On Thu, May 23, 2019 at 2:42 PM Alexander Potapenko <glider@xxxxxxxxxx> 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 only exception is slab caches with
> constructors. Those are never zero-initialized to preserve their semantics.
>
> For SLOB allocator init_on_free=1 also implies init_on_alloc=1 behavior,
> i.e. objects are zeroed at both allocation and deallocation time.
> This is done because SLOB may otherwise return multiple freelist pointers
> in the allocated object. For SLAB and SLUB enabling either init_on_alloc
> or init_on_free leads to one-time initialization of the object.
>
> 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@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> To: Christoph Lameter <cl@xxxxxxxxx>
> To: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: James Morris <jmorris@xxxxxxxxx>
> Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Sandeep Patil <sspatil@xxxxxxxxxxx>
> Cc: Laura Abbott <labbott@xxxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-security-module@xxxxxxxxxxxxxxx
> Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
> ---
> 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
> ---
> .../admin-guide/kernel-parameters.txt | 8 +++
> 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 | 16 +++++
> mm/slob.c | 22 ++++++-
> mm/slub.c | 27 ++++++--
> net/core/sock.c | 2 +-
> security/Kconfig.hardening | 14 +++++
> 12 files changed, 175 insertions(+), 21 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 52e6fbb042cc..68fb6fa41cc1 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1673,6 +1673,14 @@
>
> 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 3b13d3914176..14ded6620aa0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -135,6 +135,48 @@ 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
> @@ -1089,6 +1131,14 @@ 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)
> {
> @@ -1141,6 +1191,8 @@ 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);
>
> @@ -2019,8 +2071,8 @@ 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
> @@ -2074,13 +2126,10 @@ 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 2915d912e89a..d42eb11f8f50 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1853,6 +1853,14 @@ 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;
>
> @@ -3293,7 +3301,7 @@ 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);
> @@ -3350,7 +3358,7 @@ 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);
> @@ -3471,6 +3479,8 @@ 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);
>
> @@ -3558,7 +3568,7 @@ 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..24ae887359b8 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -524,4 +524,20 @@ 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))
> + return !(c->ctor);
> + else
> + 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);
> + else
> + return false;
> +}
> +
> #endif /* MM_SLAB_H */
> diff --git a/mm/slob.c b/mm/slob.c
> index 84aefd9b91ee..1b565ee7f479 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -212,6 +212,19 @@ static void slob_free_pages(void *b, int order)
> free_pages((unsigned long)b, order);
> }
>
> +/*
> + * init_on_free=1 also implies initialization at allocation time.
> + * This is because newly allocated objects may contain freelist pointers
> + * somewhere in the middle.
> + */
> +static inline bool slob_want_init_on_alloc(gfp_t flags, struct kmem_cache *c)
> +{
> + if (static_branch_unlikely(&init_on_alloc) ||
> + static_branch_unlikely(&init_on_free))
> + return c ? (!c->ctor) : true;
> + return flags & __GFP_ZERO;
> +}
> +
> /*
> * slob_page_alloc() - Allocate a slob block within a given slob_page sp.
> * @sp: Page to look in.
> @@ -353,8 +366,6 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
> BUG_ON(!b);
> spin_unlock_irqrestore(&slob_lock, flags);
> }
> - if (unlikely(gfp & __GFP_ZERO))
> - memset(b, 0, size);
> return b;
> }
>
> @@ -389,6 +400,9 @@ static void slob_free(void *block, int size)
> return;
> }
>
> + if (unlikely(want_init_on_free()))
> + memset(block, 0, size);
> +
> if (!slob_page_free(sp)) {
> /* This slob page is about to become partially free. Easy! */
> sp->units = units;
> @@ -484,6 +498,8 @@ __do_kmalloc_node(size_t size, gfp_t gfp, int node, unsigned long caller)
> }
>
> kmemleak_alloc(ret, size, 1, gfp);
> + if (unlikely(slob_want_init_on_alloc(gfp, 0)))
> + memset(ret, 0, size);
> return ret;
> }
>
> @@ -582,6 +598,8 @@ static void *slob_alloc_node(struct kmem_cache *c, gfp_t flags, int node)
> WARN_ON_ONCE(flags & __GFP_ZERO);
> c->ctor(b);
> }
> + if (unlikely(slob_want_init_on_alloc(flags, c)))
> + memset(b, 0, c->size);
>
> kmemleak_alloc_recursive(b, c->size, 1, c->flags, flags);
> return b;
> diff --git a/mm/slub.c b/mm/slub.c
> index cd04dbd2b5d0..5fcb3f71cf84 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1424,6 +1424,19 @@ 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 +1446,7 @@ 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 +2752,14 @@ 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 (slab_want_init_on_free(s))
> + *(void **)object = 0;
Ugh, I forgot to s/0/NULL/ here.
There also must be a check for object being nonnull itself.
I'll send a follow-up.
>
> - 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 +3180,7 @@ 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_cachffffff80081dd078e_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 0a1d4ca314f4..87883e3e3c2a 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -159,6 +159,20 @@ 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 "Set init_on_alloc=1 by default"
> + help
> + Enable init_on_alloc=1 by default, making the kernel initialize every
> + page and heap allocation with zeroes.
> + init_on_alloc can be overridden via command line.
> +
> +config INIT_ON_FREE_DEFAULT_ON
> + bool "Set init_on_free=1 by default"
> + help
> + Enable init_on_free=1 by default, making the kernel initialize freed
> + pages and slab memory with zeroes.
> + init_on_free can be overridden via command line.
> +
> endmenu
>
> endmenu
> --
> 2.21.0.1020.gf2820cf01a-goog
>
--
Alexander Potapenko
Software Engineer
Google Germany GmbH
Erika-Mann-Straße, 33
80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
