Re: [PATCH v4 09/11] x86/vmscape: Deploy BHB clearing mitigation

Pawan Gupta
2025-11-21T21:29:50Z

On Fri, Nov 21, 2025 at 08:53:38PM +0200, Nikolay Borisov wrote:
>
>
> On 11/21/25 20:41, Pawan Gupta wrote:
> > 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 <pawan.kumar.gupta@linux.intel.com>
> > > > ---
> > > > 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.
>
> From where I'm standing bugs.c is already rather hairy even after multiple
> rounds of cleanups and brushups, if we can remove code - I'll be up for it.
> At the very least in the commit message you can explicitly mention that you
> handle every case on-principle, and you expect that some of it is dead code.
> Still, I think the best code is the one which doesn't exist and you won't
> have to worry about it.

Makes sense. I will get rid of those cases.