Re: [PATCH] mm/swapfile: unuse_pte can map random data if swap read fails

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/4/4 22:11, Matthew Wilcox wrote:
> On Mon, Apr 04, 2022 at 03:37:36PM +0200, David Hildenbrand wrote:
>> On 01.04.22 09:29, Miaohe Lin wrote:
>>> There is a bug in unuse_pte(): when swap page happens to be unreadable,
>>> page filled with random data is mapped into user address space. The fix
>>> is to check for PageUptodate and fail swapoff in case of error.
>>>
>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>> ---
>>>  mm/swapfile.c | 4 ++++
>>>  1 file changed, 4 insertions(+)
>>>
>>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>>> index 63c61f8b2611..e72a35de7a0f 100644
>>> --- a/mm/swapfile.c
>>> +++ b/mm/swapfile.c
>>> @@ -1795,6 +1795,10 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>>>  		ret = 0;
>>>  		goto out;
>>>  	}
>>> +	if (unlikely(!PageUptodate(page))) {
>>> +		ret = -EIO;
>>> +		goto out;
>>> +	}
>>
>> Yeah, we have the same handling in do_swap_page(), whereby we send a
>> SIGBUS because we're dealing with an actual access.
>>
>> Interestingly, folio_test_uptodate() states:
>>
>> "Anonymous and CoW folios are always uptodate."
>>
>> @Willy, is that true or is the swapin case not documented there?
> 
> Why do we keep a !Uptodate page in the swap cache?  If it can't be
> read in from swap, I thought we just freed the page.  Since Miaohe

We could free the bad page. But we still need a way to prevent user from
accessing the wrong data.

> has observed that not happening, I guess it doesn't work that way,
> but why not make it work that way?

How could we make it work that way? Could you please tell me in more detail?
Or any suggestions?

Many thanks!

> 
> .
>