CVE-2025-40270: mm, swap: fix potential UAF issue for VMA readahead

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

mm, swap: fix potential UAF issue for VMA readahead

Since commit 78524b05f1a3 ("mm, swap: avoid redundant swap device pinning"), the common helper for allocating and preparing a folio in the swap cache layer no longer tries to get a swap device reference internally, because all callers of readswapcacheasync are already holding a swap entry reference. The repeated swap device pinning isn't needed on the same swap device.

Caller of VMA readahead is also holding a reference to the target entry's swap device, but VMA readahead walks the page table, so it might encounter swap entries from other devices, and call readswapcacheasync on another device without holding a reference to it.

So it is possible to cause a UAF when swapoff of device A raced with swapin on device B, and VMA readahead tries to read swap entries from device A. It's not easy to trigger, but in theory, it could cause real issues.

Make VMA readahead try to get the device reference first if the swap device is a different one from the target entry.