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

From: Alexey Kardashevskiy <aik@amd.com>
To: Joerg Roedel <joro@8bytes.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	kvm@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Sean Christopherson <seanjc@google.com>,
	Jiri Kosina <jkosina@suse.cz>, Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [Question PATCH kernel] x86/amd/sev/nmi+vc: Fix stack handling (why is this happening?)
Date: Sat, 28 Jan 2023 22:24:56 +1100	[thread overview]
Message-ID: <3bb3e080-caee-8bc8-7de9-f44969f16e75@amd.com> (raw)
In-Reply-To: <Y9QI9JwCVvRmtbr+@8bytes.org>



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
