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

[Andrew Morton]

On Mon,  1 Aug 2016 16:59:23 +0200 Alexander Potapenko <glider@xxxxxxxxxx> 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;
_