The patch titled
Subject: mm: cachestat: fix two shmem bugs
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-cachestat-fix-two-shmem-bugs.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-cachestat-fix-two-shmem-bugs.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: mm: cachestat: fix two shmem bugs
Date: Fri, 15 Mar 2024 05:55:56 -0400
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@xxxxxxxxxxx
Fixes: cf264e1329fb ("cachestat: implement cachestat syscall")
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Reported-by: Chengming Zhou <chengming.zhou@xxxxxxxxx> [Bug #1]
Reported-by: Jann Horn <jannh@xxxxxxxxxx> [Bug #2]
Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>
Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [v6.5+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
--- a/mm/filemap.c~mm-cachestat-fix-two-shmem-bugs
+++ a/mm/filemap.c
@@ -4197,7 +4197,23 @@ static void filemap_cachestat(struct add
/* 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))
_
Patches currently in -mm which might be from hannes@xxxxxxxxxxx are
mm-cachestat-fix-two-shmem-bugs.patch
