From: Josh Poimboeuf <jpoimboe@kernel.org>
Date: Tue, 14 Jun 2022 23:16:14 +0200
Subject: KVM: VMX: Fix IBRS handling after vmexit
Git-commit: bea7e31a5caccb6fe8ed989c065072354f0ecb52
Patch-mainline: v5.19-rc7
References: bsc#1199657 CVE-2022-29900 CVE-2022-29901

For legacy IBRS to work, the IBRS bit needs to be always re-written
after vmexit, even if it's already on.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/kvm/vmx/vmx.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b81000cc826a..421db354b1ab 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6844,8 +6844,13 @@ void noinstr vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx,
 
 	/*
 	 * If the guest/host SPEC_CTRL values differ, restore the host value.
+	 *
+	 * For legacy IBRS, the IBRS bit always needs to be written after
+	 * transitioning from a less privileged predictor mode, regardless of
+	 * whether the guest/host values differ.
 	 */
-	if (vmx->spec_ctrl != hostval)
+	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) ||
+	    vmx->spec_ctrl != hostval)
 		native_wrmsrl(MSR_IA32_SPEC_CTRL, hostval);
 
 	barrier_nospec();