Patch series "add init_on_alloc/init_on_free boot options", v10.
Provide init_on_alloc and init_on_free boot options.
These are aimed at preventing possible information leaks and making the
control-flow bugs that depend on uninitialized values more deterministic.
Enabling either of the options guarantees that the memory returned by the
page allocator and SL[AU]B is initialized with zeroes. SLOB allocator
isn't supported at the moment, as its emulation of kmem caches complicates
handling of SLAB_TYPESAFE_BY_RCU caches correctly.
Enabling init_on_free also guarantees that pages and heap objects are
initialized right after they're freed, so it won't be possible to access
stale data by using a dangling pointer.
As suggested by Michal Hocko, right now we don't let the heap users to
disable initialization for certain allocations. There's not enough
evidence that doing so can speed up real-life cases, and introducing ways
to opt-out may result in things going out of control.
This patch (of 2):
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.
[glider@google.com: v8]
Link: http://lkml.kernel.org/r/20190626121943.131390-2-glider@google.com
[glider@google.com: v9]
Link: http://lkml.kernel.org/r/20190627130316.254309-2-glider@google.com
[glider@google.com: v10]
Link: http://lkml.kernel.org/r/20190628093131.199499-2-glider@google.com
Link: http://lkml.kernel.org/r/20190617151050.92663-2-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Michal Hocko <mhocko@suse.cz> [page and dmapool parts
Acked-by: James Morris <jamorris@linux.microsoft.com>]
Cc: Christoph Lameter <cl@linux.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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
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;
}
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();
+}
+
#ifdef CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT
DECLARE_STATIC_KEY_TRUE(_debug_pagealloc_enabled);
#else
#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;
}
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);
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 && page_poisoning_enabled())
+ pr_info("mem auto-init: CONFIG_PAGE_POISONING is on, will take precedence over init_on_alloc\n");
+ 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 && page_poisoning_enabled())
+ pr_info("mem auto-init: CONFIG_PAGE_POISONING is on, will take precedence over init_on_free\n");
+ 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
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)
{
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);
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
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);
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;
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);
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);
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);
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);
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 */
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_info("mem auto-init: SLAB_POISON will take precedence over init_on_alloc/init_on_free\n");
return 1;
}
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;
+ int rsize;
+
+ if (slab_want_init_on_free(s))
+ do {
+ object = next;
+ next = get_freepointer(s, object);
+ /*
+ * Clear the object and the metadata, but don't touch
+ * the redzone.
+ */
+ memset(object, 0, s->object_size);
+ rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad
+ : 0;
+ memset((char *)object + s->inuse, 0,
+ s->size - s->inuse - rsize);
+ 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.
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;
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)
+ memset(object + s->offset, 0, sizeof(void *));
- 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);
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++)
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);
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%.
+
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