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-J2rm... 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) Signed-off-by: David Hildenbrand david@redhat.com --- include/linux/secretmem.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h index 988528b5da43..48ffe325184c 100644 --- a/include/linux/secretmem.h +++ b/include/linux/secretmem.h @@ -14,10 +14,10 @@ static inline bool page_is_secretmem(struct page *page) * Using page_mapping() is quite slow because of the actual call * instruction and repeated compound_head(page) inside the * page_mapping() function. - * 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 (PageCompound(page) || !PageLRU(page)) + if (PageCompound(page)) return false;
mapping = (struct address_space *)