This is a note to let you know that I've just added the patch titled
mm: cachestat: fix two shmem bugs
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mm-cachestat-fix-two-shmem-bugs.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From d5d39c707a4cf0bcc84680178677b97aa2cb2627 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Date: Fri, 15 Mar 2024 05:55:56 -0400
Subject: mm: cachestat: fix two shmem bugs
From: Johannes Weiner <hannes@xxxxxxxxxxx>
commit d5d39c707a4cf0bcc84680178677b97aa2cb2627 upstream.
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -4201,7 +4201,23 @@ 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 stable-queue which might be from hannes@xxxxxxxxxxx are
queue-6.6/init-kconfig-lower-gcc-version-check-for-warray-bounds.patch
queue-6.6/mm-cachestat-fix-two-shmem-bugs.patch
queue-6.6/drm-amdgpu-fix-deadlock-while-reading-mqd-from-debugfs.patch
