Re: [PATCH v10 1/2] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 28 Jun 2019 11:31:30 +0200 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.
> 
> 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.
>
> ...
>
>  v10:
>   - added Acked-by: tags
>   - converted pr_warn() to pr_info()

There are unchangelogged alterations between v9 and v10.  The
replacement of IS_ENABLED(CONFIG_PAGE_POISONING)) with
page_poisoning_enabled().


--- a/mm/page_alloc.c~mm-security-introduce-init_on_alloc=1-and-init_on_free=1-boot-options-v10
+++ a/mm/page_alloc.c
@@ -157,8 +157,8 @@ static int __init early_init_on_alloc(ch
 	if (!buf)
 		return -EINVAL;
 	ret = kstrtobool(buf, &bool_result);
-	if (bool_result && IS_ENABLED(CONFIG_PAGE_POISONING))
-		pr_warn("mem auto-init: CONFIG_PAGE_POISONING is on, will take precedence over init_on_alloc\n");
+	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
@@ -175,8 +175,8 @@ static int __init early_init_on_free(cha
 	if (!buf)
 		return -EINVAL;
 	ret = kstrtobool(buf, &bool_result);
-	if (bool_result && IS_ENABLED(CONFIG_PAGE_POISONING))
-		pr_warn("mem auto-init: CONFIG_PAGE_POISONING is on, will take precedence over init_on_free\n");
+	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
--- a/mm/slub.c~mm-security-introduce-init_on_alloc=1-and-init_on_free=1-boot-options-v10
+++ a/mm/slub.c
@@ -1281,9 +1281,8 @@ check_slabs:
 out:
 	if ((static_branch_unlikely(&init_on_alloc) ||
 	     static_branch_unlikely(&init_on_free)) &&
-	    (slub_debug & SLAB_POISON)) {
-		pr_warn("mem auto-init: SLAB_POISON will take precedence over init_on_alloc/init_on_free\n");
-	}
+	    (slub_debug & SLAB_POISON))
+		pr_info("mem auto-init: SLAB_POISON will take precedence over init_on_alloc/init_on_free\n");
 	return 1;
 }
 
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