linux-kernel - Re: [PATCH 0/3] KASAN: clean stale poison upon cold re-entry to kernel

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Date:	Thu, 3 Mar 2016 13:02:27 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, akpm@...ux-foundation.org,
	aryabinin@...tuozzo.com, catalin.marinas@....com,
	glider@...gle.com, lorenzo.pieralisi@....com, mingo@...hat.com,
	peterz@...radead.org, will.deacon@....com
Subject: Re: [PATCH 0/3] KASAN: clean stale poison upon cold re-entry to
 kernel


* Mark Rutland <mark.rutland@....com> wrote:

> Functions which the compiler has instrumented for ASAN place poison on
> the stack shadow upon entry and remove this poison prior to returning.
> 
> In some cases (e.g. hotplug and idle), CPUs may exit the kernel a number
> of levels deep in C code. If there are any instrumented functions on
> this critical path, these will leave portions of the idle thread stack
> shadow poisoned.
> 
> If a CPU returns to the kernel via a different path (e.g. a cold entry),
> then depending on stack frame layout subsequent calls to instrumented
> functions may use regions of the stack with stale poison, resulting in
> (spurious) KASAN splats to the console.
> 
> Contemporary GCCs always add stack shadow poisoning when ASAN is
> enabled, even when asked to not instrument a function [1], so we can't
> simply annotate functions on the critical path to avoid poisoning.
> 
> Instead, this series explicitly removes any stale poison before it can
> be hit. In the common hotplug case we clear the entire stack shadow in
> common code, before a CPU is brought online.
> 
> On architectures which perform a cold return as part of cpu idle may
> retain an architecture-specific amount of stack contents. To retain the
> poison for this retained context, the arch code must call the core KASAN
> code, passing a "watermark" stack pointer value beyond which shadow will
> be cleared. Architectures which don't perform a cold return as part of
> idle do not need any additional code.
> 
> This is a combination of previous approaches [2,3], attempting to keep
> as much as possible generic.
> 
> Thanks,
> Mark.
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69863
> [2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/409466.html
> [3] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/411850.html
> 
> Mark Rutland (3):
>   kasan: add functions to clear stack poison
>   sched/kasan: remove stale KASAN poison after hotplug
>   arm64: kasan: clear stale stack poison
> 
>  arch/arm64/kernel/sleep.S |  4 ++++
>  include/linux/kasan.h     |  6 +++++-
>  kernel/sched/core.c       |  3 +++
>  mm/kasan/kasan.c          | 20 ++++++++++++++++++++
>  4 files changed, 32 insertions(+), 1 deletion(-)

Looks good to me - via which tree would you like to see this merged upstream?

Thanks,

	Ingo