Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault handler

[Lance Yang <lance.yang@linux.dev>]

On 2025/10/31 18:24, Lorenzo Stoakes wrote:
> Small thing, sorry to be a pain buuuut could we please not send patches
> in-reply to another mail, it makes it harder for people to see :)
> 
> On Fri, Oct 31, 2025 at 11:59:16AM +0200, Mike Rapoport wrote:
>> On Fri, Oct 31, 2025 at 05:18:18PM +0800, Lance Yang wrote:
>>> From: Lance Yang <lance.yang@linux.dev>
>>>
>>> The error path in secretmem_fault() frees a folio before restoring its
>>> direct map status, which is a race leading to a panic.
>>
>> Let's use the issue description from the report:
>>
>> When a page fault occurs in a secret memory file created with
>> `memfd_secret(2)`, the kernel will allocate a new folio for it, mark
>> the underlying page as not-present in the direct map, and add it to
>> the file mapping.
>>
>> If two tasks cause a fault in the same page concurrently, both could
>> end up allocating a folio and removing the page from the direct map,
>> but only one would succeed in adding the folio to the file
>> mapping. The task that failed undoes the effects of its attempt by (a)
>> freeing the folio again and (b) putting the page back into the direct
>> map. However, by doing these two operations in this order, the page
>> becomes available to the allocator again before it is placed back in
>> the direct mapping.
>>
>> If another task attempts to allocate the page between (a) and (b), and
>> the kernel tries to access it via the direct map, it would result in a
>> supervisor not-present page fault.
>>
>>> Fix the ordering to restore the map before the folio is freed.
>>
>> ... restore the direct map
>>
>> With these changes
>>
>> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
> Agree with David, Mike this looks 'obviously correct' thanks for addressing
> it.
> 
> But also as per Mike, please update message accordingly and send v2
> not-in-reply-to-anything :P

Sure. V2 is on the way ;)

> 
> With that said:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks!
Lance

> 
>>
>>>
>>> Cc: <stable@vger.kernel.org>
>>> Reported-by: Google Big Sleep <big-sleep-vuln-reports@google.com>
>>> Closes: https://lore.kernel.org/linux-mm/CAEXGt5QeDpiHTu3K9tvjUTPqo+d-=wuCNYPa+6sWKrdQJ-ATdg@mail.gmail.com/
>>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>>> ---
>>>   mm/secretmem.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/secretmem.c b/mm/secretmem.c
>>> index c1bd9a4b663d..37f6d1097853 100644
>>> --- a/mm/secretmem.c
>>> +++ b/mm/secretmem.c
>>> @@ -82,13 +82,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
>>>   		__folio_mark_uptodate(folio);
>>>   		err = filemap_add_folio(mapping, folio, offset, gfp);
>>>   		if (unlikely(err)) {
>>> -			folio_put(folio);
>>>   			/*
>>>   			 * If a split of large page was required, it
>>>   			 * already happened when we marked the page invalid
>>>   			 * which guarantees that this call won't fail
>>>   			 */
>>>   			set_direct_map_default_noflush(folio_page(folio, 0));
>>> +			folio_put(folio);
>>>   			if (err == -EEXIST)
>>>   				goto retry;
>>>
>>> --
>>> 2.49.0
>>>
>>
>> --
>> Sincerely yours,
>> Mike.