x86/bugs: Rework spec_ctrl base and mask logic

x86_spec_ctrL_mask is intended to mask out bits from a MSR_SPEC_CTRL value
which are not to be modified. However the implementation is not really used
and the bitmask was inverted to make a check easier, which was removed in
"x86/bugs: Remove x86_spec_ctrl_set()"

Aside of that it is missing the STIBP bit if it is supported by the
platform, so if the mask would be used in x86_virt_spec_ctrl() then it
would prevent a guest from setting STIBP.

Add the STIBP bit if supported and use the mask in x86_virt_spec_ctrl() to
sanitize the value which is supplied by the guest.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>

arch/x86/kernel/cpu/bugs.c

@@ -42,7 +42,7 @@ EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
  * The vendor and possibly platform specific bits which can be modified in
  * x86_spec_ctrl_base.
  */
-static u64 __ro_after_init x86_spec_ctrl_mask = ~SPEC_CTRL_IBRS;
+static u64 __ro_after_init x86_spec_ctrl_mask = SPEC_CTRL_IBRS;
 
 /*
  * AMD specific MSR info for Speculative Store Bypass control.
@@ -68,6 +68,10 @@ void __init check_bugs(void)
 	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
 		rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
 
+	/* Allow STIBP in MSR_SPEC_CTRL if supported */
+	if (boot_cpu_has(X86_FEATURE_STIBP))
+		x86_spec_ctrl_mask |= SPEC_CTRL_STIBP;
+
 	/* Select the proper spectre mitigation before patching alternatives */
 	spectre_v2_select_mitigation();
 
@@ -136,18 +140,26 @@ static enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init =
 void
 x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)
 {
+	u64 msrval, guestval, hostval = x86_spec_ctrl_base;
 	struct thread_info *ti = current_thread_info();
-	u64 msr, host = x86_spec_ctrl_base;
 
 	/* Is MSR_SPEC_CTRL implemented ? */
 	if (static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
+		/*
+		 * Restrict guest_spec_ctrl to supported values. Clear the
+		 * modifiable bits in the host base value and or the
+		 * modifiable bits from the guest value.
+		 */
+		guestval = hostval & ~x86_spec_ctrl_mask;
+		guestval |= guest_spec_ctrl & x86_spec_ctrl_mask;
+
 		/* SSBD controlled in MSR_SPEC_CTRL */
 		if (static_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD))
-			host |= ssbd_tif_to_spec_ctrl(ti->flags);
+			hostval |= ssbd_tif_to_spec_ctrl(ti->flags);
 
-		if (host != guest_spec_ctrl) {
-			msr = setguest ? guest_spec_ctrl : host;
-			wrmsrl(MSR_IA32_SPEC_CTRL, msr);
+		if (hostval != guestval) {
+			msrval = setguest ? guestval : hostval;
+			wrmsrl(MSR_IA32_SPEC_CTRL, msrval);
 		}
 	}
 }
@@ -493,7 +505,7 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
 		switch (boot_cpu_data.x86_vendor) {
 		case X86_VENDOR_INTEL:
 			x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
-			x86_spec_ctrl_mask &= ~SPEC_CTRL_SSBD;
+			x86_spec_ctrl_mask |= SPEC_CTRL_SSBD;
 			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
 			break;
 		case X86_VENDOR_AMD: