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

Alexander Potapenko
2019-05-29T12:38:33Z

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>
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