linux-kernel - Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling (why is this happening?)

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Date:   Sat, 28 Jan 2023 22:24:56 +1100
From:   Alexey Kardashevskiy <aik@....com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Jiri Kosina <jkosina@...e.cz>, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling
 (why is this happening?)



On 28/1/23 04:25, Joerg Roedel wrote:
> On Fri, Jan 27, 2023 at 10:56:26PM +1100, Alexey Kardashevskiy wrote:
>> https://github.com/aik/linux/commit/d0d6bbb58fcd927ddd1f8e9d42ab121920c7eafc
> 
> Okay, I reproduced the problem here and the root cause turned out to be
> that the compiler moved the DR7 read instruction before the 5-byte NOP
> which becomes the call to sev_es_ist_enter() in SEV-ES guests. This is
> guaranteed to cause #VC exception stack recursion if the NMI was
> triggered on the #VC stack, and that leads to all kinds of undefined
> behavior.

Cool!

(out of curiosity) where do you see these NOPs? "objdump -D vmlinux" 
does not show any, is this after lifepatching?

Meanwhile, this seems to be doing the right thing:


diff --git a/arch/x86/include/asm/debugreg.h 
b/arch/x86/include/asm/debugreg.h
index b049d950612f..687b15297057 100644
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -39,7 +39,7 @@ static __always_inline unsigned long 
native_get_debugreg(int regno)
                 asm("mov %%db6, %0" :"=r" (val));
                 break;
         case 7:
-               asm("mov %%db7, %0" :"=r" (val));
+               asm volatile ("mov %%db7, %0" :"=r" (val));



-- 
Alexey