linux-kernel - Re: [PATCH] ext4: fix infinite loop when replaying fast

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Message-ID: <87pltniimq.fsf@brahms.olymp>
Date: Wed, 15 May 2024 09:28:29 +0100
From: Luis Henriques <luis.henriques@...ux.dev>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: Luis Henriques <luis.henriques@...ux.dev>,  linux-ext4@...r.kernel.org,
  linux-kernel@...r.kernel.org,  Theodore Ts'o <tytso@....edu>,  Andreas
 Dilger <adilger@...ger.ca>,  Harshad Shirwadkar
 <harshadshirwadkar@...il.com>
Subject: Re: [PATCH] ext4: fix infinite loop when replaying fast_commit

On Wed 15 May 2024 12:59:26 PM +08, Zhang Yi wrote;

> On 2024/5/14 21:04, Luis Henriques wrote:
>> On Sat 11 May 2024 02:24:17 PM +08, Zhang Yi wrote;
>> 
>>> On 2024/5/10 19:52, Luis Henriques (SUSE) wrote:
>>>> When doing fast_commit replay an infinite loop may occur due to an
>>>> uninitialized extent_status struct.  ext4_ext_determine_insert_hole() does
>>>> not detect the replay and calls ext4_es_find_extent_range(), which will
>>>> return immediately without initializing the 'es' variable.
>>>>
>>>> Because 'es' contains garbage, an integer overflow may happen causing an
>>>> infinite loop in this function, easily reproducible using fstest generic/039.
>>>>
>>>> This commit fixes this issue by detecting the replay in function
>>>> ext4_ext_determine_insert_hole().  It also adds initialization code to the
>>>> error path in function ext4_es_find_extent_range().
>>>>
>>>> Thanks to Zhang Yi, for figuring out the real problem!
>>>>
>>>> Fixes: 8016e29f4362 ("ext4: fast commit recovery path")
>>>> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@...ux.dev>
>>>> ---
>>>> Hi!
>>>>
>>>> Two comments:
>>>> 1) The change in ext4_ext_map_blocks() could probably use the min_not_zero
>>>>    macro instead.  I decided not to do so simply because I wasn't sure if
>>>>    that would be safe, but I'm fine changing that if you think it is.
>>>>
>>>> 2) I thought about returning 'EXT_MAX_BLOCKS' instead of '0' in
>>>>    ext4_lblk_t ext4_ext_determine_insert_hole(), which would then avoid
>>>>    the extra change to ext4_ext_map_blocks().  '0' sounds like the right
>>>>    value to return, but I'm also OK using 'EXT_MAX_BLOCKS' instead.
>>>>
>>>> And again thanks to Zhang Yi for pointing me the *real* problem!
>>>>
>>>>  fs/ext4/extents.c        | 6 +++++-
>>>>  fs/ext4/extents_status.c | 5 ++++-
>>>>  2 files changed, 9 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>>>> index e57054bdc5fd..b5bfcb6c18a0 100644
>>>> --- a/fs/ext4/extents.c
>>>> +++ b/fs/ext4/extents.c
>>>> @@ -4052,6 +4052,9 @@ static ext4_lblk_t ext4_ext_determine_insert_hole(struct inode *inode,
>>>>  	ext4_lblk_t hole_start, len;
>>>>  	struct extent_status es;
>>>>  
>>>> +	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
>>>> +		return 0;
>>>> +
>>>
>>> Sorry, I think it's may not correct. When replaying the jouranl, although
>>> we don't use the extent statue tree, we still need to query the accurate
>>> hole length, e.g. please see skip_hole(). If you do this, the hole length
>>> becomes incorrect, right?
>> 
>> Thank you for your review (and sorry for my delay replying).
>> 
>> So, I see three different options to follow your suggestion:
>> 
>> 1) Initialize 'es' immediately when declaring it in function
>>    ext4_ext_determine_insert_hole():
>> 
>> 	es.es_lblk = es.es_len = es.es_pblk = 0;
>> 
>> 2) Initialize 'es' only in ext4_es_find_extent_range() when checking if an
>>    fc replay is in progress (my patch was already doing something like
>>    that):
>> 
>> 	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY) {
>> 		/* Initialize extent to zero */
>> 		es->es_lblk = es->es_len = es->es_pblk = 0;
>> 		return;
>> 	}
>> 
>> 3) Remove the check for fc replay in function ext4_es_find_extent_range(),
>>    which will then unconditionally call __es_find_extent_range().  This
>>    will effectively also initialize the 'es' fields to '0' and, because
>>    __es_tree_search() will return NULL (at least in generic/039 test!),
>>    nothing else will be done.
>> 
>> Since all these 3 options seem to have the same result, I believe option
>> 1) is probably the best as it initializes the structure shortly after it's
>> declaration.  Would you agree?  Or did I misunderstood you?
>> 
>
> Both 1 and 2 are looks fine to me, but I would prefer to initialize it
> unconditionally in ext4_es_find_extent_range().
>
> @@ -310,6 +310,8 @@ void ext4_es_find_extent_range(struct inode *inode,
> 				ext4_lblk_t lblk, ext4_lblk_t end,
> 				struct extent_status *es)
>  {
> +	es->es_lblk = es->es_len = es->es_pblk = 0;
> +
> 	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
> 		return;

Thank you, Yi.  I'll send out v2 shortly.  Although, to be fair, the real
patch author shouldn't be me. :-)

Cheers,
-- 
Luis