linux-kernel - Re: [PATCH v3] x86/speculation, KVM: only IBPB for switch_mm_always

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Message-ID: <YmxlHBsxcIy8uYaB@zn.tnic>
Date:   Sat, 30 Apr 2022 00:22:20 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Jon Kohler <jon@...anix.com>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Balbir Singh <sblbir@...zon.com>,
        Kim Phillips <kim.phillips@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v3] x86/speculation, KVM: only IBPB for
 switch_mm_always_ibpb on vCPU load

On Fri, Apr 29, 2022 at 09:59:52PM +0000, Sean Christopherson wrote:
> Correct, but KVM also doesn't do IBPB on VM-Exit (or VM-Entry),

Why doesn't it do that? Not needed?

> nor does KVM do IBPB before exiting to userspace.

Same question.

> The IBPB we want to whack is issued only when KVM is switching vCPUs.

Then please document it properly as I've already requested.

> Except that _none_ of that documentation explains why the hell KVM
> does IBPB when switching betwen vCPUs.

Probably because the folks involved in those patches weren't the hell
mainly virt people. Although I see a bunch of virt people on CC on that
patch.

>   : But stepping back, why does KVM do its own IBPB in the first place?  The goal is
>   : to prevent one vCPU from attacking the next vCPU run on the same pCPU.  But unless
>   : userspace is running multiple VMs in the same process/mm_struct, switching vCPUs,
>   : i.e. switching tasks, will also switch mm_structs and thus do IPBP via cond_mitigation.
>   :
>   : If userspace runs multiple VMs in the same process,

This keeps popping up. Who does that? Can I get a real-life example to
such VM-based containers or what the hell that is, pls?

> enables cond_ipbp, _and_ sets
>   : TIF_SPEC_IB, then it's being stupid and isn't getting full protection in any case,
>   : e.g. if userspace is handling an exit-to-userspace condition for two vCPUs from
>   : different VMs, then the kernel could switch between those two vCPUs' tasks without
>   : bouncing through KVM and thus without doing KVM's IBPB.
>   :
>   : I can kinda see doing this for always_ibpb, e.g. if userspace is unaware of spectre
>   : and is naively running multiple VMs in the same process.

So this needs a clearer definition: what protection are we even talking
about when the address spaces of processes are shared? My naïve
thinking would be: none. They're sharing address space - branch pred.
poisoning between the two is the least of their worries.

So to cut to the chase: it sounds to me like you don't want to do IBPB
at all on vCPU switch. And the process switch case is taken care of by
switch_mm().

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette