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

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

On 2025/10/31 17:59, 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:

Will do. I'll also add the missing Fixes: tag.

> 
> 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>

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
>>
>