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: cf264e1329
("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>
This commit is contained in:
parent
9cecde80aa
commit
d5d39c707a
16
mm/filemap.c
16
mm/filemap.c
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@ -4197,7 +4197,23 @@ static void filemap_cachestat(struct address_space *mapping,
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/* shmem file - in swap cache */
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swp_entry_t swp = radix_to_swp_entry(folio);
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/* swapin error results in poisoned entry */
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if (non_swap_entry(swp))
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goto resched;
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/*
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* Getting a swap entry from the shmem
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* inode means we beat
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* shmem_unuse(). rcu_read_lock()
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* ensures swapoff waits for us before
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* freeing the swapper space. However,
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* we can race with swapping and
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* invalidation, so there might not be
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* a shadow in the swapcache (yet).
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*/
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shadow = get_shadow_from_swap_cache(swp);
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if (!shadow)
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goto resched;
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}
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#endif
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if (workingset_test_recent(shadow, true, &workingset))
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