[PATCH v3 0/3] RFC: add init_on_alloc/init_on_free boot options

[PATCH v3 0/3] RFC: add init_on_alloc/init_on_free boot options

[Alexander Potapenko]

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[AOU]B is initialized with zeroes.

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.

Alexander Potapenko (3):
  mm: security: introduce init_on_alloc=1 and init_on_free=1 boot
    options
  mm: init: report memory auto-initialization features at boot time
  lib: introduce test_meminit module

 .../admin-guide/kernel-parameters.txt         |   8 +
 drivers/infiniband/core/uverbs_ioctl.c        |   2 +-
 include/linux/mm.h                            |  22 ++
 init/main.c                                   |  24 ++
 kernel/kexec_core.c                           |   2 +-
 lib/Kconfig.debug                             |   8 +
 lib/Makefile                                  |   1 +
 lib/test_meminit.c                            | 208 ++++++++++++++++++
 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 ++
 16 files changed, 416 insertions(+), 21 deletions(-)
 create mode 100644 lib/test_meminit.c
---
 v3: dropped __GFP_NO_AUTOINIT patches

-- 
2.21.0.1020.gf2820cf01a-goog

[PATCH v3 1/3] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

[Alexander Potapenko]

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@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: 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
---
 .../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;
 
-	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_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 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

[PATCH 2/3] mm: init: report memory auto-initialization features at boot time

[Alexander Potapenko]

Print the currently enabled stack and heap initialization modes.

The possible options for stack are:
 - "all" for CONFIG_INIT_STACK_ALL;
 - "byref_all" for CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL;
 - "byref" for CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF;
 - "__user" for CONFIG_GCC_PLUGIN_STRUCTLEAK_USER;
 - "off" otherwise.

Depending on the values of init_on_alloc and init_on_free boottime
options we also report "heap alloc" and "heap free" as "on"/"off".

In the init_on_free mode initializing pages at boot time may take some
time, so print a notice about that as well.

Signed-off-by: Alexander Potapenko <glider@google.com>
Suggested-by: Kees Cook <keescook@chromium.org>
To: Andrew Morton <akpm@linux-foundation.org>
To: Christoph Lameter <cl@linux.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jann Horn <jannh@google.com>
Cc: Kostya Serebryany <kcc@google.com>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Sandeep Patil <sspatil@android.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Cc: kernel-hardening@lists.openwall.com
Cc: linux-mm@kvack.org
Cc: linux-security-module@vger.kernel.org
---
 init/main.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/init/main.c b/init/main.c
index 5a2c69b4d7b3..90f721c58e61 100644
--- a/init/main.c
+++ b/init/main.c
@@ -519,6 +519,29 @@ static inline void initcall_debug_enable(void)
 }
 #endif
 
+/* Report memory auto-initialization states for this boot. */
+void __init report_meminit(void)
+{
+	const char *stack;
+
+	if (IS_ENABLED(CONFIG_INIT_STACK_ALL))
+		stack = "all";
+	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL))
+		stack = "byref_all";
+	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF))
+		stack = "byref";
+	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_USER))
+		stack = "__user";
+	else
+		stack = "off";
+
+	pr_info("mem auto-init: stack:%s, heap alloc:%s, heap free:%s\n",
+		stack, want_init_on_alloc(GFP_KERNEL) ? "on" : "off",
+		want_init_on_free() ? "on" : "off");
+	if (want_init_on_free())
+		pr_info("Clearing system memory may take some time...\n");
+}
+
 /*
  * Set up kernel memory allocators
  */
@@ -529,6 +552,7 @@ static void __init mm_init(void)
 	 * bigger than MAX_ORDER unless SPARSEMEM.
 	 */
 	page_ext_init_flatmem();
+	report_meminit();
 	mem_init();
 	kmem_cache_init();
 	pgtable_init();
-- 
2.21.0.1020.gf2820cf01a-goog

[PATCH 3/3] lib: introduce test_meminit module

[Alexander Potapenko]

Add tests for heap and pagealloc initialization.
These can be used to check init_on_alloc and init_on_free implementations
as well as other approaches to initialization.

Expected test output in the case the kernel provides heap initialization
(e.g. when running with either init_on_alloc=1 or init_on_free=1):

  test_meminit: all 10 tests in test_pages passed
  test_meminit: all 40 tests in test_kvmalloc passed
  test_meminit: all 20 tests in test_kmemcache passed
  test_meminit: all 70 tests passed!

Signed-off-by: Alexander Potapenko <glider@google.com>
To: Kees Cook <keescook@chromium.org>
To: Andrew Morton <akpm@linux-foundation.org>
To: Christoph Lameter <cl@linux.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: Jann Horn <jannh@google.com>
Cc: linux-mm@kvack.org
Cc: linux-security-module@vger.kernel.org
Cc: kernel-hardening@lists.openwall.com
---
 v3:
  - added example test output to the description
  - fixed a missing include spotted by kbuild test robot <lkp@intel.com>
  - added a missing MODULE_LICENSE
  - call do_kmem_cache_size() with size >= sizeof(void*) to unbreak
  debug builds
---
 lib/Kconfig.debug  |   8 ++
 lib/Makefile       |   1 +
 lib/test_meminit.c | 208 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 217 insertions(+)
 create mode 100644 lib/test_meminit.c

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index fdfa173651eb..036e8ef03831 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2043,6 +2043,14 @@ config TEST_STACKINIT
 
 	  If unsure, say N.
 
+config TEST_MEMINIT
+	tristate "Test level of heap/page initialization"
+	help
+	  Test if the kernel is zero-initializing heap and page allocations.
+	  This can be useful to test init_on_alloc and init_on_free features.
+
+	  If unsure, say N.
+
 endif # RUNTIME_TESTING_MENU
 
 config MEMTEST
diff --git a/lib/Makefile b/lib/Makefile
index fb7697031a79..05980c802500 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -91,6 +91,7 @@ obj-$(CONFIG_TEST_DEBUG_VIRTUAL) += test_debug_virtual.o
 obj-$(CONFIG_TEST_MEMCAT_P) += test_memcat_p.o
 obj-$(CONFIG_TEST_OBJAGG) += test_objagg.o
 obj-$(CONFIG_TEST_STACKINIT) += test_stackinit.o
+obj-$(CONFIG_TEST_MEMINIT) += test_meminit.o
 
 obj-$(CONFIG_TEST_LIVEPATCH) += livepatch/
 
diff --git a/lib/test_meminit.c b/lib/test_meminit.c
new file mode 100644
index 000000000000..d46e2b8c8e8e
--- /dev/null
+++ b/lib/test_meminit.c
@@ -0,0 +1,208 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test cases for SL[AOU]B/page initialization at alloc/free time.
+ */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/vmalloc.h>
+
+#define GARBAGE_INT (0x09A7BA9E)
+#define GARBAGE_BYTE (0x9E)
+
+#define REPORT_FAILURES_IN_FN() \
+	do {	\
+		if (failures)	\
+			pr_info("%s failed %d out of %d times\n",	\
+				__func__, failures, num_tests);		\
+		else		\
+			pr_info("all %d tests in %s passed\n",		\
+				num_tests, __func__);			\
+	} while (0)
+
+/* Calculate the number of uninitialized bytes in the buffer. */
+static int count_nonzero_bytes(void *ptr, size_t size)
+{
+	int i, ret = 0;
+	unsigned char *p = (unsigned char *)ptr;
+
+	for (i = 0; i < size; i++)
+		if (p[i])
+			ret++;
+	return ret;
+}
+
+static void fill_with_garbage(void *ptr, size_t size)
+{
+	unsigned int *p = (unsigned int *)ptr;
+	int i = 0;
+
+	while (size >= sizeof(*p)) {
+		p[i] = GARBAGE_INT;
+		i++;
+		size -= sizeof(*p);
+	}
+	if (size)
+		memset(&p[i], GARBAGE_BYTE, size);
+}
+
+static int __init do_alloc_pages_order(int order, int *total_failures)
+{
+	struct page *page;
+	void *buf;
+	size_t size = PAGE_SIZE << order;
+
+	page = alloc_pages(GFP_KERNEL, order);
+	buf = page_address(page);
+	fill_with_garbage(buf, size);
+	__free_pages(page, order);
+
+	page = alloc_pages(GFP_KERNEL, order);
+	buf = page_address(page);
+	if (count_nonzero_bytes(buf, size))
+		(*total_failures)++;
+	fill_with_garbage(buf, size);
+	__free_pages(page, order);
+	return 1;
+}
+
+static int __init test_pages(int *total_failures)
+{
+	int failures = 0, num_tests = 0;
+	int i;
+
+	for (i = 0; i < 10; i++)
+		num_tests += do_alloc_pages_order(i, &failures);
+
+	REPORT_FAILURES_IN_FN();
+	*total_failures += failures;
+	return num_tests;
+}
+
+static int __init do_kmalloc_size(size_t size, int *total_failures)
+{
+	void *buf;
+
+	buf = kmalloc(size, GFP_KERNEL);
+	fill_with_garbage(buf, size);
+	kfree(buf);
+
+	buf = kmalloc(size, GFP_KERNEL);
+	if (count_nonzero_bytes(buf, size))
+		(*total_failures)++;
+	fill_with_garbage(buf, size);
+	kfree(buf);
+	return 1;
+}
+
+static int __init do_vmalloc_size(size_t size, int *total_failures)
+{
+	void *buf;
+
+	buf = vmalloc(size);
+	fill_with_garbage(buf, size);
+	vfree(buf);
+
+	buf = vmalloc(size);
+	if (count_nonzero_bytes(buf, size))
+		(*total_failures)++;
+	fill_with_garbage(buf, size);
+	vfree(buf);
+	return 1;
+}
+
+static int __init test_kvmalloc(int *total_failures)
+{
+	int failures = 0, num_tests = 0;
+	int i, size;
+
+	for (i = 0; i < 20; i++) {
+		size = 1 << i;
+		num_tests += do_kmalloc_size(size, &failures);
+		num_tests += do_vmalloc_size(size, &failures);
+	}
+
+	REPORT_FAILURES_IN_FN();
+	*total_failures += failures;
+	return num_tests;
+}
+
+#define CTOR_BYTES 4
+/* Initialize the first 4 bytes of the object. */
+void some_ctor(void *obj)
+{
+	memset(obj, 'A', CTOR_BYTES);
+}
+
+static int __init do_kmem_cache_size(size_t size, bool want_ctor,
+				     int *total_failures)
+{
+	struct kmem_cache *c;
+	void *buf;
+	int iter, bytes = 0;
+	int fail = 0;
+
+	c = kmem_cache_create("test_cache", size, 1, 0,
+			      want_ctor ? some_ctor : NULL);
+	for (iter = 0; iter < 10; iter++) {
+		buf = kmem_cache_alloc(c, GFP_KERNEL);
+		if (!want_ctor || iter == 0)
+			bytes = count_nonzero_bytes(buf, size);
+		if (want_ctor) {
+			/*
+			 * Newly initialized memory must be initialized using
+			 * the constructor.
+			 */
+			if (iter == 0 && bytes < CTOR_BYTES)
+				fail = 1;
+		} else {
+			if (bytes)
+				fail = 1;
+		}
+		fill_with_garbage(buf, size);
+		kmem_cache_free(c, buf);
+	}
+	kmem_cache_destroy(c);
+
+	*total_failures += fail;
+	return 1;
+}
+
+static int __init test_kmemcache(int *total_failures)
+{
+	int failures = 0, num_tests = 0;
+	int i, size;
+
+	for (i = 0; i < 10; i++) {
+		size = 8 << i;
+		num_tests += do_kmem_cache_size(size, false, &failures);
+		num_tests += do_kmem_cache_size(size, true, &failures);
+	}
+	REPORT_FAILURES_IN_FN();
+	*total_failures += failures;
+	return num_tests;
+}
+
+static int __init test_meminit_init(void)
+{
+	int failures = 0, num_tests = 0;
+
+	num_tests += test_pages(&failures);
+	num_tests += test_kvmalloc(&failures);
+	num_tests += test_kmemcache(&failures);
+
+	if (failures == 0)
+		pr_info("all %d tests passed!\n", num_tests);
+	else
+		pr_info("failures: %d out of %d\n", failures, num_tests);
+
+	return failures ? -EINVAL : 0;
+}
+module_init(test_meminit_init);
+
+MODULE_LICENSE("GPL");
-- 
2.21.0.1020.gf2820cf01a-goog

Re: [PATCH v3 1/3] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

[Alexander Potapenko]

On Thu, May 23, 2019 at 2:42 PM Alexander Potapenko <glider@google.com> 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@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: 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
> ---
>  .../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

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