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

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 08/01/2016 05:59 PM, Alexander Potapenko 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.
> 

Just curious, how did find this?
Overflow is possible if system has more than 32 cpus per GB of memory. AFIAK this quite unusual. 

> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Fixes: 55834c59098d ("mm: kasan: initial memory quarantine
> implementation")
> Signed-off-by: Alexander Potapenko <glider@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",
> +		);

Why WARN? I'd suggest pr_warn_once();

> +		new_quarantine_size = 0;
> +	} else {
> +		new_quarantine_size -= percpu_quarantines;
> +	}
>  	WRITE_ONCE(quarantine_size, new_quarantine_size);
>  
>  	last = global_quarantine.head;
> 

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