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From:   Andy Lutomirski <luto@amacapital.net>
Mime-Version: 1.0 (1.0)
Subject: Re: [RFC PATCH 00/62] Linux as SEV-ES Guest Support
Date:   Tue, 11 Feb 2020 19:48:12 -0800
Message-Id: <BD48E405-8E3F-4EEE-A72A-8A7EDCB6A376@amacapital.net>
References: <20200211135256.24617-1-joro@8bytes.org>
Cc:     x86@kernel.org, hpa@zytor.com, Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Jiri Slaby <jslaby@suse.cz>,
        Dan Williams <dan.j.williams@intel.com>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        Juergen Gross <JGross@suse.com>,
        Kees Cook <keescook@chromium.org>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Joerg Roedel <jroedel@suse.de>
In-Reply-To: <20200211135256.24617-1-joro@8bytes.org>
To:     Joerg Roedel <joro@8bytes.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Feb 11, 2020, at 5:53 AM, Joerg Roedel <joro@8bytes.org> wrote:

>=20
>=20
>    * Putting some NMI-load on the guest will make it crash usually
>      within a minute

Suppose you do CPUID or some MMIO and get #VC. You fill in the GHCB to ask f=
or help. Some time between when you start filling it out and when you do VMG=
EXIT, you get NMI. If the NMI does
its own GHCB access [0], it will clobber the outer #VC=E2=80=99a state, resu=
lting in a failure when VMGEXIT happens. There=E2=80=99s a related failure m=
ode if the NMI is after the VMGEXIT but before the result is read.

I suspect you can fix this by saving the GHCB at the beginning of do_nmi and=
 restoring it at the end. This has the major caveat that it will not work if=
 do_nmi comes from user mode and schedules, but I don=E2=80=99t believe this=
 can happen.

[0] Due to the NMI_COMPLETE catastrophe, there is a 100% chance that this ha=
ppens.=