mm/secretmem: fix GUP-fast succeeding on secretmem folios

commit 65291dcfcf8936e1b23cfd7718fdfde7cfaf7706 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
index 35f3a4a8ceb1e3276f3be9ebb68a1dd275f50de0..acf7e1a3f3def9fd4027aa659ba1d4dec8e944b0 100644 (file)
--- a/include/linux/secretmem.h
+++ b/include/linux/secretmem.h
@@ -13,10 +13,10 @@ static inline bool folio_is_secretmem(struct folio *folio)
        /*
         * Using folio_mapping() is quite slow because of the actual call
         * instruction.
-        * We know that secretmem pages are not compound and LRU so we can
+        * We know that secretmem pages are not compound, so we can
         * save a couple of cycles here.
         */
-       if (folio_test_large(folio) || !folio_test_lru(folio))
+       if (folio_test_large(folio))
                return false;
 
        mapping = (struct address_space *)