mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

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>

1 files changed, 35 insertions, 5 deletions

diff --git a/mm/slub.c b/mm/slub.c
index c9541a480627..e6c030e47364 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1279,6 +1279,10 @@ check_slabs:
 	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;
 }
 
@@ -1422,6 +1426,28 @@ 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;
+	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.
@@ -1431,9 +1457,7 @@ 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;
@@ -2729,8 +2753,14 @@ redo:
 		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);
@@ -3151,7 +3181,7 @@ 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++)