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Date: Thu, 27 Jun 2019 09:25:11 -0400
From: Qian Cai <cai@....pw>
To: Alexander Potapenko <glider@...gle.com>, Andrew Morton
<akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>, Kees Cook
<keescook@...omium.org>
Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>, Michal Hocko
<mhocko@...nel.org>, James Morris <jmorris@...ei.org>, "Serge E. Hallyn"
<serge@...lyn.com>, Nick Desaulniers <ndesaulniers@...gle.com>, Kostya
Serebryany <kcc@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Sandeep
Patil <sspatil@...roid.com>, Laura Abbott <labbott@...hat.com>, Randy
Dunlap <rdunlap@...radead.org>, Jann Horn <jannh@...gle.com>, Mark Rutland
<mark.rutland@....com>, Marco Elver <elver@...gle.com>, linux-mm@...ck.org,
linux-security-module@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v9 1/2] mm: security: introduce init_on_alloc=1 and
init_on_free=1 boot options
On Thu, 2019-06-27 at 15:03 +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.
>
> This is expected to be on-by-default on Android and Chrome OS. And it
> gives the opportunity for anyone else to use it under distros too via
> the boot args. (The init_on_free feature is regularly requested by
> folks where memory forensics is included in their threat models.)
>
> 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.
>
> If either SLUB poisoning or page poisoning is enabled, those options
> take precedence over init_on_alloc and init_on_free: initialization is
> only applied to unpoisoned allocations.
>
> 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 infrastructure 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@...gle.com>
> Acked-by: Kees Cook <keescook@...omium.org>
> To: Andrew Morton <akpm@...ux-foundation.org>
> To: Christoph Lameter <cl@...ux.com>
> To: Kees Cook <keescook@...omium.org>
> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: James Morris <jmorris@...ei.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Kostya Serebryany <kcc@...gle.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Sandeep Patil <sspatil@...roid.com>
> Cc: Laura Abbott <labbott@...hat.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Qian Cai <cai@....pw>
> Cc: linux-mm@...ck.org
> Cc: linux-security-module@...r.kernel.org
> Cc: kernel-hardening@...ts.openwall.com
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
> 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
> v7:
> - rebase the patch, added the Acked-by: tag
> v8:
> - addressed comments by Michal Hocko: revert kernel/kexec_core.c and
> apply initialization in dma_pool_free()
> - disable init_on_alloc/init_on_free if slab poisoning or page
> poisoning are enabled, as requested by Qian Cai
> - skip the redzone when initializing a freed heap object, as requested
> by Qian Cai and Kees Cook
> - use s->offset to address the freeptr (suggested by Kees Cook)
> - updated the patch description, added Signed-off-by: tag
> v9:
> - picked up -mm fixes from Qian Cai and Andrew Morton (order of calls
> in free_pages_prepare(), export init_on_alloc)
> - exported init_on_free
> - allowed using init_on_alloc/init_on_free with SLUB poisoning and
> page poisoning. Poisoning supersedes zero-initialization, so some
> tests may behave differently with poisoning enabled.
> ---
> .../admin-guide/kernel-parameters.txt | 9 +++
> drivers/infiniband/core/uverbs_ioctl.c | 2 +-
> include/linux/mm.h | 24 +++++++
> mm/dmapool.c | 4 +-
> mm/page_alloc.c | 71 +++++++++++++++++--
> mm/slab.c | 16 ++++-
> mm/slab.h | 20 ++++++
> mm/slub.c | 41 +++++++++--
> net/core/sock.c | 2 +-
> security/Kconfig.hardening | 29 ++++++++
> 10 files changed, 200 insertions(+), 18 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 dd0b5f4e1e45..81b582657854 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2696,6 +2696,30 @@ 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) &&
> + !page_poisoning_enabled())
> + 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) &&
> + !page_poisoning_enabled();
> +}
> +
> extern bool _debug_pagealloc_enabled;
>
> static inline bool debug_pagealloc_enabled(void)
> diff --git a/mm/dmapool.c b/mm/dmapool.c
> index 8c94c89a6f7e..fe5d33060415 100644
> --- a/mm/dmapool.c
> +++ b/mm/dmapool.c
> @@ -378,7 +378,7 @@ 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;
> @@ -428,6 +428,8 @@ void dma_pool_free(struct dma_pool *pool, void *vaddr,
> dma_addr_t dma)
> }
>
> offset = vaddr - page->vaddr;
> + if (want_init_on_free())
> + memset(vaddr, 0, pool->size);
> #ifdef DMAPOOL_DEBUG
> if ((dma - page->dma) != offset) {
> spin_unlock_irqrestore(&pool->lock, flags);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d66bc8abe0af..c3123fa41bba 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -136,6 +136,55 @@ 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
> +EXPORT_SYMBOL(init_on_alloc);
> +
> +#ifdef CONFIG_INIT_ON_FREE_DEFAULT_ON
> +DEFINE_STATIC_KEY_TRUE(init_on_free);
> +#else
> +DEFINE_STATIC_KEY_FALSE(init_on_free);
> +#endif
> +EXPORT_SYMBOL(init_on_free);
> +
> +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 && IS_ENABLED(CONFIG_PAGE_POISONING))
> + pr_warn("mem auto-init: CONFIG_PAGE_POISONING is on, will
> take precedence over init_on_alloc\n");
I don't like the warning here. It makes people think it is bug that need to be
fixed, but actually it is just information. People could enable both in a debug
kernel.
> + 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 && IS_ENABLED(CONFIG_PAGE_POISONING))
> + pr_warn("mem auto-init: CONFIG_PAGE_POISONING is on, will
> take precedence over init_on_free\n");
Ditto.
> + 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 +1139,14 @@ 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 +1198,9 @@ static __always_inline bool free_pages_prepare(struct
> page *page,
> PAGE_SIZE << order);
> }
> arch_free_page(page, order);
> + if (want_init_on_free())
> + kernel_init_free_pages(page, 1 << order);
> +
> kernel_poison_pages(page, 1 << order, 0);
> if (debug_pagealloc_enabled())
> kernel_map_pages(page, 1 << order, 0);
> @@ -2020,8 +2080,8 @@ 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 +2135,10 @@ 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..d3f585e604bb 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -524,4 +524,24 @@ 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 | SLAB_POISON))
> + 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 | SLAB_POISON)));
> + return false;
> +}
> +
> #endif /* MM_SLAB_H */
> diff --git a/mm/slub.c b/mm/slub.c
> index cd04dbd2b5d0..3ccdab86f253 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1279,6 +1279,11 @@ 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("mem auto-init: SLAB_POISON will take precedence over
> init_on_alloc/init_on_free\n");
> + }
Ditto
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