mm: cachestat: fix two shmem bugs

Vulnerability Description

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 get_shadow_from_swap_cache() on it will result in an out-of-bounds access to swapper_spaces[]. Validate the entry with non_swap_entry() 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 __remove_mapping; swapin, invalidation, or swapoff have removed the shadow from swapcache after we saw the shmem swap entry. This will send a NULL to workingset_test_recent(). 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 get_shadow_from_swap_cache() returning NULL.

References

• https://git.kernel.org/stable/c/b79f9e1ff27c994a4c452235ba09e672ec698e23
• https://git.kernel.org/stable/c/d962f6c583458037dc7e529659b2b02b9dd3d94b
• https://git.kernel.org/stable/c/24a0e73d544439bb9329fbbafac44299e548a677
• https://git.kernel.org/stable/c/d5d39c707a4cf0bcc84680178677b97aa2cb2627