Date:   Fri, 27 Jan 2023 18:25:08 +0100
From:   Joerg Roedel <joro@8bytes.org>
To:     Alexey Kardashevskiy <aik@amd.com>
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?)
Message-ID: <Y9QI9JwCVvRmtbr+@8bytes.org>
References: <20230127035616.508966-1-aik@amd.com>
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 <Y9OpcoSacyOkPkvl@8bytes.org>
 <b7880f0b-a592-cf2d-03b9-1ccfd83f8223@amd.com>
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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.

Regards,

	Joerg