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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 19 Mar 2020 08:44:03 -0700
Message-ID: <CALCETrW9EYi5dzCKNtKkxM18CC4n5BZxTp1=qQ5qZccwstXjzg@mail.gmail.com>
Subject: Re: [PATCH 41/70] x86/sev-es: Add Runtime #VC Exception Handler
To:     Joerg Roedel <joro@8bytes.org>
Cc:     X86 ML <x86@kernel.org>, "H. Peter Anvin" <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>,
        LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>
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On Thu, Mar 19, 2020 at 2:14 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> From: Tom Lendacky <thomas.lendacky@amd.com>
>
> Add the handler for #VC exceptions invoked at runtime.

If I read this correctly, this does not use IST.  If that's true, I
don't see how this can possibly work.  There at least two nasty cases
that come to mind:

1. SYSCALL followed by NMI.  The NMI IRET hack gets to #VC and we
explode.  This is fixable by getting rid of the NMI EFLAGS.TF hack.

2. tools/testing/selftests/x86/mov_ss_trap_64.  User code does MOV
(addr), SS; SYSCALL, where addr has a data breakpoint.  We get #DB
promoted to #VC with no stack.