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 <dvyu...@google.com>
Fixes: 55834c59098d ("mm: kasan: initial memory quarantine
implementation")
Signed-off-by: Alexander Potapenko <gli...@google.com>
---
 mm/kasan/quarantine.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
index 65793f1..416d3b0 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 +214,15 @@ 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();
+       if (new_quarantine_size < percpu_quarantines) {
+               WARN_ONCE(1,
+                       "Too little memory, disabling global KASAN 
quarantine.\n",
+               );
+               new_quarantine_size = 0;
+       } else {
+               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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