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

[Kees Cook]

On Fri, Sep 30, 2016 at 9:41 AM, Mark Rutland <mark.rutland@xxxxxxx> 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@xxxxxxx> 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