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 folio 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 folio and removing the page from the direct map, but only one would succeed in adding the folio to the file mapping. The task that failed undoes the effects of its attempt by (a) freeing the folio 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 folio 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+6sWKr... 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) 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 4bedf491a8a7..f64ea1cde2f9 100644 --- a/mm/secretmem.c +++ b/mm/secretmem.c @@ -84,13 +84,13 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf) __folio_mark_uptodate(folio); err = filemap_add_folio(mapping, folio, offset, gfp); if (unlikely(err)) { - folio_put(folio); /* * 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); + folio_put(folio); if (err == -EEXIST) goto retry;