linux-kernel - Re: [PATCH] x86_64: test that userspace stack is in fact NX

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Date:   Tue, 3 Oct 2023 21:06:51 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86_64: test that userspace stack is in fact NX

* Dave Hansen <dave.hansen@...el.com> wrote:

> On 10/3/23 06:00, Alexey Dobriyan wrote:
> > On Mon, Oct 02, 2023 at 07:23:10AM -0700, Dave Hansen wrote:
> >> Basically, could you spend a moment in the changelog to talk about:
> >>
> >> 1. 32-bit kernels on NX hardware
> >> and
> >> 2. 64-bit kernels on non-NX hardware
> > 
> > Sure. My logic whas that i386 is dead arch, but this test is easy to
> > port to i386, only 2 simple functions.
> 
> I honestly don't feel strongly about it one way or the other.  But
> whatever we do, let's explain it, please.
> 
> > I don't want to parse /proc/cpuinfo. If someone knows they're shipping
> > NX-incapable hardware, just let them disable the test.
> 
> Other than clearcpuid=nx, I don't _think_ we have any way to clear the
> X86_FEATURE_NX bit right now.  That should mean that you can use regular
> old CPUID to see if the booted kernel supports NX. [...]

I think that's probably overkill - the test should report a failure if
NX is not available for whatever reason.

Because not having NX in 2023 on any system that is threatened is a
big security vulnerability in itself, and whether the vendor or owner
intentionally did that or not doesn't really matter, and a failing
kernel testcase will be the least of their problems.

In fact I'd argue that we should fail this testcase in that situation
as a matter of principle: NX clearly doesn't work and there's very
few situations where that's acceptable.

Anyone who doesn't want or have NX can skip paying attention to this
failing testcase just fine.

Thanks,

	Ingo