CVE-2024-35797: mm: cachestat: fix two shmem bugs

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

mm: cachestat: fix two shmem bugs

When cachestat on shmem races with swapping and invalidation, there are two possible bugs:

1) A swapin error can have resulted in a poisoned swap entry in the shmem inode's xarray. Calling getshadowfromswapcache() on it will result in an out-of-bounds access to swapperspaces[].

Validate the entry with nonswapentry() before going further.

2) When we find a valid swap entry in the shmem's inode, the shadow entry in the swapcache might not exist yet: swap IO is still in progress and we're before removemapping; swapin, invalidation, or swapoff have removed the shadow from swapcache after we saw the shmem swap entry.

This will send a NULL to workingsettestrecent(). The latter purely operates on pointer bits, so it won't crash - node 0, memcg ID 0, eviction timestamp 0, etc. are all valid inputs - but it's a bogus test. In theory that could result in a false "recently evicted" count.

Such a false positive wouldn't be the end of the world. But for code clarity and (future) robustness, be explicit about this case.

Bail on getshadowfromswapcache() returning NULL.

— NVD