents are freed twice in ext4_mb_release_inode_pa() and
>> ext4_process_freed_data() (as described in commit 94d7c16cbbbd), and
>> bb_free is incremented twice. Hence when trim is executed, a zero-division
>> bug is triggered in mb_update_avg_fragment_size() because bb_free is not
>> zero and bb_fragments is zero.
>>
>> Therefore, update move_len after each extent move to avoid the issue.
>>
>> Reported-by: Wei Chen <harperchen1110@...il.com>
>> Reported-by: xingwei lee <xrivendell7@...il.com>
>> Closes: https://lore.kernel.org/r/CAO4mrferzqBUnCag8R3m2zf897ts9UEuhjFQGPtODT92rYyR2Q@mail.gmail.com
>> Fixes: fcf6b1b729bc ("ext4: refactor ext4_move_extents code base")
>> CC: stable@...r.kernel.org # 3.18
>> Signed-off-by: Baokun Li <libaokun1@...wei.com>
>> ---
>>   fs/ext4/move_extent.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
>> index 3aa57376d9c2..4b9b503c6346 100644
>> --- a/fs/ext4/move_extent.c
>> +++ b/fs/ext4/move_extent.c
>> @@ -672,7 +672,7 @@ ext4_move_extents(struct file *o_filp, struct file *d_filp, __u64 orig_blk,
>>   		 */
>>   		ext4_double_up_write_data_sem(orig_inode, donor_inode);
>>   		/* Swap original branches with new branches */
>> -		move_extent_per_page(o_filp, donor_inode,
>> +		*moved_len += move_extent_per_page(o_filp, donor_inode,
>>   				     orig_page_index, donor_page_index,
>>   				     offset_in_page, cur_len,
>>   				     unwritten, &ret);
> Although this is currently fine, I think ext4_move_extents() should be
> careful and zero out *moved_len before it starts using it.
Totally agree, now __ext4_ioctl zeroes it out, but to avoid code logic
changes or new callers afterward,  I'll zero it out before using it in
ext4_move_extents() in the next version.
>
>> @@ -682,7 +682,6 @@ ext4_move_extents(struct file *o_filp, struct file *d_filp, __u64 orig_blk,
>>   		o_start += cur_len;
>>   		d_start += cur_len;
>>   	}
>> -	*moved_len = o_start - orig_blk;
>>   	if (*moved_len > len)
>>   		*moved_len = len;
> So I'm not sure the *moved_len > len condition can ever trigger but if it
> does, we'd need to check it whenever we are returning moved_len. So either
> I'd delete the condition or move it to the out label.
>
> 								Honza
As the code stands now, the *moved_len > len condition never
occurs, and is supposed to be code left over from the refactoring
in [Fixes], which I'll remove in the next version.

Thank you for your review!
-- 
With Best Regards,
Baokun Li
.