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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id n1sm12732251pfu.2.2020.08.15.09.52.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 15 Aug 2020 09:52:30 -0700 (PDT) Date: Sat, 15 Aug 2020 09:52:29 -0700 From: Kees Cook To: Alexander Popov Cc: Jann Horn , Will Deacon , Andrey Ryabinin , Alexander Potapenko , Dmitry Vyukov , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Masahiro Yamada , Masami Hiramatsu , Steven Rostedt , Peter Zijlstra , Krzysztof Kozlowski , Patrick Bellasi , David Howells , Eric Biederman , Johannes Weiner , Laura Abbott , Arnd Bergmann , Greg Kroah-Hartman , kasan-dev@googlegroups.com, linux-mm@kvack.org, kernel-hardening@lists.openwall.com, linux-kernel@vger.kernel.org, notify@kernel.org Subject: Re: [PATCH RFC 1/2] mm: Extract SLAB_QUARANTINE from KASAN Message-ID: <202008150939.A994680@keescook> References: <20200813151922.1093791-1-alex.popov@linux.com> <20200813151922.1093791-2-alex.popov@linux.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200813151922.1093791-2-alex.popov@linux.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 13, 2020 at 06:19:21PM +0300, Alexander Popov wrote: > Heap spraying is an exploitation technique that aims to put controlled > bytes at a predetermined memory location on the heap. Heap spraying for > exploiting use-after-free in the Linux kernel relies on the fact that on > kmalloc(), the slab allocator returns the address of the memory that was > recently freed. Allocating a kernel object with the same size and > controlled contents allows overwriting the vulnerable freed object. > > Let's extract slab freelist quarantine from KASAN functionality and > call it CONFIG_SLAB_QUARANTINE. This feature breaks widespread heap > spraying technique used for exploiting use-after-free vulnerabilities > in the kernel code. > > If this feature is enabled, freed allocations are stored in the quarantine > and can't be instantly reallocated and overwritten by the exploit > performing heap spraying. It may be worth clarifying that this is specifically only direct UAF and doesn't help with spray-and-overflow-into-a-neighboring-object attacks (i.e. both tend to use sprays, but the former doesn't depend on a write overflow). > Signed-off-by: Alexander Popov > --- > include/linux/kasan.h | 107 ++++++++++++++++++++----------------- > include/linux/slab_def.h | 2 +- > include/linux/slub_def.h | 2 +- > init/Kconfig | 11 ++++ > mm/Makefile | 3 +- > mm/kasan/Makefile | 2 + > mm/kasan/kasan.h | 75 +++++++++++++------------- > mm/kasan/quarantine.c | 2 + > mm/kasan/slab_quarantine.c | 99 ++++++++++++++++++++++++++++++++++ > mm/slub.c | 2 +- > 10 files changed, 216 insertions(+), 89 deletions(-) > create mode 100644 mm/kasan/slab_quarantine.c > > diff --git a/include/linux/kasan.h b/include/linux/kasan.h > index 087fba34b209..b837216f760c 100644 > --- a/include/linux/kasan.h > +++ b/include/linux/kasan.h > @@ -42,32 +42,14 @@ void kasan_unpoison_task_stack(struct task_struct *task); > void kasan_alloc_pages(struct page *page, unsigned int order); > void kasan_free_pages(struct page *page, unsigned int order); > > -void kasan_cache_create(struct kmem_cache *cache, unsigned int *size, > - slab_flags_t *flags); > - > void kasan_poison_slab(struct page *page); > void kasan_unpoison_object_data(struct kmem_cache *cache, void *object); > void kasan_poison_object_data(struct kmem_cache *cache, void *object); > void * __must_check kasan_init_slab_obj(struct kmem_cache *cache, > const void *object); > > -void * __must_check kasan_kmalloc_large(const void *ptr, size_t size, > - gfp_t flags); > void kasan_kfree_large(void *ptr, unsigned long ip); > void kasan_poison_kfree(void *ptr, unsigned long ip); > -void * __must_check kasan_kmalloc(struct kmem_cache *s, const void *object, > - size_t size, gfp_t flags); > -void * __must_check kasan_krealloc(const void *object, size_t new_size, > - gfp_t flags); > - > -void * __must_check kasan_slab_alloc(struct kmem_cache *s, void *object, > - gfp_t flags); > -bool kasan_slab_free(struct kmem_cache *s, void *object, unsigned long ip); > - > -struct kasan_cache { > - int alloc_meta_offset; > - int free_meta_offset; > -}; > > /* > * These functions provide a special case to support backing module > @@ -107,10 +89,6 @@ static inline void kasan_disable_current(void) {} > static inline void kasan_alloc_pages(struct page *page, unsigned int order) {} > static inline void kasan_free_pages(struct page *page, unsigned int order) {} > > -static inline void kasan_cache_create(struct kmem_cache *cache, > - unsigned int *size, > - slab_flags_t *flags) {} > - > static inline void kasan_poison_slab(struct page *page) {} > static inline void kasan_unpoison_object_data(struct kmem_cache *cache, > void *object) {} > @@ -122,17 +100,65 @@ static inline void *kasan_init_slab_obj(struct kmem_cache *cache, > return (void *)object; > } > > +static inline void kasan_kfree_large(void *ptr, unsigned long ip) {} > +static inline void kasan_poison_kfree(void *ptr, unsigned long ip) {} > +static inline void kasan_free_shadow(const struct vm_struct *vm) {} > +static inline void kasan_remove_zero_shadow(void *start, unsigned long size) {} > +static inline void kasan_unpoison_slab(const void *ptr) {} > + > +static inline int kasan_module_alloc(void *addr, size_t size) > +{ > + return 0; > +} > + > +static inline int kasan_add_zero_shadow(void *start, unsigned long size) > +{ > + return 0; > +} > + > +static inline size_t kasan_metadata_size(struct kmem_cache *cache) > +{ > + return 0; > +} > + > +#endif /* CONFIG_KASAN */ > + > +struct kasan_cache { > + int alloc_meta_offset; > + int free_meta_offset; > +}; > + > +#if defined(CONFIG_KASAN) || defined(CONFIG_SLAB_QUARANTINE) > + > +void kasan_cache_create(struct kmem_cache *cache, unsigned int *size, > + slab_flags_t *flags); > +void * __must_check kasan_kmalloc_large(const void *ptr, size_t size, > + gfp_t flags); > +void * __must_check kasan_kmalloc(struct kmem_cache *s, const void *object, > + size_t size, gfp_t flags); > +void * __must_check kasan_krealloc(const void *object, size_t new_size, > + gfp_t flags); > +void * __must_check kasan_slab_alloc(struct kmem_cache *s, void *object, > + gfp_t flags); > +bool kasan_slab_free(struct kmem_cache *s, void *object, unsigned long ip); > + > +#else /* CONFIG_KASAN || CONFIG_SLAB_QUARANTINE */ > + > +static inline void kasan_cache_create(struct kmem_cache *cache, > + unsigned int *size, > + slab_flags_t *flags) {} > + > static inline void *kasan_kmalloc_large(void *ptr, size_t size, gfp_t flags) > { > return ptr; > } > -static inline void kasan_kfree_large(void *ptr, unsigned long ip) {} > -static inline void kasan_poison_kfree(void *ptr, unsigned long ip) {} > + > static inline void *kasan_kmalloc(struct kmem_cache *s, const void *object, > size_t size, gfp_t flags) > { > return (void *)object; > } > + > static inline void *kasan_krealloc(const void *object, size_t new_size, > gfp_t flags) > { > @@ -144,43 +170,28 @@ static inline void *kasan_slab_alloc(struct kmem_cache *s, void *object, > { > return object; > } > + > static inline bool kasan_slab_free(struct kmem_cache *s, void *object, > unsigned long ip) > { > return false; > } > - > -static inline int kasan_module_alloc(void *addr, size_t size) { return 0; } > -static inline void kasan_free_shadow(const struct vm_struct *vm) {} > - > -static inline int kasan_add_zero_shadow(void *start, unsigned long size) > -{ > - return 0; > -} > -static inline void kasan_remove_zero_shadow(void *start, > - unsigned long size) > -{} > - > -static inline void kasan_unpoison_slab(const void *ptr) { } > -static inline size_t kasan_metadata_size(struct kmem_cache *cache) { return 0; } > - > -#endif /* CONFIG_KASAN */ > +#endif /* CONFIG_KASAN || CONFIG_SLAB_QUARANTINE */ > > #ifdef CONFIG_KASAN_GENERIC > - > #define KASAN_SHADOW_INIT 0 > - > -void kasan_cache_shrink(struct kmem_cache *cache); > -void kasan_cache_shutdown(struct kmem_cache *cache); > void kasan_record_aux_stack(void *ptr); > - > #else /* CONFIG_KASAN_GENERIC */ > +static inline void kasan_record_aux_stack(void *ptr) {} > +#endif /* CONFIG_KASAN_GENERIC */ > > +#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_SLAB_QUARANTINE) > +void kasan_cache_shrink(struct kmem_cache *cache); > +void kasan_cache_shutdown(struct kmem_cache *cache); > +#else /* CONFIG_KASAN_GENERIC || CONFIG_SLAB_QUARANTINE */ > static inline void kasan_cache_shrink(struct kmem_cache *cache) {} > static inline void kasan_cache_shutdown(struct kmem_cache *cache) {} > -static inline void kasan_record_aux_stack(void *ptr) {} > - > -#endif /* CONFIG_KASAN_GENERIC */ > +#endif /* CONFIG_KASAN_GENERIC || CONFIG_SLAB_QUARANTINE */ In doing this extraction, I wonder if function naming should be changed? If it's going to live a new life outside of KASAN proper, maybe call these functions quarantine_cache_*()? But perhaps that's too much churn... > #ifdef CONFIG_KASAN_SW_TAGS > > diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h > index 9eb430c163c2..fc7548f27512 100644 > --- a/include/linux/slab_def.h > +++ b/include/linux/slab_def.h > @@ -72,7 +72,7 @@ struct kmem_cache { > int obj_offset; > #endif /* CONFIG_DEBUG_SLAB */ > > -#ifdef CONFIG_KASAN > +#if defined(CONFIG_KASAN) || defined(CONFIG_SLAB_QUARANTINE) > struct kasan_cache kasan_info; > #endif > > diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h > index 1be0ed5befa1..71020cee9fd2 100644 > --- a/include/linux/slub_def.h > +++ b/include/linux/slub_def.h > @@ -124,7 +124,7 @@ struct kmem_cache { > unsigned int *random_seq; > #endif > > -#ifdef CONFIG_KASAN > +#if defined(CONFIG_KASAN) || defined(CONFIG_SLAB_QUARANTINE) > struct kasan_cache kasan_info; > #endif > > diff --git a/init/Kconfig b/init/Kconfig > index d6a0b31b13dc..de5aa061762f 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -1931,6 +1931,17 @@ config SLAB_FREELIST_HARDENED > sanity-checking than others. This option is most effective with > CONFIG_SLUB. > > +config SLAB_QUARANTINE > + bool "Enable slab freelist quarantine" > + depends on !KASAN && (SLAB || SLUB) > + help > + Enable slab freelist quarantine to break heap spraying technique > + used for exploiting use-after-free vulnerabilities in the kernel > + code. If this feature is enabled, freed allocations are stored > + in the quarantine and can't be instantly reallocated and > + overwritten by the exploit performing heap spraying. > + This feature is a part of KASAN functionality. > + To make this available to distros, I think this needs to be more than just a CONFIG. I'd love to see this CONFIG control the availability, but have a boot param control a ro-after-init static branch for these functions (like is done for init_on_alloc, hardened usercopy, etc). Then the branch can be off by default for regular distro users, and more cautious folks could enable it with a boot param without having to roll their own kernels. > [...] > +struct kasan_track { > + u32 pid; pid_t? > + depot_stack_handle_t stack; > +}; > [...] > +#if defined(CONFIG_KASAN_GENERIC) && \ > + (defined(CONFIG_SLAB) || defined(CONFIG_SLUB)) || \ > + defined(CONFIG_SLAB_QUARANTINE) This seems a bit messy. Perhaps an invisible CONFIG to do this logic and then the files can test for that? CONFIG_USE_SLAB_QUARANTINE or something? > [...] > + * Heap spraying is an exploitation technique that aims to put controlled > + * bytes at a predetermined memory location on the heap. Heap spraying for > + * exploiting use-after-free in the Linux kernel relies on the fact that on > + * kmalloc(), the slab allocator returns the address of the memory that was > + * recently freed. Allocating a kernel object with the same size and > + * controlled contents allows overwriting the vulnerable freed object. > + * > + * If freed allocations are stored in the quarantine, they can't be > + * instantly reallocated and overwritten by the exploit performing > + * heap spraying. I would clarify this with the details of what is actually happening: the allocation isn't _moved_ to a quarantine, yes? It's only marked as not available for allocation? > + */ > + > +#include > +#include > +#include > +#include > +#include "../slab.h" > +#include "kasan.h" > + > +void kasan_cache_create(struct kmem_cache *cache, unsigned int *size, > + slab_flags_t *flags) > +{ > + cache->kasan_info.alloc_meta_offset = 0; > + > + if (cache->flags & SLAB_TYPESAFE_BY_RCU || cache->ctor || > + cache->object_size < sizeof(struct kasan_free_meta)) { > + cache->kasan_info.free_meta_offset = *size; > + *size += sizeof(struct kasan_free_meta); > + BUG_ON(*size > KMALLOC_MAX_SIZE); Please don't use BUG_ON()[1]. Interesting! -Kees [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on -- Kees Cook