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Peter Anvin" , Josh Poimboeuf , Sean Christopherson , Paolo Bonzini , Borislav Petkov , Dave Hansen , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Asit Mallick , Tao Zhang Subject: Re: [PATCH v4 09/11] x86/vmscape: Deploy BHB clearing mitigation Message-ID: <20251121184148.hi6ye2trohwjm3oe@desk> References: <20251119-vmscape-bhb-v4-0-1adad4e69ddc@linux.intel.com> <20251119-vmscape-bhb-v4-9-1adad4e69ddc@linux.intel.com> <5cdca004-5228-4f07-b9b8-901880f59bb7@suse.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5cdca004-5228-4f07-b9b8-901880f59bb7@suse.com> On Fri, Nov 21, 2025 at 04:23:56PM +0200, Nikolay Borisov wrote: > > > On 11/20/25 08:19, Pawan Gupta wrote: > > IBPB mitigation for VMSCAPE is an overkill on CPUs that are only affected > > by the BHI variant of VMSCAPE. On such CPUs, eIBRS already provides > > indirect branch isolation between guest and host userspace. However, branch > > history from guest may also influence the indirect branches in host > > userspace. > > > > To mitigate the BHI aspect, use clear_bhb_loop(). > > > > Signed-off-by: Pawan Gupta > > --- > > Documentation/admin-guide/hw-vuln/vmscape.rst | 4 ++++ > > arch/x86/include/asm/nospec-branch.h | 2 ++ > > arch/x86/kernel/cpu/bugs.c | 30 ++++++++++++++++++++------- > > 3 files changed, 29 insertions(+), 7 deletions(-) > > > > diff --git a/Documentation/admin-guide/hw-vuln/vmscape.rst b/Documentation/admin-guide/hw-vuln/vmscape.rst > > index d9b9a2b6c114c05a7325e5f3c9d42129339b870b..dc63a0bac03d43d1e295de0791dd6497d101f986 100644 > > --- a/Documentation/admin-guide/hw-vuln/vmscape.rst > > +++ b/Documentation/admin-guide/hw-vuln/vmscape.rst > > @@ -86,6 +86,10 @@ The possible values in this file are: > > run a potentially malicious guest and issues an IBPB before the first > > exit to userspace after VM-exit. > > + * 'Mitigation: Clear BHB before exit to userspace': > > + > > + As above, conditional BHB clearing mitigation is enabled. > > + > > * 'Mitigation: IBPB on VMEXIT': > > IBPB is issued on every VM-exit. This occurs when other mitigations like > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > > index 15a2fa8f2f48a066e102263513eff9537ac1d25f..1e8c26c37dbed4256b35101fb41c0e1eb6ef9272 100644 > > --- a/arch/x86/include/asm/nospec-branch.h > > +++ b/arch/x86/include/asm/nospec-branch.h > > @@ -388,6 +388,8 @@ extern void write_ibpb(void); > > #ifdef CONFIG_X86_64 > > extern void clear_bhb_loop(void); > > +#else > > +static inline void clear_bhb_loop(void) {} > > #endif > > extern void (*x86_return_thunk)(void); > > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c > > index cbb3341b9a19f835738eda7226323d88b7e41e52..d12c07ccf59479ecf590935607394492c988b2ff 100644 > > --- a/arch/x86/kernel/cpu/bugs.c > > +++ b/arch/x86/kernel/cpu/bugs.c > > @@ -109,9 +109,8 @@ DEFINE_PER_CPU(u64, x86_spec_ctrl_current); > > EXPORT_PER_CPU_SYMBOL_GPL(x86_spec_ctrl_current); > > /* > > - * Set when the CPU has run a potentially malicious guest. An IBPB will > > - * be needed to before running userspace. That IBPB will flush the branch > > - * predictor content. > > + * Set when the CPU has run a potentially malicious guest. Indicates that a > > + * branch predictor flush is needed before running userspace. > > */ > > DEFINE_PER_CPU(bool, x86_predictor_flush_exit_to_user); > > EXPORT_PER_CPU_SYMBOL_GPL(x86_predictor_flush_exit_to_user); > > @@ -3200,13 +3199,15 @@ enum vmscape_mitigations { > > VMSCAPE_MITIGATION_AUTO, > > VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER, > > VMSCAPE_MITIGATION_IBPB_ON_VMEXIT, > > + VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER, > > }; > > static const char * const vmscape_strings[] = { > > - [VMSCAPE_MITIGATION_NONE] = "Vulnerable", > > + [VMSCAPE_MITIGATION_NONE] = "Vulnerable", > > /* [VMSCAPE_MITIGATION_AUTO] */ > > - [VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER] = "Mitigation: IBPB before exit to userspace", > > - [VMSCAPE_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT", > > + [VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER] = "Mitigation: IBPB before exit to userspace", > > + [VMSCAPE_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT", > > + [VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER] = "Mitigation: Clear BHB before exit to userspace", > > }; > > static enum vmscape_mitigations vmscape_mitigation __ro_after_init = > > @@ -3253,8 +3254,19 @@ static void __init vmscape_select_mitigation(void) > > vmscape_mitigation = VMSCAPE_MITIGATION_NONE; > > break; > > + case VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER: > > + if (!boot_cpu_has(X86_FEATURE_BHI_CTRL)) > > + vmscape_mitigation = VMSCAPE_MITIGATION_NONE; > > + break; > > Am I missing something or this case can never execute because > VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER is only ever set if mitigation is > VMSCAPE_MITIGATION_AUTO in the below branch? Perhaps just remove it? This > just shows how confusing the logic for choosing the mitigations has > become.... The goal was not make any assumptions on what vmscape_parse_cmdline() can and cannot set. If you feel strongly about it, I can remove this case. > > case VMSCAPE_MITIGATION_AUTO: > > - if (boot_cpu_has(X86_FEATURE_IBPB)) > > + /* > > + * CPUs with BHI_CTRL(ADL and newer) can avoid the IBPB and use BHB > > + * clear sequence. These CPUs are only vulnerable to the BHI variant > > + * of the VMSCAPE attack and does not require an IBPB flush. > > + */ > > + if (boot_cpu_has(X86_FEATURE_BHI_CTRL)) > > + vmscape_mitigation = VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER; > > + else if (boot_cpu_has(X86_FEATURE_IBPB)) > > vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER; > > else > > vmscape_mitigation = VMSCAPE_MITIGATION_NONE; > > > >