kasan: fix quarantine_size accounting during cache removal

quarantine_size tracks the total number of bytes stored in
global_quarantine[].  It is incremented when per-CPU quarantine objects
are moved into the global quarantine and decremented when a global batch
is evicted by kasan_quarantine_reduce().

kasan_quarantine_remove_cache() also removes objects from the global
quarantine.  qlist_move_cache() rebuilds the source batch and updates its
.bytes field, but quarantine_size is not adjusted accordingly.

As a result, quarantine_size remains over-counted by the size of the
removed objects.  The stale accounting accumulates across cache removals.
Once the inflated value exceeds quarantine_max_size,
kasan_quarantine_reduce() can evict a batch even though the actual number
of bytes in global_quarantine[] is still below quarantine_max_size,
shortening the quarantine window.

Fix the accounting by recording each batch's size before
qlist_move_cache() and subtracting the number of bytes actually removed
from quarantine_size while holding quarantine_lock.

A KUnit reproducer used during testing observed the over-count grow by
4698864 bytes after one kasan_quarantine_remove_cache() call with the fix
reverted.  With this change applied, the over-count did not grow.

Link: https://lore.kernel.org/20260811073332.1351893-1-sh_def@163.com
Fixes: 64abdcb24351 ("kasan: eliminate long stalls during quarantine reduction")
Signed-off-by: Hui Su <sh_def@163.com>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260808031459.3032812-1-sh_def%40163.com
Reviewed-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
index 16f4e67..c9944fd 100644 (file)
--- a/mm/kasan/quarantine.c
+++ b/mm/kasan/quarantine.c
@@ -370,9 +370,14 @@ void kasan_quarantine_remove_cache(struct kmem_cache *cache)
 
        raw_spin_lock_irqsave(&quarantine_lock, flags);
        for (i = 0; i < QUARANTINE_BATCHES; i++) {
+               size_t old_bytes;
+
                if (qlist_empty(&global_quarantine[i]))
                        continue;
+               old_bytes = global_quarantine[i].bytes;
                qlist_move_cache(&global_quarantine[i], &to_free, cache);
+               WRITE_ONCE(quarantine_size, quarantine_size -
+                          (old_bytes - global_quarantine[i].bytes));
                /* Scanning whole quarantine can take a while. */
                raw_spin_unlock_irqrestore(&quarantine_lock, flags);
                cond_resched();