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From:   Jim Mattson <jmattson@google.com>
Date:   Fri, 11 Nov 2022 16:54:27 -0800
Message-ID: <CALMp9eT-XHz2GyWsQt+5eeGGm-9kvCj5PxC8GibEyc9rXoUcEw@mail.gmail.com>
Subject: Re: [PATCH 2/3] x86/speculation: Support Automatic IBRS
To:     Kim Phillips <kim.phillips@amd.com>
Cc:     Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Joao Martins <joao.m.martins@oracle.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Juergen Gross <jgross@suse.com>,
        Tony Luck <tony.luck@intel.com>,
        Babu Moger <Babu.Moger@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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On Fri, Nov 11, 2022 at 4:46 PM Kim Phillips <kim.phillips@amd.com> wrote:
>
> On 11/11/22 6:40 AM, Thadeu Lima de Souza Cascardo wrote:
> > On Fri, Nov 11, 2022 at 01:09:37PM +0100, Borislav Petkov wrote:
> >> On Mon, Nov 07, 2022 at 04:39:02PM -0600, Kim Phillips wrote:
> >>> I've started a version that has AUTOIBRS reuse SPECTRE_V2_EIBRS
> >>> spectre_v2_mitigation enum, but, so far, it's change to bugs.c
> >>> looks bigger: 58 lines changed vs. 34 (see below).
> >>
> >> It can be smaller. You simply do:
> >>
> >>      if (cpu_has(c, X86_FEATURE_AUTOIBRS))
> >>              setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
> >>
> >> and the rest should just work - see below.
> >>
> >> And yes, as Peter says, when the user requests something, the user
> >> should get it. No matter whether it makes sense or not.
>
> OK & thanks.
>
> >> @@ -1474,11 +1477,19 @@ static void __init spectre_v2_select_mitigation(void)
> >>              break;
> >>
> >>      case SPECTRE_V2_CMD_EIBRS_LFENCE:
> >> -            mode = SPECTRE_V2_EIBRS_LFENCE;
> >> +            if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> >> +                    pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
> >> +                    mode = SPECTRE_V2_EIBRS;
> >> +            } else
> >> +                    mode = SPECTRE_V2_EIBRS_LFENCE;
> >>              break;
> >>
> >>      case SPECTRE_V2_CMD_EIBRS_RETPOLINE:
> >> -            mode = SPECTRE_V2_EIBRS_RETPOLINE;
> >> +            if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> >> +                    pr_err(SPECTRE_V2_EIBRS_AMD_MSG);
> >> +                    mode = SPECTRE_V2_EIBRS;
> >> +            } else
> >> +                    mode = SPECTRE_V2_EIBRS_RETPOLINE;
> >>              break;
> >>      }
> >>
> >
> > I am confused here. Isn't the agreement that the user should get what they
> > asked for? That is, instead of warning and changing the mode to
> > SPECTRE_V2_EIBRS, the kernel should still use lfence or retpoline as requested?
> >
> > The point of those options was to protect against Branch History Injection
> > attacks and Intra-Mode Branch Target Injection attacks. The first one might not
> > affect the CPUs that support AUTOIBRS, though we haven't heard that.
> >
> > The second one (IMBTI) is very likely still possible with AUTOIBRS and
> > retpolines should still protect against those attacks. So users who want to be
> > paranoid should still be able to opt for "eibrs,retpoline" and have retpolines
> > enabled.
>
> I've removed the above and have the complete diff below.  It includes patch 1/3 and
> drops 3/3 for now due to Jim Mattson's comments.  After some more testing, I'll
> resubmit.

I bought the argument that AutoIBRS => Same Mode IBRS, so L2 should
not be able to steer L1's indirect branches, even if they share a
predictor mode.