linux-kernel - Re: [PATCH] kasan: skip quarantine if object is still accessible under RCU

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Message-ID: <45cd4505-39a0-404d-9840-a0a75fcc707f@suse.cz>
Date: Thu, 24 Jul 2025 12:14:34 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Jann Horn <jannh@...gle.com>, Andrey Ryabinin <ryabinin.a.a@...il.com>,
 Alexander Potapenko <glider@...gle.com>,
 Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>,
 Vincenzo Frascino <vincenzo.frascino@....com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: kasan-dev@...glegroups.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kasan: skip quarantine if object is still accessible
 under RCU

On 7/23/25 16:59, Jann Horn wrote:
> Currently, enabling KASAN masks bugs where a lockless lookup path gets a
> pointer to a SLAB_TYPESAFE_BY_RCU object that might concurrently be
> recycled and is insufficiently careful about handling recycled objects:
> KASAN puts freed objects in SLAB_TYPESAFE_BY_RCU slabs onto its quarantine
> queues, even when it can't actually detect UAF in these objects, and the
> quarantine prevents fast recycling.
> 
> When I introduced CONFIG_SLUB_RCU_DEBUG, my intention was that enabling
> CONFIG_SLUB_RCU_DEBUG should cause KASAN to mark such objects as freed
> after an RCU grace period and put them on the quarantine, while disabling
> CONFIG_SLUB_RCU_DEBUG should allow such objects to be reused immediately;
> but that hasn't actually been working.

Was the "allow reuse immediately" not working also before you introduced
CONFIG_SLUB_RCU_DEBUG, or is it a side-effect of that? IOW should we add a
Fixes: here?

> I discovered such a UAF bug involving SLAB_TYPESAFE_BY_RCU yesterday; I
> could only trigger this bug in a KASAN build by disabling
> CONFIG_SLUB_RCU_DEBUG and applying this patch.
> 
> Signed-off-by: Jann Horn <jannh@...gle.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/kasan/common.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index ed4873e18c75..9142964ab9c9 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -230,16 +230,12 @@ static bool check_slab_allocation(struct kmem_cache *cache, void *object,
>  }
>  
>  static inline void poison_slab_object(struct kmem_cache *cache, void *object,
> -				      bool init, bool still_accessible)
> +				      bool init)
>  {
>  	void *tagged_object = object;
>  
>  	object = kasan_reset_tag(object);
>  
> -	/* RCU slabs could be legally used after free within the RCU period. */
> -	if (unlikely(still_accessible))
> -		return;
> -
>  	kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
>  			KASAN_SLAB_FREE, init);
>  
> @@ -261,7 +257,22 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
>  	if (!kasan_arch_is_ready() || is_kfence_address(object))
>  		return false;
>  
> -	poison_slab_object(cache, object, init, still_accessible);
> +	/*
> +	 * If this point is reached with an object that must still be
> +	 * accessible under RCU, we can't poison it; in that case, also skip the
> +	 * quarantine. This should mostly only happen when CONFIG_SLUB_RCU_DEBUG
> +	 * has been disabled manually.
> +	 *
> +	 * Putting the object on the quarantine wouldn't help catch UAFs (since
> +	 * we can't poison it here), and it would mask bugs caused by
> +	 * SLAB_TYPESAFE_BY_RCU users not being careful enough about object
> +	 * reuse; so overall, putting the object into the quarantine here would
> +	 * be counterproductive.
> +	 */
> +	if (still_accessible)
> +		return false;
> +
> +	poison_slab_object(cache, object, init);
>  
>  	/*
>  	 * If the object is put into quarantine, do not let slab put the object
> @@ -519,7 +530,7 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
>  	if (check_slab_allocation(slab->slab_cache, ptr, ip))
>  		return false;
>  
> -	poison_slab_object(slab->slab_cache, ptr, false, false);
> +	poison_slab_object(slab->slab_cache, ptr, false);
>  	return true;
>  }
>  
> 
> ---
> base-commit: 89be9a83ccf1f88522317ce02f854f30d6115c41
> change-id: 20250723-kasan-tsbrcu-noquarantine-e207bb990e24
>