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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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From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Mon, 20 Apr 2020 09:46:01 +0530
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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;


> 
>>
>> 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.
>>>
>>
>