linux-kernel - Re: [PATCH 3/3] arm64: dump: Add checking for writable and exectuable pages

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Date:   Fri, 30 Sep 2016 10:16:28 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Laura Abbott <labbott@...hat.com>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        David Brown <david.brown@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 3/3] arm64: dump: Add checking for writable and exectuable pages

On Fri, Sep 30, 2016 at 9:41 AM, Mark Rutland <mark.rutland@....com> wrote:
> On Fri, Sep 30, 2016 at 09:25:45AM -0700, Kees Cook wrote:
>> On Fri, Sep 30, 2016 at 8:58 AM, Mark Rutland <mark.rutland@....com> wrote:
>
>> > Would it be worth verifying that all kernel mappings are UXN, too?
>> >
>> > ARMv8 allows execute-only mappings, and a !UXN mapping could result in an info
>> > leak (e.g. pointers in MOVZ+MOVK sequences), or potential asynchronous issues
>> > (e.g. user instruction fetches accessing read-destructive device registers).
>> > All kernel mappings *should* be UXN.
>>
>> I love this idea, but based on what came up with hardened usercopy,
>> there are a lot of readers of kernel memory still. I think the
>> expectations around UXN need to be clarified so we can reason about
>> things like perf that want to read the kernel text.
>
> The UXN (User eXecute Never) bit only controls whether userspace can execute a
> page, not whether the kernel can read it. The RW permissions come from the AP
> bits regardless.

Ah! Sorry, I misunderstood. Yeah, UXN checking makes sense there then. :)

> We already try to ensure that all kernel memory is UXN by construction, so this
> would just be a sanity check, as with the rest of the W^X checks.
>
> The MOVZ+MOVK case above is where a sequence of 16-bit immediate MOVs are used
> to encode a pointer. If a kernel mapping lacked UXN, userspace could execute it
> (unprivileged), and extract the pointer generated into a GPR.
>
> Having kernel exec-only memory is a different story entirely, though I agree
> it's something to look into.

Yeah, this'll need to get sorted out for x86 too.

-Kees

-- 
Kees Cook
Nexus Security