From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Raj, Ashok" <ashok.raj@intel.com>
Subject: Re: [PATCH v3 0/4] KVM: Expose speculation control feature to guests
Date: Tue, 30 Jan 2018 15:48:22 -0800
Message-ID: <20180130234822.GA62797@otc-nc-03>
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Cc: David Woodhouse <dwmw2@infradead.org>,
        KarimAllah Ahmed <karahmed@amazon.de>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Linus Torvalds <torvalds@linux-foundati
To: Paolo Bonzini <pbonzini@redhat.com>
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On Tue, Jan 30, 2018 at 06:36:20PM -0500, Paolo Bonzini wrote:
> On 30/01/2018 04:00, David Woodhouse wrote:
> > I believe Ashok sent you a change which made us do IBPB on *every*
> > vmexit; I don't think we need that. It's currently done in vcpu_load()
> > which means we'll definitely have done it between running one vCPU and
> > the next, and when vCPUs are pinned we basically never need to do it.
> > 
> > We know that VMM (e.g. qemu) userspace could be vulnerable to attacks
> > from guest ring 3, because there is no flush between the vmexit and the
> > host kernel "returning" to the userspace thread. Doing a full IBPB on
> > *every* vmexit would protect from that, but it's overkill. If that's
> > the reason, let's come up with something better.
> 
> Certainly not every vmexit!  But doing it on every userspace vmexit and
> every sched_out would not be *that* bad.

Right.. agreed. We discussed the different scenarios that doing IBPB
on VMexit would help, and decided its really not required on every exit. 

One obvious case is when there is a VMexit and return back to Qemu
process (witout a real context switch) do we need that to be 
protected from any poisoned BTB from guest?

If Qemu is protected by !dumpable/retpoline that should give that gaurantee.
We do VM->VM IBPB at vmload() time that should provide that gaurantee.

Cheers,
Ashok

> 
> We try really hard to avoid userspace vmexits for everything remotely
> critical to performance (the main exception that's left is the PMTIMER
> I/O port, that Windows likes to access quite a lot), so they shouldn't
> happen that often.