[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20200715154856.GA24822@suse.de>
Date: Wed, 15 Jul 2020 17:48:56 +0200
From: Joerg Roedel <jroedel@...e.de>
To: Kees Cook <keescook@...omium.org>
Cc: Joerg Roedel <joro@...tes.org>, x86@...nel.org, hpa@...or.com,
Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>,
Jiri Slaby <jslaby@...e.cz>,
Dan Williams <dan.j.williams@...el.com>,
Tom Lendacky <thomas.lendacky@....com>,
Juergen Gross <jgross@...e.com>,
David Rientjes <rientjes@...gle.com>,
Cfir Cohen <cfir@...gle.com>,
Erdem Aktas <erdemaktas@...gle.com>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mike Stunes <mstunes@...are.com>,
Sean Christopherson <sean.j.christopherson@...el.com>,
Martin Radev <martin.b.radev@...il.com>,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 70/75] x86/head/64: Don't call verify_cpu() on
starting APs
Hi Kees,
as a general note: With SEV-ES the guest kernel will get #VC exceptions
for events that, without SEV-ES, would just cause a #VMEXIT to the
hypervisor.
On Wed, Jul 15, 2020 at 08:26:14AM -0700, Kees Cook wrote:
> On Wed, Jul 15, 2020 at 11:26:38AM +0200, Joerg Roedel wrote:
> > That MSR is Intel-only, right? The boot-path installed here is only used
> > for SEV-ES guests, running on AMD systems, so this MSR is not even
> > accessed during boot on those VMs.
>
> Oh, hrm, yes, that's true. If other x86 maintainers are comfortable with
> this, then okay. My sense is that changing the early CPU startup paths
> will cause trouble down the line.
The AP startup path does not change for non SEV-ES guests. But under
SEV-ES everything that might cause a #VC exception must be avoided until
the kernel is ready to handle them. With the current patches this
happens when the AP runs in 64bit long-mode and loaded TSS and IDT.
Therefore a slightly different AP boot-path is needed for SEV-ES guests.
> So, going back to the requirements here ... what things in verify_cpu()
> can cause exceptions? AFAICT, cpuid is safely handled (i.e. it is
> detected and only run in a way to avoid exceptions and the MSR
> reads/writes are similarly bound by CPU family/id range checks). I must
> be missing something. :)
It is actually the CPUID instructions that cause #VC exceptions. The
MSRs that are accessed on AMD processors are not intercepted in the
hypervisors this code has been tested on, so these will not cause #VC
exceptions.
Regards,
Joerg
Powered by blists - more mailing lists