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

[Andrew Morton]

On Mon,  1 Aug 2016 16:59:23 +0200 Alexander Potapenko <gli...@google.com> 
wrote:

> 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.
> 
> --- a/mm/kasan/quarantine.c
> +++ b/mm/kasan/quarantine.c
> @@ -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;

This is a little tidier:

--- 
a/mm/kasan/quarantine.c~kasan-avoid-overflowing-quarantine-size-on-low-memory-systems-fix
+++ a/mm/kasan/quarantine.c
@@ -217,14 +217,11 @@ void quarantine_reduce(void)
        new_quarantine_size = (READ_ONCE(totalram_pages) << PAGE_SHIFT) /
                QUARANTINE_FRACTION;
        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",
-               );
+       if (WARN_ONCE(new_quarantine_size < percpu_quarantines,
+               "Too little memory, disabling global KASAN quarantine.\n"))
                new_quarantine_size = 0;
-       } else {
+       else
                new_quarantine_size -= percpu_quarantines;
-       }
        WRITE_ONCE(quarantine_size, new_quarantine_size);
 
        last = global_quarantine.head;
_