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

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Message-ID: <38f0ff98-3dcd-4dc2-87f1-3ea34bb9935a@redhat.com>
Date: Fri, 31 Oct 2025 11:19:50 +0100
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...nel.org>, Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, big-sleep-vuln-reports@...gle.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lorenzo.stoakes@...cle.com, willy@...radead.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/secretmem: fix use-after-free race in fault
 handler

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@...ux.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@...nel.org>

Fully agreed

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb