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

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

On 2025/10/31 18:19, David Hildenbrand wrote:
> On 31.10.25 10:59, 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:
>>
>> 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>
> 
> Fully agreed
> 
> Acked-by: David Hildenbrand <david@redhat.com>

Cheers!