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Tue, 13 Aug 2024 09:03:03 +0000 From: Shinichiro Kawasaki To: Jann Horn CC: Andrey Ryabinin , Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , Roman Gushchin , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Marco Elver , "kasan-dev@googlegroups.com" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , David Sterba , "syzbot+263726e59eab6b442723@syzkaller.appspotmail.com" Subject: Re: [PATCH v8 2/2] slub: Introduce CONFIG_SLUB_RCU_DEBUG Thread-Topic: [PATCH v8 2/2] slub: Introduce CONFIG_SLUB_RCU_DEBUG Thread-Index: AQHa7V+aNJWuNJQxqEGdTCA84xSRzA== Date: Tue, 13 Aug 2024 09:03:03 +0000 Message-ID: References: <20240809-kasan-tsbrcu-v8-0-aef4593f9532@google.com> <20240809-kasan-tsbrcu-v8-2-aef4593f9532@google.com> In-Reply-To: <20240809-kasan-tsbrcu-v8-2-aef4593f9532@google.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-publictraffictype: Email x-ms-traffictypediagnostic: DM8PR04MB8037:EE_|CYYPR04MB9030:EE_ x-ms-office365-filtering-correlation-id: 46bf5952-472f-443a-99a9-08dcbb76bd43 wdcipoutbound: EOP-TRUE x-ms-exchange-senderadcheck: 1 x-ms-exchange-antispam-relay: 0 x-microsoft-antispam: BCL:0;ARA:13230040|7416014|366016|1800799024|376014|38070700018; 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When I tested the next-20240808 kernel which includes this patch, I observed that slab_free_after_rcu_debug() reports many WARNs. Please find my question in = line. On Aug 09, 2024 / 17:36, Jann Horn wrote: > Currently, KASAN is unable to catch use-after-free in SLAB_TYPESAFE_BY_RC= U > slabs because use-after-free is allowed within the RCU grace period by > design. >=20 > Add a SLUB debugging feature which RCU-delays every individual > kmem_cache_free() before either actually freeing the object or handing it > off to KASAN, and change KASAN to poison freed objects as normal when thi= s > option is enabled. >=20 > For now I've configured Kconfig.debug to default-enable this feature in t= he > KASAN GENERIC and SW_TAGS modes; I'm not enabling it by default in HW_TAG= S > mode because I'm not sure if it might have unwanted performance degradati= on > effects there. >=20 > Note that this is mostly useful with KASAN in the quarantine-based GENERI= C > mode; SLAB_TYPESAFE_BY_RCU slabs are basically always also slabs with a > ->ctor, and KASAN's assign_tag() currently has to assign fixed tags for > those, reducing the effectiveness of SW_TAGS/HW_TAGS mode. > (A possible future extension of this work would be to also let SLUB call > the ->ctor() on every allocation instead of only when the slab page is > allocated; then tag-based modes would be able to assign new tags on every > reallocation.) [...] > diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug > index afc72fde0f03..41a58536531d 100644 > --- a/mm/Kconfig.debug > +++ b/mm/Kconfig.debug > @@ -67,12 +67,44 @@ config SLUB_DEBUG_ON > equivalent to specifying the "slab_debug" parameter on boot. > There is no support for more fine grained debug control like > possible with slab_debug=3Dxxx. SLUB debugging may be switched > off in a kernel built with CONFIG_SLUB_DEBUG_ON by specifying > "slab_debug=3D-". > =20 > +config SLUB_RCU_DEBUG > + bool "Enable UAF detection in TYPESAFE_BY_RCU caches (for KASAN)" > + depends on SLUB_DEBUG > + # SLUB_RCU_DEBUG should build fine without KASAN, but is currently usel= ess > + # without KASAN, so mark it as a dependency of KASAN for now. > + depends on KASAN > + default KASAN_GENERIC || KASAN_SW_TAGS When I tested the next-20240808 kernel which includes this patch, I saw the SLUB_RCU_DEBUG was enabled because I enable KASAN_GENERIC and KASAN_SW_TAGS for my test target kernels. I also enable KFENCE. [...] > +#ifdef CONFIG_SLUB_RCU_DEBUG > +static void slab_free_after_rcu_debug(struct rcu_head *rcu_head) > +{ > + struct rcu_delayed_free *delayed_free =3D > + container_of(rcu_head, struct rcu_delayed_free, head); > + void *object =3D delayed_free->object; > + struct slab *slab =3D virt_to_slab(object); > + struct kmem_cache *s; > + > + kfree(delayed_free); > + > + if (WARN_ON(is_kfence_address(object))) > + return; With the kernel configs above, I see the many WARNs are reported here. When SLUB_RCU_DEBUG is enabled, should I disable KFENCE?=