linux-kernel - Re: [kernel-hardening] [PATCH] arm64: vdso: Mark vDSO code as read-only

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Message-ID: <CAKv+Gu-5OdN7C_SvDytsoSn2QFkVZmonv2ffjpiuEXJA8tc0GA@mail.gmail.com>
Date:	Sat, 13 Feb 2016 10:00:56 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	David Brown <david.brown@...aro.org>
Cc:	kernel-hardening@...ts.openwall.com,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [kernel-hardening] [PATCH] arm64: vdso: Mark vDSO code as read-only

On 13 February 2016 at 01:31, David Brown <david.brown@...aro.org> wrote:
> On Thu, Feb 11, 2016 at 03:19:20PM +0100, Ard Biesheuvel wrote:
>
>>> diff --git a/arch/arm64/kernel/vdso/vdso.S
>>> b/arch/arm64/kernel/vdso/vdso.S
>>> index 60c1db5..db7c0f2 100644
>>> --- a/arch/arm64/kernel/vdso/vdso.S
>>> +++ b/arch/arm64/kernel/vdso/vdso.S
>>> @@ -24,6 +24,7 @@
>>>         __PAGE_ALIGNED_DATA
>>
>>
>> ^^ You can get rid of this now as well
>
>
> Can we?  The page is getting mapped to userspace, and if we didn't
> page align it, we could leak kernel read-only data to every userspace
> process.
>

That's what the .balign is for. __PAGE_ALIGNED_DATA only sets the
section, and you're overriding that right after.