mm/secretmem: fix use-after-free race in fault handler

From: Lance Yang <[email protected]>

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.

---
v1 -> v2:
- Collect Reviewed-by from Mike and Lorenzo - thanks!
- Collect Acked-by from David - thanks!
- Update the changelog as Mike suggested
- https://lore.kernel.org/linux-mm/[email protected]/

 mm/secretmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index c1bd9a4b663d..37f6d1097853 100644
@@ -82,13 +82,13 @@ 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(folio_page(folio, 0));
+			folio_put(folio);
 			if (err == -EEXIST)
 				goto retry;
 

2.49.0