Re: [PATCH 2/5] brd: synchronize using page and free page with rcu

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/04/21 13:24, Christoph Hellwig 写道:
> On Fri, Apr 18, 2025 at 05:38:23PM +0800, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> Currently, after fetching the page by xa_load() in IO path, there is no
>> protection and page can be freed concurrently by discard:
> 
> Ah, I guess this helps with the race I mentioned in reply to the
> previous patch.  Is the rcu_head in struct page available for use
> by subsystems freeing the page?

Take a look at other union fileds int the struct page, in this case,
the page:
  - not used for pagecache or anonymous page
  - not used for page_pool
  - not used for compound page
  - not used for zone device page

So, I think it's fine to use the rcu_head.

We may want to avoid the page reference here since it's atomic and will
affect IO performance.

BTW, perhaps this patch should be the first patch in this set. :)

Thanks,
Kuai
> 
>>
>> cpu0
>> brd_submit_bio
>>   brd_do_bvec
>>    page = brd_lookup_page
>>                            cpu1
>>                            brd_submit_bio
>>                             brd_do_discard
>>                              page = __xa_erase()
>>                              __free_page()
>>    // page UAF
>>
>> Fix the problem by protecting page with rcu.
>>
>> Fixes: 9ead7efc6f3f ("brd: implement discard support")
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>> ---
>>   drivers/block/brd.c | 11 ++++++++++-
>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/brd.c b/drivers/block/brd.c
>> index a6e4f005cb76..740ed13faaff 100644
>> --- a/drivers/block/brd.c
>> +++ b/drivers/block/brd.c
>> @@ -208,6 +208,7 @@ static int brd_do_bvec(struct brd_device *brd, struct page *page,
>>   			goto out;
>>   	}
>>   
>> +	rcu_read_lock();
>>   	mem = kmap_local_page(page);
>>   	if (!op_is_write(opf)) {
>>   		copy_from_brd(mem + off, brd, sector, len);
>> @@ -217,11 +218,19 @@ static int brd_do_bvec(struct brd_device *brd, struct page *page,
>>   		copy_to_brd(brd, mem + off, sector, len);
>>   	}
>>   	kunmap_local(mem);
>> +	rcu_read_unlock();
>>   
>>   out:
>>   	return err;
>>   }
>>   
>> +static void brd_free_one_page(struct rcu_head *head)
>> +{
>> +	struct page *page = container_of(head, struct page, rcu_head);
>> +
>> +	__free_page(page);
>> +}
>> +
>>   static void brd_do_discard(struct brd_device *brd, sector_t sector, u32 size)
>>   {
>>   	sector_t aligned_sector = (sector + PAGE_SECTORS) & ~PAGE_SECTORS;
>> @@ -232,7 +241,7 @@ static void brd_do_discard(struct brd_device *brd, sector_t sector, u32 size)
>>   	while (size >= PAGE_SIZE && aligned_sector < rd_size * 2) {
>>   		page = __xa_erase(&brd->brd_pages, aligned_sector >> PAGE_SECTORS_SHIFT);
>>   		if (page) {
>> -			__free_page(page);
>> +			call_rcu(&page->rcu_head, brd_free_one_page);
>>   			brd->brd_nr_pages--;
>>   		}
>>   		aligned_sector += PAGE_SECTORS;
>> -- 
>> 2.39.2
>>
>>
> ---end quoted text---
> 
> .
>