From 2f8f173413f1cbf52660d04df92d0069c4306d25 Mon Sep 17 00:00:00 2001 From: Pawan Gupta Date: Thu, 14 Aug 2025 10:20:42 -0700 Subject: 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 Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Reviewed-by: Dave Hansen Reviewed-by: Borislav Petkov (AMD) Acked-by: Sean Christopherson --- arch/x86/include/asm/entry-common.h | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'arch/x86/include/asm/entry-common.h') diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h index d535a97c7284..ce3eb6d5fdf9 100644 --- a/arch/x86/include/asm/entry-common.h +++ b/arch/x86/include/asm/entry-common.h @@ -93,6 +93,13 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs, * 8 (ia32) bits. */ choose_random_kstack_offset(rdtsc()); + + /* Avoid unnecessary reads of 'x86_ibpb_exit_to_user' */ + if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER) && + this_cpu_read(x86_ibpb_exit_to_user)) { + indirect_branch_prediction_barrier(); + this_cpu_write(x86_ibpb_exit_to_user, false); + } } #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare -- cgit v1.2.3