Re: [PATCH 6.1.y] mm/secretmem: fix GUP-fast succeeding on secretmem folios

[David Hildenbrand <david@redhat.com>]

On 08.04.24 12:34, David Hildenbrand wrote:
> folio_is_secretmem() currently relies on secretmem folios being LRU
> folios, to save some cycles.
> 
> However, folios might reside in a folio batch without the LRU flag set, or
> temporarily have their LRU flag cleared.  Consequently, the LRU flag is
> unreliable for this purpose.
> 
> In particular, this is the case when secretmem_fault() allocates a fresh
> page and calls filemap_add_folio()->folio_add_lru().  The folio might be
> added to the per-cpu folio batch and won't get the LRU flag set until the
> batch was drained using e.g., lru_add_drain().
> 
> Consequently, folio_is_secretmem() might not detect secretmem folios and
> GUP-fast can succeed in grabbing a secretmem folio, crashing the kernel
> when we would later try reading/writing to the folio, because the folio
> has been unmapped from the directmap.
> 
> Fix it by removing that unreliable check.
> 
> Link: https://lkml.kernel.org/r/20240326143210.291116-2-david@redhat.com
> Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas")
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Reported-by: xingwei lee <xrivendell7@gmail.com>
> Reported-by: yue sun <samsun1006219@gmail.com>
> Closes: https://lore.kernel.org/lkml/CABOYnLyevJeravW=QrH0JUPYEcDN160aZFb7kwndm-J2rmz0HQ@mail.gmail.com/
> Debugged-by: Miklos Szeredi <miklos@szeredi.hu>
> Tested-by: Miklos Szeredi <mszeredi@redhat.com>
> Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 65291dcfcf8936e1b23cfd7718fdfde7cfaf7706)

Forgot to add when cherry-picking

Signed-off-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb