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authorJohannes Weiner <hannes@cmpxchg.org>2024-03-15 05:55:56 -0400
committerAndrew Morton <akpm@linux-foundation.org>2024-03-26 11:07:20 -0700
commitd5d39c707a4cf0bcc84680178677b97aa2cb2627 (patch)
treed0eb07a289d1765abe38aa2bb1265d519842651f /mm
parent9cecde80aae0fb0aa44425575d5aca71bc646d89 (diff)
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: cf264e1329fb ("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>
Diffstat (limited to 'mm')
-rw-r--r--mm/filemap.c16
1 files changed, 16 insertions, 0 deletions
diff --git a/mm/filemap.c b/mm/filemap.c
index 7437b2bd75c1..30de18c4fd28 100644
--- a/mm/filemap.c
+++ b/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))