Fix the "IBPB mitigated RETBleed" mode of operation on AMD CPUs

(not turned on by default), which also need STIBP enabled (if
 available) to be '100% safe' on even the shortest speculation
 windows.
 
 Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'x86-urgent-2022-08-13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fix from Ingo Molnar:
 "Fix the 'IBPB mitigated RETBleed' mode of operation on AMD CPUs (not
  turned on by default), which also need STIBP enabled (if available) to
  be '100% safe' on even the shortest speculation windows"

* tag 'x86-urgent-2022-08-13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/bugs: Enable STIBP for IBPB mitigated RETBleed

Documentation/admin-guide/kernel-parameters.txt

@@ -5274,20 +5274,33 @@
 			Speculative Code Execution with Return Instructions)
 			vulnerability.
 
+			AMD-based UNRET and IBPB mitigations alone do not stop
+			sibling threads from influencing the predictions of other
+			sibling threads. For that reason, STIBP is used on pro-
+			cessors that support it, and mitigate SMT on processors
+			that don't.
+
 			off          - no mitigation
 			auto         - automatically select a migitation
 			auto,nosmt   - automatically select a mitigation,
 				       disabling SMT if necessary for
 				       the full mitigation (only on Zen1
 				       and older without STIBP).
-			ibpb	     - mitigate short speculation windows on
-				       basic block boundaries too. Safe, highest
-				       perf impact.
-			unret        - force enable untrained return thunks,
-				       only effective on AMD f15h-f17h
-				       based systems.
-			unret,nosmt  - like unret, will disable SMT when STIBP
-			               is not available.
+			ibpb         - On AMD, mitigate short speculation
+				       windows on basic block boundaries too.
+				       Safe, highest perf impact. It also
+				       enables STIBP if present. Not suitable
+				       on Intel.
+			ibpb,nosmt   - Like "ibpb" above but will disable SMT
+				       when STIBP is not available. This is
+				       the alternative for systems which do not
+				       have STIBP.
+			unret        - Force enable untrained return thunks,
+				       only effective on AMD f15h-f17h based
+				       systems.
+			unret,nosmt  - Like unret, but will disable SMT when STIBP
+				       is not available. This is the alternative for
+				       systems which do not have STIBP.
 
 			Selecting 'auto' will choose a mitigation method at run
 			time according to the CPU.

arch/x86/kernel/cpu/bugs.c

@@ -152,7 +152,7 @@ void __init check_bugs(void)
 	/*
 	 * spectre_v2_user_select_mitigation() relies on the state set by
 	 * retbleed_select_mitigation(); specifically the STIBP selection is
-	 * forced for UNRET.
+	 * forced for UNRET or IBPB.
 	 */
 	spectre_v2_user_select_mitigation();
 	ssb_select_mitigation();
@@ -1179,7 +1179,8 @@ spectre_v2_user_select_mitigation(void)
 	    boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
 		mode = SPECTRE_V2_USER_STRICT_PREFERRED;
 
-	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {
+	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET ||
+	    retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
 		if (mode != SPECTRE_V2_USER_STRICT &&
 		    mode != SPECTRE_V2_USER_STRICT_PREFERRED)
 			pr_info("Selecting STIBP always-on mode to complement retbleed mitigation\n");
@@ -2360,10 +2361,11 @@ static ssize_t srbds_show_state(char *buf)
 
 static ssize_t retbleed_show_state(char *buf)
 {
-	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {
+	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET ||
+	    retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
 	    if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
 		boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)
-		    return sprintf(buf, "Vulnerable: untrained return thunk on non-Zen uarch\n");
+		    return sprintf(buf, "Vulnerable: untrained return thunk / IBPB on non-AMD based uarch\n");
 
 	    return sprintf(buf, "%s; SMT %s\n",
 			   retbleed_strings[retbleed_mitigation],