Re: [PATCH 6.1.y] mm/secretmem: fix use-after-free race in fault handler

[Mike Rapoport <rppt@kernel.org>]

Oops, copied the wrong git send-email command, sorry for the noise

On Thu, Nov 20, 2025 at 09:15:47PM +0200, Mike Rapoport wrote:
> From: Lance Yang <lance.yang@linux.dev>
> 
> When a page fault occurs in a secret memory file created with
> `memfd_secret(2)`, the kernel will allocate a new page 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 page and removing the page from the direct map, but only
> one would succeed in adding the page to the file mapping.  The task that
> failed undoes the effects of its attempt by (a) freeing the page 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 direct map before the page is freed.
> 
> Link: https://lkml.kernel.org/r/20251031120955.92116-1-lance.yang@linux.dev
> Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas")
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> 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/
> Acked-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 6f86d0534fddfbd08687fa0f01479d4226bc3c3d)
> [rppt: replaced folio with page in the patch and in the changelog]
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>  mm/secretmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/secretmem.c b/mm/secretmem.c
> index 624663a94808..0c86133ad33f 100644
> --- a/mm/secretmem.c
> +++ b/mm/secretmem.c
> @@ -82,13 +82,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
>  		__SetPageUptodate(page);
>  		err = add_to_page_cache_lru(page, mapping, offset, gfp);
>  		if (unlikely(err)) {
> -			put_page(page);
>  			/*
>  			 * 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(page);
> +			put_page(page);
>  			if (err == -EEXIST)
>  				goto retry;
>  
> -- 
> 2.50.1
> 

-- 
Sincerely yours,
Mike.