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

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Message-ID: <1d5223b8-0275-619d-db1c-e2aaaddb173e@intel.com>
Date:   Mon, 2 Oct 2023 07:23:10 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     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>
Cc:     linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86_64: test that userspace stack is in fact NX

On 10/1/23 09:31, Alexey Dobriyan wrote:
> Here is how it works:
> 
> * fault and fill the stack from rsp with int3 down until rlimit allows,
> * fill upwards with int3 too, overwrite libc stuff, argv, envp,
> * try to exec int3 on each page and catch it with either SIGSEGV or
>   SIGTRAP handler.
> 
> Note: trying to execute _every_ int3 takes 30-40 seconds even on fast
> machine, so only 1 int3 per page is tried.
> 
> Tested on F37 kernel and on custom kernel which did
> 
> 	vm_flags |= VM_EXEC;
> 
> to stack VMA.

I guess the subject implies it, but it's probably worth a sentence or
two in the changelog about this being 64-bit only.

IIRC, there _are_ x86_64 CPUs that don't support NX.  It's also entirely
possible for a hypervisor to disable NX enumeration for a guest.  Those
two are (probably) rare enough that they can be ignored for now.  But it
might mean adding a CPUID check at some point.

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

?