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.

Link: https://lkml.kernel.org/r/20240315095556.GC581298@cmpxchg.org
Fixes: cf264e1 ("cachestat: implement cachestat syscall")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Chengming Zhou <chengming.zhou@linux.dev>	[Bug #1]
Reported-by: Jann Horn <jannh@google.com>		[Bug #2]
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: <stable@vger.kernel.org>				[v6.5+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/filemap.c

@@ -4197,7 +4197,23 @@ static void filemap_cachestat(struct address_space *mapping,
 				/* shmem file - in swap cache */
 				swp_entry_t swp = radix_to_swp_entry(folio);
 
+				/* swapin error results in poisoned entry */
+				if (non_swap_entry(swp))
+					goto resched;
+
+				/*
+				 * Getting a swap entry from the shmem
+				 * inode means we beat
+				 * shmem_unuse(). rcu_read_lock()
+				 * ensures swapoff waits for us before
+				 * freeing the swapper space. However,
+				 * we can race with swapping and
+				 * invalidation, so there might not be
+				 * a shadow in the swapcache (yet).
+				 */
 				shadow = get_shadow_from_swap_cache(swp);
+				if (!shadow)
+					goto resched;
 			}
 #endif
 			if (workingset_test_recent(shadow, true, &workingset))