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Subject: Re: [PATCH] ext4: validate fiemap iomap begin offset and length value
To:     Murphy Zhou <jencce.kernel@gmail.com>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Jan Kara <jack@suse.cz>,
        "Darrick J. Wong" <darrick.wong@oracle.com>
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 <20200420072735.krkt2mundqguhqpl@xzhoux.usersys.redhat.com>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Mon, 20 Apr 2020 19:18:19 +0530
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Hello Murphy,


On 4/20/20 12:57 PM, Murphy Zhou wrote:
> On Mon, Apr 20, 2020 at 09:46:01AM +0530, Ritesh Harjani wrote:
>>
>>
>> On 4/20/20 8:27 AM, Murphy Zhou wrote:
>>> On Sun, Apr 19, 2020 at 09:49:27PM +0530, Ritesh Harjani wrote:
>>>> Hello Ted,
>>>>
>>>> On 4/19/20 10:16 AM, Theodore Y. Ts'o wrote:
>>>>
>>>>> ext4_map_block() is returning EFSCORRUPTED when lblk is
>>>>> EXT4_MAX_LOGICAL_BLOCK, which is why he's constraining lblk to
>>>>> EXT4_MAX_LOGICAL_BLOCK.  I haven't looked into this more closely yet,
>>>>
>>>> Yes, I did mention about this case in point 2 in below link though.
>>>> But maybe I was only focused on testing syzcaller reproducer, so
>>>> couldn't test this reported case.
>>>>
>>>> https://www.spinics.net/lists/linux-ext4/msg71387.html
>>>>
>>>>
>>>>> On Sun, Apr 19, 2020 at 12:42:24AM -0400, Theodore Y. Ts'o wrote:
>>>>>> I think we need to take his patch, and make a simialr change to
>>>>>> ext4_iomap_begin().   Ritesh, do you agree?
>>>>>
>>>>> For example...
>>>>>
>>>>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>>>>> index 2a4aae6acdcb..adce3339d697 100644
>>>>> --- a/fs/ext4/inode.c
>>>>> +++ b/fs/ext4/inode.c
>>>>> @@ -3424,8 +3424,10 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>>>>>     	int ret;
>>>>>     	struct ext4_map_blocks map;
>>>>>     	u8 blkbits = inode->i_blkbits;
>>>>> +	ext4_lblk_t lblk = offset >> blkbits;
>>>>> +	ext4_lblk_t last_lblk = (offset + length - 1) >> blkbits;
>>>>
>>>> Why play with last_lblk but?
>>>>
>>>>
>>>>
>>>>> -	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
>>>>> +	if (lblk > EXT4_MAX_LOGICAL_BLOCK)
>>>>>     		return -EINVAL;
>>>>>     	if (WARN_ON_ONCE(ext4_has_inline_data(inode)))
>>>>> @@ -3434,9 +3436,15 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>>>>>     	/*
>>>>>     	 * Calculate the first and last logical blocks respectively.
>>>>>     	 */
>>>>> -	map.m_lblk = offset >> blkbits;
>>>>> -	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
>>>>> -			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
>>>>> +	if (last_lblk >= EXT4_MAX_LOGICAL_BLOCK)
>>>>> +		last_lblk = EXT4_MAX_LOGICAL_BLOCK - 1;
>>>>> +	if (lblk >= EXT4_MAX_LOGICAL_BLOCK)
>>>>> +		lblk = EXT4_MAX_LOGICAL_BLOCK - 1;
>>>>> +
>>>>> +	map.m_lblk = lblk;
>>>>> +	map.m_len = last_lblk - lblk + 1;
>>>>> +	if (map.m_len == 0 )
>>>>> +		map.m_len = 1;
>>>>
>>>> Not sure but with above changes map.m_len will never be
>>>> 0. Right?
>>>
>>> Yes. If it's 0, in ext4_iomap_is_delalloc we will get an "end" that
>>> is less then m_lblk, causing another WARN in ext4_es_find_extent_range.
>>
>> Sorry lost you. Ok so what I meant above is.
>> With your changes made in above code to truncate last_lblk
>> and lblk, we may never end up in a situation where map.m_len will be 0.
>> So the below check in your code, isn't it redundant?
>> I wanted to double confirm this with you.
>>
>> +	if (map.m_len == 0 )
>> +		map.m_len = 1;
> 
> No it's not redundant. I hit and said that wo/ these two lines we will
> hit a WARN later.

Ok, so thanks for the logs. I should have figured this out earlier but
duh, missed it again.

Your values are too big and your variable 'last_lblk' is of type
'ext4_lblk_t' (which is u32).
So what you maybe seeing is an overflow case where sometimes your
last_lblk is becoming just 1 less than map.m_lblk and thus your are 
getting map.m_len to be 0.

Can you pls carefully review and confirm now this at your end?

_(My reasoning behind was this)_
Because for m_len to become 0, we should have lblk = last_lblk + 1.
This can only happen if length passed is 0. Or last_lblk got
overflowed and become less then lblk. Now AFAIU, length passed
as argument cannot be 0.


-ritesh

> 
> At first I thought truncating values is enough, but it's not.
> generic/013 (fsstress) can hit the WARN in fs/ext4/extents_status.c:266
> easily.
> 
> By printk values confirmed that at that time  m_len is zero.
> 
> Found some debug notes showing how crazy these numbers are:
> 
>   offset 80000395000 length 3533d50a37ee6ddb, lblk 80000395 llblk d0a3827b
>   lblk 80000395 llblk d0a3827b, m_lblk 80000395 m_len 50a37ee7
>   end d0a3827b, m_lblk 80000395 m_len 50a37ee7
>   offset d0a3827c000 length 3533cffffffffddb, lblk d0a3827c llblk d0a3827b
>   lblk d0a3827c llblk d0a3827b, m_lblk d0a3827c m_len 0
>   end d0a3827b, m_lblk d0a3827c m_len 0
>   ------------[ cut here ]------------
>   WARNING: CPU: 6 PID: 7962 at fs/ext4/extents_status.c:266 __es_find_extent_range+0x102/0x120 [ext4]
> 
> Thanks.
>>
>>
>>>
>>>>
>>>> Ok, so the problem mainly is coming since ext4_map_blocks()
>>>> is returning -EFSCORRUPTED in case if lblk >= EXT4_MAX_LOGICAL_BLOCK.
>>>>
>>>> So why change last_lblk?
>>>
>>> I guess because we need to make sure a sane length value. In the loop
>>> in iomap_fiemap, start and length are not checked, assuming be checked
>>> by caller. If length get overflowed, the start value for the next loop
>>> can also be affected, which makes lblk last_lblk and m_len to go crazy.
>>
>> Sorry I didn't it explain it right maybe. So if we are anyway changing
>> lblk by truncating it and making sure map.m_len is not getting
>> overflowed (as we did in my previous patch), then we need not play with
>> last_lblk anyways.
>>
>> And FWIW, instead of truncating lblk just so that ext4_map_blocks()
>> doesn't WARN, we can as well just return -ENOENT for
>> lblk >= EXT4_MAX_LOGICAL_BLOCK. ENOENT makes more sense to me,
>> but please feel free to correct me here.
>>
>> Thoughts?
>>
>> Meanwhile, I will also play this change (-ENOENT) a bit to at least get
>> few of the known test cases covered.
>>
>>
>> Also I do had this question for ext4.
>> EXT4_MAX_BLOCKS explaination says that's the max *number* of logical
>> blocks in a file. So since it is the number of blocks, it is equivalent
>> of length. Whereas the EXT4_MAX_LOGICAL_BLOCK says the max logical block
>> of a file, which is equivalent of offset.
>> Considering the logical offset starts from 0, so as Ted was saying
>> having both values same doesn't make sense. Ideally maybe
>> EXT4_MAX_LOGICAL_BLOCK should be 0xFFFFFFFFE.
>>
>> But that may also require some careful checking of all bounds of length
>> and offset across the code. So maybe we can revisit this later.
>> /*
>>   * Maximum number of logical blocks in a file; ext4_extent's ee_block is
>>   * __le32.
>>   */
>> #define EXT_MAX_BLOCKS	0xffffffff
>>
>>
>> /* Max logical block we can support */
>> #define EXT4_MAX_LOGICAL_BLOCK		0xFFFFFFFF
>>
>>
>> -ritesh
>>
>>>
>>> Thanks.
>>>
>>>> Shouldn't we just change the logic to return -ENOENT in case
>>>> if (lblk >= EXT4_MAX_LOGICAL_BLOCK)? ENOENT can be handled by
>>>> IOMAP APIs to abort the loop properly.
>>>> This along with the map.m_len overlflow patch which I had submitted
>>>> before. (since the overflow patch is anyway a valid fix which we anyways
>>>> need).
>>>>
>>>> -ritesh
>>>>
>>>>
>>>>>     	if (flags & IOMAP_WRITE)
>>>>>     		ret = ext4_iomap_alloc(inode, &map, flags);
>>>>> @@ -3524,8 +3532,10 @@ static int ext4_iomap_begin_report(struct inode *inode, loff_t offset,
>>>>>     	bool delalloc = false;
>>>>>     	struct ext4_map_blocks map;
>>>>>     	u8 blkbits = inode->i_blkbits;
>>>>> +	ext4_lblk_t lblk = offset >> blkbits;
>>>>> +	ext4_lblk_t last_lblk = (offset + length - 1) >> blkbits;
>>>>> -	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
>>>>> +	if (lblk > EXT4_MAX_LOGICAL_BLOCK)
>>>>>     		return -EINVAL;
>>>>>     	if (ext4_has_inline_data(inode)) {
>>>>> @@ -3540,9 +3550,15 @@ static int ext4_iomap_begin_report(struct inode *inode, loff_t offset,
>>>>>     	/*
>>>>>     	 * Calculate the first and last logical block respectively.
>>>>>     	 */
>>>>> -	map.m_lblk = offset >> blkbits;
>>>>> -	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
>>>>> -			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
>>>>> +	if (last_lblk >= EXT4_MAX_LOGICAL_BLOCK)
>>>>> +		last_lblk = EXT4_MAX_LOGICAL_BLOCK - 1;
>>>>> +	if (lblk >= EXT4_MAX_LOGICAL_BLOCK)
>>>>> +		lblk = EXT4_MAX_LOGICAL_BLOCK - 1;
>>>>> +
>>>>> +	map.m_lblk = lblk;
>>>>> +	map.m_len = last_lblk - lblk + 1;
>>>>> +	if (map.m_len == 0 )
>>>>> +		map.m_len = 1;
>>>>>     	/*
>>>>>     	 * Fiemap callers may call for offset beyond s_bitmap_maxbytes.
>>>>>
>>>>
>>>
>>
>