Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <CA+55aFxBh3LvsQq9wy313NQCn3iu+yuAmsi0zNVxWmGDUUds-A@mail.gmail.com>
References: <1516476182-5153-6-git-send-email-karahmed@amazon.de>
 <20180129201404.GA1588@localhost.localdomain> <1517257022.18619.30.camel@infradead.org>
 <20180129204256.GV25150@localhost.localdomain> <31415b7f-9c76-c102-86cd-6bf4e23e3aee@linux.intel.com>
 <1517259759.18619.38.camel@infradead.org> <CA+55aFxBh3LvsQq9wy313NQCn3iu+yuAmsi0zNVxWmGDUUds-A@mail.gmail.com>
From:   Jim Mattson <jmattson@google.com>
Date:   Mon, 29 Jan 2018 17:03:14 -0800
Message-ID: <CALMp9eSM6WqX5wstMPgx64d+Dwh4A9+9ngf70qjDVx-_2_g4WQ@mail.gmail.com>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Woodhouse <dwmw2@infradead.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Eduardo Habkost <ehabkost@redhat.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ashok Raj <ashok.raj@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>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        KVM list <kvm@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

The guest OS is responsible for protecting itself from intra-guest
attacks. The hypervisor can't do that. We want to give the guest OS
the tools it needs to make reasonable decisions about the intra-guest
protections it wants to enable, in an environment where the virtual
processor and the physical processor may not actually have the same
F/M/S (and in fact, where the physical processor may change at any
time).

Right now, we are dealing with one workaround, which is tied to
Skylake-era model numbers. Yes, we could report a Skylake model
number, and Linux guests would use IBRS instead of retpoline. But this
approach doesn't scale. What happens when someone introduces a
workaround tied to some other model numbers?

On Mon, Jan 29, 2018 at 4:23 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Mon, Jan 29, 2018 at 1:02 PM, David Woodhouse <dwmw2@infradead.org> wrote:
>>
>> On Mon, 2018-01-29 at 12:44 -0800, Arjan van de Ven wrote:
>>>
>>> the objective is to have retpoline be safe everywhere and never use IBRS
>>> (Linus was also pretty clear about that) so I'm confused by your question
>
> Note on the unhappiness with some of the patches involved: what I do
> *not* want to see is the "on every kernel entry" kind of garbage.
>
> So my unhappiness with the intel microcode patches is two-fold:
>
>  (a) the interface is nasty and wrong, and I absolutely detest how Intel did it.
>
>  (b) the write to random MSR's on every kernel entry/exit is wrong
>
> but that doesn't mean that I will necessarily end up NAK'ing every
> single IBRS/IBPB patch.
>
> My concern with (a) is that unlike meltdown, the intel work-around
> isn't forward-looking, and doesn't have a "we fixed it" bit. Instead,
> it has a "we have a nasty workaround that may or may not be horribly
> expensive" bit, and isn't all that well-defined.
>
> My dislike of (b) comes from "we have retpoline and various wondrous
> RSB filling crud already, we're smarter than that". So it's not that I
> refuse any IBRS/IBPB use, I refuse the stupid and _mindless_ kind of
> use.
>
>> The question is about all the additional RSB-frobbing and call depth
>> counting and other bits that don't really even exist for Skylake yet in
>> a coherent form.
>>
>> If a guest doesn't have those, because it's running some future kernel
>> where they *are* implemented but not enabled because at *boot* time it
>> discovered it wasn't on Skylake, the question is what happens if that
>> guest is subsequently migrated to a Skylake-class machine.
>
> So I actually have a _different_ question to the virtualization
> people. This includes the vmware people, but it also obviously
> incldues the Amazon AWS kind of usage.
>
> When you're a hypervisor (whether vmware or Amazon), why do you even
> end up caring about these things so much? You're protected from
> meltdown thanks to the virtual environment already having separate
> page tables.  And the "big hammer" approach to spectre would seem to
> be to just make sure the BTB and RSB are flushed at vmexit time - and
> even then you might decide that you really want to just move it to
> vmenter time, and only do it if the VM has changed since last time
> (per CPU).
>
> Why do you even _care_ about the guest, and how it acts wrt Skylake?
> What you should care about is not so much the guests (which do their
> own thing) but protect guests from each other, no?
>
> So I'm a bit mystified by some of this discussion within the context
> of virtual machines. I think that is separate from any measures that
> the guest machine may then decide to partake in.
>
> If you are ever going to migrate to Skylake, I think you should just
> always tell the guests that you're running on Skylake. That way the
> guests will always assume the worst case situation wrt Specte.
>
> Maybe that mystification comes from me missing something.
>
>                Linus