🛡️ CVE-2025-40272 — kernel

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

In the Linux kernel, the following vulnerability has been resolved:

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

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.