Re: [PATCH v4 02/34] ext4: check the extent status again before inserting delalloc block

Ritesh Harjani
2024-05-01T12:19:59Z

Dave Chinner <david@fromorbit.com> writes:

> On Wed, Apr 10, 2024 at 10:29:16PM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@huawei.com>
>>
>> Now we lookup extent status entry without holding the i_data_sem before
>> inserting delalloc block, it works fine in buffered write path and
>> because it holds i_rwsem and folio lock, and the mmap path holds folio
>> lock, so the found extent locklessly couldn't be modified concurrently.
>> But it could be raced by fallocate since it allocate block whitout
>> holding i_rwsem and folio lock.
>>
>> ext4_page_mkwrite() ext4_fallocate()
>> block_page_mkwrite()
>> ext4_da_map_blocks()
>> //find hole in extent status tree
>> ext4_alloc_file_blocks()
>> ext4_map_blocks()
>> //allocate block and unwritten extent
>> ext4_insert_delayed_block()
>> ext4_da_reserve_space()
>> //reserve one more block
>> ext4_es_insert_delayed_block()
>> //drop unwritten extent and add delayed extent by mistake
>
> Shouldn't this be serialised by the file invalidation lock? Hole
> punching via fallocate must do this to avoid data use-after-free
> bugs w.r.t racing page faults and all the other fallocate ops need
> to serialise page faults to avoid page cache level data corruption.
> Yet here we see a problem resulting from a fallocate operation
> racing with a page fault....

IIUC, fallocate operations which invalidates the page cache contents needs
to take th invalidate_lock in exclusive mode to prevent page fault
operations from loading pages for stale mappings (blocks which were
marked free might get reused). This can cause stale data exposure.

Here the fallocate operation require allocation of unwritten extents and
does not require truncate of pagecache range. So I guess, it is not
strictly necessary to hold the invalidate lock here.
But I see XFS does take IOLOCK_EXCL AND MMAPLOCK_EXCL even for this operation.

I guess we could use the invalidate lock for fallocate operation in ext4
too. However, I think we still require the current patch. The reason is
ext4_da_map_blocks() call here first tries to lookup the extent status
cache w/o any i_data_sem lock in the fastpath. If it finds a hole, it
takes the i_data_sem in write mode and just inserts an entry into extent
status cache w/o re-checking for the same under the exclusive lock.
...So I believe we still should have this patch which re-verify under
the write lock if whether any other operation has inserted any entry
already or not.

>
> Ah, I see that the invalidation lock is only picked up deep inside
> ext4_punch_hole(), ext4_collapse_range(), ext4_insert_range() and
> ext4_zero_range(). They all do the same flush, lock, and dio wait
> preamble but each do it just a little bit differently. The allocation path does
> it just a little bit differently again and does not take the
> invalidate lock...

Yes, I think it is not stricly required to take invalidate lock in the
allocation path of fallocate. Hence it could expose such a problem which
existed in ext4_da_map_blocks(), right?

>
> Perhaps the ext4 fallocate code should be factored so that all the
> fallocate operations run the same flush, lock and wait code rather
> than having 5 slightly different copies of the same code?

Yes. I agree. These paths can be refactored and if we are doing so, we
may as well just use the invalidate lock as you suggested.

-ritesh