x86/vmscape: Add conditional IBPB mitigation

VMSCAPE is a vulnerability that exploits insufficient branch predictor
isolation between a guest and a userspace hypervisor (like QEMU). Existing
mitigations already protect kernel/KVM from a malicious guest. Userspace
can additionally be protected by flushing the branch predictors after a
VMexit.

Since it is the userspace that consumes the poisoned branch predictors,
conditionally issue an IBPB after a VMexit and before returning to
userspace. Workloads that frequently switch between hypervisor and
userspace will incur the most overhead from the new IBPB.

This new IBPB is not integrated with the existing IBPB sites. For
instance, a task can use the existing speculation control prctl() to
get an IBPB at context switch time. With this implementation, the
IBPB is doubled up: one at context switch and another before running
userspace.

The intent is to integrate and optimize these cases post-embargo.

[ dhansen: elaborate on suboptimal IBPB solution ]

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Sean Christopherson <seanjc@google.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index b74bf937cd9f..410f8df8b77a 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -105,6 +105,14 @@ EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
 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.
+ */
+DEFINE_PER_CPU(bool, x86_ibpb_exit_to_user);
+EXPORT_PER_CPU_SYMBOL_GPL(x86_ibpb_exit_to_user);
+
 u64 x86_pred_cmd __ro_after_init = PRED_CMD_IBPB;
 
 static u64 __ro_after_init x86_arch_cap_msr;