Re: [PATCH v7 1/2] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

[Michal Hocko <mhocko@kernel.org>]

On Mon 17-06-19 17:10:49, Alexander Potapenko 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 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.

Thanks for reworking the original implemenation. This looks much better!

> Signed-off-by: Alexander Potapenko <glider@google.com>
> Acked-by: Kees Cook <keescook@chromium.org>
> 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

Acked-by: Michal Hocko <mhocko@suse.cz> # page allocator parts.

kmalloc based parts look good to me as well but I am not sure I fill
qualified to give my ack there without much more digging and I do not
have much time for that now.

[...]
> 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);
>  	}

I am not really sure I follow here. Why do we want to handle
want_init_on_alloc here? The allocated memory comes from the page
allocator and so it will get zeroed there. arch_kexec_post_alloc_pages
might touch the content there but is there any actual risk of any kind
of leak?

> diff --git a/mm/dmapool.c b/mm/dmapool.c
> index 8c94c89a6f7e..e164012d3491 100644
> --- a/mm/dmapool.c
> +++ b/mm/dmapool.c
> @@ -378,7 +378,7 @@ 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;

Don't you miss dma_pool_free and want_init_on_free?
-- 
Michal Hocko
SUSE Labs