From bb1c19636aedae39360e6fdbcaef4f2bcff25785 Mon Sep 17 00:00:00 2001 From: Lance Yang Date: Thu, 20 Nov 2025 21:38:21 +0200 Subject: [PATCH] mm/secretmem: fix use-after-free race in fault handler [ Upstream commit 6f86d0534fddfbd08687fa0f01479d4226bc3c3d ] 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 Reported-by: Google Big Sleep Closes: https://lore.kernel.org/linux-mm/CAEXGt5QeDpiHTu3K9tvjUTPqo+d-=wuCNYPa+6sWKrdQJ-ATdg@mail.gmail.com/ Acked-by: David Hildenbrand Reviewed-by: Mike Rapoport (Microsoft) Reviewed-by: Lorenzo Stoakes Cc: Matthew Wilcox (Oracle) Cc: Signed-off-by: Andrew Morton (cherry picked from commit 6f86d0534fddfbd08687fa0f01479d4226bc3c3d) [rppt: replaced folio with page in the patch and in the changelog] Signed-off-by: Mike Rapoport (Microsoft) Signed-off-by: Sasha Levin --- 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 @@ retry: __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;