Re: [PATCH v5 1/3] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

Kees Cook
2019-05-29T18:42:03Z

On Wed, May 29, 2019 at 02:38:10PM +0200, Alexander Potapenko wrote:
> The new options are needed to prevent possible information leaks and
> make control-flow bugs that depend on uninitialized values more
> deterministic.
>
> init_on_alloc=1 makes the kernel initialize newly allocated pages and heap
> objects with zeroes. Initialization is done at allocation time at the
> places where checks for __GFP_ZERO are performed.
>
> init_on_free=1 makes the kernel initialize freed pages and heap objects
> with zeroes upon their deletion. This helps to ensure sensitive data
> doesn't leak via use-after-free accesses.
>
> Both init_on_alloc=1 and init_on_free=1 guarantee that the allocator
> returns zeroed memory. The two exceptions are slab caches with
> constructors and SLAB_TYPESAFE_BY_RCU flag. Those are never
> zero-initialized to preserve their semantics.
>
> Both init_on_alloc and init_on_free default to zero, but those defaults
> can be overridden with CONFIG_INIT_ON_ALLOC_DEFAULT_ON and
> CONFIG_INIT_ON_FREE_DEFAULT_ON.
>
> Slowdown for the new features compared to init_on_free=0,
> init_on_alloc=0:
>
> hackbench, init_on_free=1: +7.62% sys time (st.err 0.74%)
> hackbench, init_on_alloc=1: +7.75% sys time (st.err 2.14%)
>
> Linux build with -j12, init_on_free=1: +8.38% wall time (st.err 0.39%)
> Linux build with -j12, init_on_free=1: +24.42% sys time (st.err 0.52%)
> Linux build with -j12, init_on_alloc=1: -0.13% wall time (st.err 0.42%)
> Linux build with -j12, init_on_alloc=1: +0.57% sys time (st.err 0.40%)
>
> The slowdown for init_on_free=0, init_on_alloc=0 compared to the
> baseline is within the standard error.
>
> The new features are also going to pave the way for hardware memory
> tagging (e.g. arm64's MTE), which will require both on_alloc and on_free
> hooks to set the tags for heap objects. With MTE, tagging will have the
> same cost as memory initialization.
>
> Although init_on_free is rather costly, there are paranoid use-cases where
> in-memory data lifetime is desired to be minimized. There are various
> arguments for/against the realism of the associated threat models, but
> given that we'll need the infrastructre for MTE anyway, and there are
> people who want wipe-on-free behavior no matter what the performance cost,
> it seems reasonable to include it in this series.
>
> Signed-off-by: Alexander Potapenko <glider@google.com>

I'm really looking forward to having this available! :)

Acked-by: Kees Cook <keescook@chromium.org>

> To: Andrew Morton <akpm@linux-foundation.org>
> To: Christoph Lameter <cl@linux.com>
> To: Kees Cook <keescook@chromium.org>
> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: James Morris <jmorris@namei.org>
> Cc: "Serge E. Hallyn" <serge@hallyn.com>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Kostya Serebryany <kcc@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Sandeep Patil <sspatil@android.com>
> Cc: Laura Abbott <labbott@redhat.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Jann Horn <jannh@google.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Marco Elver <elver@google.com>
> Cc: linux-mm@kvack.org
> Cc: linux-security-module@vger.kernel.org
> Cc: kernel-hardening@lists.openwall.com
> ---
> v2:
> - unconditionally initialize pages in kernel_init_free_pages()
> - comment from Randy Dunlap: drop 'default false' lines from Kconfig.hardening
> v3:
> - don't call kernel_init_free_pages() from memblock_free_pages()
> - adopted some Kees' comments for the patch description
> v4:
> - use NULL instead of 0 in slab_alloc_node() (found by kbuild test robot)
> - don't write to NULL object in slab_alloc_node() (found by Android
> testing)
> v5:
> - adjusted documentation wording as suggested by Kees
> - disable SLAB_POISON if auto-initialization is on
> - don't wipe RCU cache allocations made without __GFP_ZERO
> - dropped SLOB support
> ---
> .../admin-guide/kernel-parameters.txt | 9 +++
> drivers/infiniband/core/uverbs_ioctl.c | 2 +-
> include/linux/mm.h | 22 +++++++
> kernel/kexec_core.c | 2 +-
> mm/dmapool.c | 2 +-
> mm/page_alloc.c | 63 ++++++++++++++++---
> mm/slab.c | 16 ++++-
> mm/slab.h | 19 ++++++
> mm/slub.c | 33 ++++++++--
> net/core/sock.c | 2 +-
> security/Kconfig.hardening | 29 +++++++++
> 11 files changed, 180 insertions(+), 19 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 138f6664b2e2..84ee1121a2b9 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1673,6 +1673,15 @@
>
> initrd= [BOOT] Specify the location of the initial ramdisk
>
> + init_on_alloc= [MM] Fill newly allocated pages and heap objects with
> + zeroes.
> + Format: 0 | 1
> + Default set by CONFIG_INIT_ON_ALLOC_DEFAULT_ON.
> +
> + init_on_free= [MM] Fill freed pages and heap objects with zeroes.
> + Format: 0 | 1
> + Default set by CONFIG_INIT_ON_FREE_DEFAULT_ON.
> +
> init_pkru= [x86] Specify the default memory protection keys rights
> register contents for all processes. 0x55555554 by
> default (disallow access to all but pkey 0). Can
> diff --git a/drivers/infiniband/core/uverbs_ioctl.c b/drivers/infiniband/core/uverbs_ioctl.c
> index 829b0c6944d8..61758201d9b2 100644
> --- a/drivers/infiniband/core/uverbs_ioctl.c
> +++ b/drivers/infiniband/core/uverbs_ioctl.c
> @@ -127,7 +127,7 @@ __malloc void *_uverbs_alloc(struct uverbs_attr_bundle *bundle, size_t size,
> res = (void *)pbundle->internal_buffer + pbundle->internal_used;
> pbundle->internal_used =
> ALIGN(new_used, sizeof(*pbundle->internal_buffer));
> - if (flags & __GFP_ZERO)
> + if (want_init_on_alloc(flags))
> memset(res, 0, size);
> return res;
> }
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0e8834ac32b7..7733a341c0c4 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2685,6 +2685,28 @@ static inline void kernel_poison_pages(struct page *page, int numpages,
> int enable) { }
> #endif
>
> +#ifdef CONFIG_INIT_ON_ALLOC_DEFAULT_ON
> +DECLARE_STATIC_KEY_TRUE(init_on_alloc);
> +#else
> +DECLARE_STATIC_KEY_FALSE(init_on_alloc);
> +#endif
> +static inline bool want_init_on_alloc(gfp_t flags)
> +{
> + if (static_branch_unlikely(&init_on_alloc))
> + return true;
> + return flags & __GFP_ZERO;
> +}
> +
> +#ifdef CONFIG_INIT_ON_FREE_DEFAULT_ON
> +DECLARE_STATIC_KEY_TRUE(init_on_free);
> +#else
> +DECLARE_STATIC_KEY_FALSE(init_on_free);
> +#endif
> +static inline bool want_init_on_free(void)
> +{
> + return static_branch_unlikely(&init_on_free);
> +}
> +
> extern bool _debug_pagealloc_enabled;
>
> static inline bool debug_pagealloc_enabled(void)
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index fd5c95ff9251..2f75dd0d0d81 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -315,7 +315,7 @@ static struct page *kimage_alloc_pages(gfp_t gfp_mask, unsigned int order)
> arch_kexec_post_alloc_pages(page_address(pages), count,
> gfp_mask);
>
> - if (gfp_mask & __GFP_ZERO)
> + if (want_init_on_alloc(gfp_mask))
> for (i = 0; i < count; i++)
> clear_highpage(pages + i);
> }
> diff --git a/mm/dmapool.c b/mm/dmapool.c
> index 76a160083506..493d151067cb 100644
> --- a/mm/dmapool.c
> +++ b/mm/dmapool.c
> @@ -381,7 +381,7 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
> #endif
> spin_unlock_irqrestore(&pool->lock, flags);
>
> - if (mem_flags & __GFP_ZERO)
> + if (want_init_on_alloc(mem_flags))
> memset(retval, 0, pool->size);
>
> return retval;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d66bc8abe0af..50a3b104a491 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -136,6 +136,48 @@ unsigned long totalcma_pages __read_mostly;
>
> int percpu_pagelist_fraction;
> gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
> +#ifdef CONFIG_INIT_ON_ALLOC_DEFAULT_ON
> +DEFINE_STATIC_KEY_TRUE(init_on_alloc);
> +#else
> +DEFINE_STATIC_KEY_FALSE(init_on_alloc);
> +#endif
> +#ifdef CONFIG_INIT_ON_FREE_DEFAULT_ON
> +DEFINE_STATIC_KEY_TRUE(init_on_free);
> +#else
> +DEFINE_STATIC_KEY_FALSE(init_on_free);
> +#endif
> +
> +static int __init early_init_on_alloc(char *buf)
> +{
> + int ret;
> + bool bool_result;
> +
> + if (!buf)
> + return -EINVAL;
> + ret = kstrtobool(buf, &bool_result);
> + if (bool_result)
> + static_branch_enable(&init_on_alloc);
> + else
> + static_branch_disable(&init_on_alloc);
> + return ret;
> +}
> +early_param("init_on_alloc", early_init_on_alloc);
> +
> +static int __init early_init_on_free(char *buf)
> +{
> + int ret;
> + bool bool_result;
> +
> + if (!buf)
> + return -EINVAL;
> + ret = kstrtobool(buf, &bool_result);
> + if (bool_result)
> + static_branch_enable(&init_on_free);
> + else
> + static_branch_disable(&init_on_free);
> + return ret;
> +}
> +early_param("init_on_free", early_init_on_free);
>
> /*
> * A cached value of the page's pageblock's migratetype, used when the page is
> @@ -1090,6 +1132,14 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
> return ret;
> }
>
> +static void kernel_init_free_pages(struct page *page, int numpages)
> +{
> + int i;
> +
> + for (i = 0; i < numpages; i++)
> + clear_highpage(page + i);
> +}
> +
> static __always_inline bool free_pages_prepare(struct page *page,
> unsigned int order, bool check_free)
> {
> @@ -1142,6 +1192,8 @@ static __always_inline bool free_pages_prepare(struct page *page,
> }
> arch_free_page(page, order);
> kernel_poison_pages(page, 1 << order, 0);
> + if (want_init_on_free())
> + kernel_init_free_pages(page, 1 << order);
> if (debug_pagealloc_enabled())
> kernel_map_pages(page, 1 << order, 0);
>
> @@ -2020,8 +2072,8 @@ static inline int check_new_page(struct page *page)
>
> static inline bool free_pages_prezeroed(void)
> {
> - return IS_ENABLED(CONFIG_PAGE_POISONING_ZERO) &&
> - page_poisoning_enabled();
> + return (IS_ENABLED(CONFIG_PAGE_POISONING_ZERO) &&
> + page_poisoning_enabled()) || want_init_on_free();
> }
>
> #ifdef CONFIG_DEBUG_VM
> @@ -2075,13 +2127,10 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
> static void prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
> unsigned int alloc_flags)
> {
> - int i;
> -
> post_alloc_hook(page, order, gfp_flags);
>
> - if (!free_pages_prezeroed() && (gfp_flags & __GFP_ZERO))
> - for (i = 0; i < (1 << order); i++)
> - clear_highpage(page + i);
> + if (!free_pages_prezeroed() && want_init_on_alloc(gfp_flags))
> + kernel_init_free_pages(page, 1 << order);
>
> if (order && (gfp_flags & __GFP_COMP))
> prep_compound_page(page, order);
> diff --git a/mm/slab.c b/mm/slab.c
> index f7117ad9b3a3..98a89d7c922d 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1830,6 +1830,14 @@ static bool set_objfreelist_slab_cache(struct kmem_cache *cachep,
>
> cachep->num = 0;
>
> + /*
> + * If slab auto-initialization on free is enabled, store the freelist
> + * off-slab, so that its contents don't end up in one of the allocated
> + * objects.
> + */
> + if (unlikely(slab_want_init_on_free(cachep)))
> + return false;
> +
> if (cachep->ctor || flags & SLAB_TYPESAFE_BY_RCU)
> return false;
>
> @@ -3263,7 +3271,7 @@ slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid,
> local_irq_restore(save_flags);
> ptr = cache_alloc_debugcheck_after(cachep, flags, ptr, caller);
>
> - if (unlikely(flags & __GFP_ZERO) && ptr)
> + if (unlikely(slab_want_init_on_alloc(flags, cachep)) && ptr)
> memset(ptr, 0, cachep->object_size);
>
> slab_post_alloc_hook(cachep, flags, 1, &ptr);
> @@ -3320,7 +3328,7 @@ slab_alloc(struct kmem_cache *cachep, gfp_t flags, unsigned long caller)
> objp = cache_alloc_debugcheck_after(cachep, flags, objp, caller);
> prefetchw(objp);
>
> - if (unlikely(flags & __GFP_ZERO) && objp)
> + if (unlikely(slab_want_init_on_alloc(flags, cachep)) && objp)
> memset(objp, 0, cachep->object_size);
>
> slab_post_alloc_hook(cachep, flags, 1, &objp);
> @@ -3441,6 +3449,8 @@ void ___cache_free(struct kmem_cache *cachep, void *objp,
> struct array_cache *ac = cpu_cache_get(cachep);
>
> check_irq_off();
> + if (unlikely(slab_want_init_on_free(cachep)))
> + memset(objp, 0, cachep->object_size);
> kmemleak_free_recursive(objp, cachep->flags);
> objp = cache_free_debugcheck(cachep, objp, caller);
>
> @@ -3528,7 +3538,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> cache_alloc_debugcheck_after_bulk(s, flags, size, p, _RET_IP_);
>
> /* Clear memory outside IRQ disabled section */
> - if (unlikely(flags & __GFP_ZERO))
> + if (unlikely(slab_want_init_on_alloc(flags, s)))
> for (i = 0; i < size; i++)
> memset(p[i], 0, s->object_size);
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 43ac818b8592..31032d488b29 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -524,4 +524,23 @@ static inline int cache_random_seq_create(struct kmem_cache *cachep,
> static inline void cache_random_seq_destroy(struct kmem_cache *cachep) { }
> #endif /* CONFIG_SLAB_FREELIST_RANDOM */
>
> +static inline bool slab_want_init_on_alloc(gfp_t flags, struct kmem_cache *c)
> +{
> + if (static_branch_unlikely(&init_on_alloc)) {
> + if (c->ctor)
> + return false;
> + if (c->flags & SLAB_TYPESAFE_BY_RCU)
> + return flags & __GFP_ZERO;
> + return true;
> + }
> + return flags & __GFP_ZERO;
> +}
> +
> +static inline bool slab_want_init_on_free(struct kmem_cache *c)
> +{
> + if (static_branch_unlikely(&init_on_free))
> + return !(c->ctor || (c->flags & SLAB_TYPESAFE_BY_RCU));
> + return false;
> +}
> +
> #endif /* MM_SLAB_H */
> diff --git a/mm/slub.c b/mm/slub.c
> index cd04dbd2b5d0..9c4a8b9a955c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1279,6 +1279,12 @@ static int __init setup_slub_debug(char *str)
> if (*str == ',')
> slub_debug_slabs = str + 1;
> out:
> + if ((static_branch_unlikely(&init_on_alloc) ||
> + static_branch_unlikely(&init_on_free)) &&
> + (slub_debug & SLAB_POISON)) {
> + pr_warn("disabling SLAB_POISON: can't be used together with memory auto-initialization\n");
> + slub_debug &= ~SLAB_POISON;
> + }
> return 1;
> }
>
> @@ -1424,6 +1430,19 @@ static __always_inline bool slab_free_hook(struct kmem_cache *s, void *x)
> static inline bool slab_free_freelist_hook(struct kmem_cache *s,
> void **head, void **tail)
> {
> +
> + void *object;
> + void *next = *head;
> + void *old_tail = *tail ? *tail : *head;
> +
> + if (slab_want_init_on_free(s))
> + do {
> + object = next;
> + next = get_freepointer(s, object);
> + memset(object, 0, s->size);
> + set_freepointer(s, object, next);
> + } while (object != old_tail);
> +
> /*
> * Compiler cannot detect this function can be removed if slab_free_hook()
> * evaluates to nothing. Thus, catch all relevant config debug options here.
> @@ -1433,9 +1452,7 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
> defined(CONFIG_DEBUG_OBJECTS_FREE) || \
> defined(CONFIG_KASAN)
>
> - void *object;
> - void *next = *head;
> - void *old_tail = *tail ? *tail : *head;
> + next = *head;
>
> /* Head and tail of the reconstructed freelist */
> *head = NULL;
> @@ -2741,8 +2758,14 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
> prefetch_freepointer(s, next_object);
> stat(s, ALLOC_FASTPATH);
> }
> + /*
> + * If the object has been wiped upon free, make sure it's fully
> + * initialized by zeroing out freelist pointer.
> + */
> + if (unlikely(slab_want_init_on_free(s)) && object)
> + *(void **)object = NULL;
>
> - if (unlikely(gfpflags & __GFP_ZERO) && object)
> + if (unlikely(slab_want_init_on_alloc(gfpflags, s)) && object)
> memset(object, 0, s->object_size);
>
> slab_post_alloc_hook(s, gfpflags, 1, &object);
> @@ -3163,7 +3186,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> local_irq_enable();
>
> /* Clear memory outside IRQ disabled fastpath loop */
> - if (unlikely(flags & __GFP_ZERO)) {
> + if (unlikely(slab_want_init_on_alloc(flags, s))) {
> int j;
>
> for (j = 0; j < i; j++)
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 75b1c950b49f..9ceb90c875bc 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1602,7 +1602,7 @@ static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority,
> sk = kmem_cache_alloc(slab, priority & ~__GFP_ZERO);
> if (!sk)
> return sk;
> - if (priority & __GFP_ZERO)
> + if (want_init_on_alloc(priority))
> sk_prot_clear_nulls(sk, prot->obj_size);
> } else
> sk = kmalloc(prot->obj_size, priority);
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index c6cb2d9b2905..a1ffe2eb4d5f 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -160,6 +160,35 @@ config STACKLEAK_RUNTIME_DISABLE
> runtime to control kernel stack erasing for kernels built with
> CONFIG_GCC_PLUGIN_STACKLEAK.
>
> +config INIT_ON_ALLOC_DEFAULT_ON
> + bool "Enable heap memory zeroing on allocation by default"
> + help
> + This has the effect of setting "init_on_alloc=1" on the kernel
> + command line. This can be disabled with "init_on_alloc=0".
> + When "init_on_alloc" is enabled, all page allocator and slab
> + allocator memory will be zeroed when allocated, eliminating
> + many kinds of "uninitialized heap memory" flaws, especially
> + heap content exposures. The performance impact varies by
> + workload, but most cases see <1% impact. Some synthetic
> + workloads have measured as high as 7%.
> +
> +config INIT_ON_FREE_DEFAULT_ON
> + bool "Enable heap memory zeroing on free by default"
> + help
> + This has the effect of setting "init_on_free=1" on the kernel
> + command line. This can be disabled with "init_on_free=0".
> + Similar to "init_on_alloc", when "init_on_free" is enabled,
> + all page allocator and slab allocator memory will be zeroed
> + when freed, eliminating many kinds of "uninitialized heap memory"
> + flaws, especially heap content exposures. The primary difference
> + with "init_on_free" is that data lifetime in memory is reduced,
> + as anything freed is wiped immediately, making live forensics or
> + cold boot memory attacks unable to recover freed memory contents.
> + The performance impact varies by workload, but is more expensive
> + than "init_on_alloc" due to the negative cache effects of
> + touching "cold" memory areas. Most cases see 3-5% impact. Some
> + synthetic workloads have measured as high as 8%.
> +
> endmenu
>
> endmenu
> --
> 2.22.0.rc1.257.g3120a18244-goog
>

--
Kees Cook