[PATCH v2] kasan: avoid overflowing quarantine size on low memory systems

[Alexander Potapenko <glider@google.com>]

If the total amount of memory assigned to quarantine is less than the
amount of memory assigned to per-cpu quarantines, |new_quarantine_size|
may overflow. Instead, set it to zero.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: 55834c59098d ("mm: kasan: initial memory quarantine
implementation")
Signed-off-by: Alexander Potapenko <glider@google.com>
---
v2: Don't print the warning if we don't have enough memory for quarantine.
---
 mm/kasan/quarantine.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
index 65793f1..8c29938 100644
--- a/mm/kasan/quarantine.c
+++ b/mm/kasan/quarantine.c
@@ -196,7 +196,7 @@ void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache)
 
 void quarantine_reduce(void)
 {
-	size_t new_quarantine_size;
+	size_t new_quarantine_size, percpu_quarantines;
 	unsigned long flags;
 	struct qlist_head to_free = QLIST_INIT;
 	size_t size_to_free = 0;
@@ -214,7 +214,9 @@ void quarantine_reduce(void)
 	 */
 	new_quarantine_size = (READ_ONCE(totalram_pages) << PAGE_SHIFT) /
 		QUARANTINE_FRACTION;
-	new_quarantine_size -= QUARANTINE_PERCPU_SIZE * num_online_cpus();
+	percpu_quarantines = QUARANTINE_PERCPU_SIZE * num_online_cpus();
+	new_quarantine_size = (new_quarantine_size < percpu_quarantines) ?
+		0 : new_quarantine_size - percpu_quarantines;
 	WRITE_ONCE(quarantine_size, new_quarantine_size);
 
 	last = global_quarantine.head;
-- 
2.8.0.rc3.226.g39d4020

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