linux-kernel - Re: [PATCH v3 2/4] x86/traps: Print non-canonical address on #GP

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Date:   Wed, 20 Nov 2019 18:37:22 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Jann Horn <jannh@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v3 2/4] x86/traps: Print non-canonical address on #GP

On Wed, Nov 20, 2019 at 08:21:43AM -0800, Sean Christopherson wrote:
> On Wed, Nov 20, 2019 at 02:39:13PM +0100, Borislav Petkov wrote:
> > On Wed, Nov 20, 2019 at 02:28:30PM +0100, Ingo Molnar wrote:
> > > I'd rather we not trust the decoder and the execution environment so much 
> > > that it never produces a 0 linear address in a #GP:
> > 
> > I was just scratching my head whether I could trigger a #GP with address
> > of 0. But yeah, I agree, let's be really cautious here. I wouldn't want
> > to debug a #GP with a wrong address reported.
> 
> It's definitely possible, there are a handful of non-SIMD instructions that
> generate #GP(0) it CPL=0 in 64-bit mode *and* have a memory operand.  Some
> of them might even be legitimately encountered in the wild.
> 
>   - CMPXCHG16B if it's not supported by the CPU.
>   - VMXON if CR4 is misconfigured or VMX isn't enabled in FEATURE_CONTROL.
>   - MONITOR if ECX has an invalid hint (although MONITOR hardcodes the
>     address in DS:RAX and so doesn't have a ModR/M byte).
> 
> Undoudbtedly there are other instructions with similar sources of #GP.

Right, we currently put our trust in the insn decoder to handle those
correctly too.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette