Re: [PATCH] mm: add ___GFP_NOINIT flag which disables zeroing on alloc

[Hyunsoon Kim]

On Mon, Mar 29, 2021 at 12:53:48PM +0200, David Hildenbrand wrote:
> On 29.03.21 07:29, Hyunsoon Kim wrote:
> >This patch allows programmer to avoid zero initialization on page
> >allocation even when the kernel config "CONFIG_INIT_ON_ALLOC_DEFAULT"
> >is enabled. The configuration is made to prevent uninitialized
> >heap memory flaws, and Android has applied this for security and
> >deterministic execution times. Please refer to below.
> >
> >https://android-review.googlesource.com/c/kernel/common/+/1235132
> >
> >However, there is a case that the zeroing page memory is unnecessary
> >when the page is used on specific purpose and will be zeroed
> >automatically by hardware that accesses the memory through DMA.
> >For instance, page allocation used for IP packet reception from Exynos
> >modem is solely used for packet reception. Although the page will be
> >freed eventually and reused for some other purpose, initialization at
> >that moment of reuse will be sufficient to avoid uninitialized heap
> >memory flaws. To support this kind of control, this patch creates new
> >gfp type called ___GFP_NOINIT, that allows no zeroing at the moment
> >of page allocation, called by many related APIs such as page_frag_alloc,
> >alloc_pages, etc.
> >
> >Signed-off-by: Hyunsoon Kim <h10.kim@xxxxxxxxxxx>
> >---
> >  include/linux/gfp.h | 2 ++
> >  include/linux/mm.h  | 4 +++-
> >  2 files changed, 5 insertions(+), 1 deletion(-)
> >
> >diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> >index 8572a14..4ddd947 100644
> >--- a/include/linux/gfp.h
> >+++ b/include/linux/gfp.h
> >@@ -58,6 +58,8 @@ struct vm_area_struct;
> >  #else
> >  #define ___GFP_NOLOCKDEP	0
> >  #endif
> >+#define ___GFP_NOINIT		0x1000000u
> >+
> >  /* If the above are modified, __GFP_BITS_SHIFT may need updating */
> >  /*
> >diff --git a/include/linux/mm.h b/include/linux/mm.h
> >index 8ba4342..06a23bb 100644
> >--- a/include/linux/mm.h
> >+++ b/include/linux/mm.h
> >@@ -2907,7 +2907,9 @@ static inline void kernel_unpoison_pages(struct page *page, int numpages) { }
> >  DECLARE_STATIC_KEY_FALSE(init_on_alloc);
> >  static inline bool want_init_on_alloc(gfp_t flags)
> >  {
> >-	if (static_branch_unlikely(&init_on_alloc))
> >+	if (flags & ___GFP_NOINIT)
> >+		return false;
> >+	else if (static_branch_unlikely(&init_on_alloc))
> >  		return true;
> >  	return flags & __GFP_ZERO;
> >  }
> >
> 
> We discussed that in the past - whatever leaves the buddy shall be
> initialized. This is a security feature, not something random kernel modules
> should be able to hack around.
> 
> We also discussed back then to allow other allocators to eventually be able
> to optimize in the future if we are sure it really makes sense. Then,
> however, we need a new API that is not available to random modules, instead
> of exposing ___GFP_NOINIT to anybody out there in the system.
> 
> Nacked-by: David Hildenbrand <david@xxxxxxxxxx>
> 
> -- 
> Thanks,
> 
> David / dhildenb

If you don't mind, may i ask you exactly what security flaws you are expecting
from uninitialized value allocation? I can think of below scenario:

1. Security related value is freed by security system.
2. Malicious module get allocation to the memory region that is freed by above.
3. Malicious module uses that uninitialized value, and breach the security.

Could you please confirm that I am think in the right way? If so, isn't it
possible to make the security system to zero on free? I am not talking about
CONFIG_INIT_ON_FREE_DEFAULT_ON. I am just suggesting that isn't it better to
make programs that generate important values to be forced to initialize on
free, instead of making whole system to zeroing on alloc always, resulting
in performance downgrade? I think this approach can make enhancement.

Thanks,