linux-kernel - Re: [PATCH] arm: Use kernel mm when updating section permissions

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Date:	Fri, 6 Nov 2015 12:28:58 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Kevin Hilman <khilman@...nel.org>
Cc:	info@...nelci.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Laura Abbott <labbott@...oraproject.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Laura Abbott <labbott@...hat.com>,
	Shuah Khan <shuahkh@....samsung.com>,
	Tyler Baker <tyler.baker@...aro.org>
Subject: Re: [PATCH] arm: Use kernel mm when updating section permissions

On Fri, Nov 6, 2015 at 12:11 PM, Kevin Hilman <khilman@...nel.org> wrote:
> On Fri, Nov 6, 2015 at 11:12 AM, Kees Cook <keescook@...omium.org> wrote:
>
> [...]
>
>> Hi Kevin and Kernel CI folks,
>>
>> Could lkdtm get added to the kernel-CI workflows? Extracting and
>> validating Oops details when poking lkdtm would be extremely valuable
>> for these cases. :)
>
> Yeah, we can add that.
>
> What arches should we expect this to be working on?  For starters

This is a great question. ;) They're a mix of CONFIG and hardware
feature specific, so probably they should be run on all architectures
and we can figure out what's missing in each case.

Everything built with CONFIG_DEBUG_RODATA should pass these:

WRITE_RO
WRITE_KERN
EXEC_DATA
EXEC_STACK
EXEC_KMALLOC
EXEC_VMALLOC

But architectures without CONFIG_DEBUG_RODATA should be shamed. ;)

Passing EXEC_USERSPACE requires SMEP on x86, and PXN on arm64.
Passing ACCESS_USERSPACE rquires SMAP on x86, and PAN on arm64.

The recent PAN emulation CONFIG_CPU_SW_DOMAIN_PAN on non-LPAE arm
should cover ACCESS_USERSPACE too, and maybe EXEC_USERSPACE, but I
haven't taken a close look.

It might be useful, frankly, to test everything in lkdtm.

> we'll get builds going with CONFIG_LKDTM=y, and then start looking at
> adding the tests on arches that should work.
>
> Thes will be an interesting failure modes to catch because a kernel
> panic is actually a PASS, and a failure to panic is a FAIL.  :)

Yup! :) And extracting the Oops message can become important too. As
recently shown with CONFIG_CPU_SW_DOMAIN_PAN, the test was wrong, and
the Oops showed it:
http://www.gossamer-threads.com/lists/linux/kernel/2293320

Thanks for looking into it!

-Kees

-- 
Kees Cook
Chrome OS Security
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