KVM: VMX: Fix IBRS handling after vmexit

commit bea7e31a upstream.

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: NJosh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Signed-off-by: NThadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: NSuleiman Souhlal <suleiman@google.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/kvm/vmx.c

@@ -10770,9 +10770,13 @@ u64 __always_inline vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx)
 	guestval = __rdmsr(MSR_IA32_SPEC_CTRL);
 
 	/*
-	 * 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 (guestval != hostval)
+	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) ||
+	    guestval != hostval)
 		native_wrmsrl(MSR_IA32_SPEC_CTRL, hostval);
 
 	barrier_nospec();