linux-kernel - [PATCH] x86/bugs: Enable STIBP for IBPB mitigated RetBleed

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Date:   Thu, 4 Aug 2022 14:22:01 -0500
From:   Kim Phillips <kim.phillips@....com>
To:     <x86@...nel.org>, <peterz@...radead.org>, <bp@...e.de>,
        <bpf@...r.kernel.org>, <jpoimboe@...hat.com>,
        <andrew.cooper3@...rix.com>
CC:     Kim Phillips <kim.phillips@....com>,
        <linux-kernel@...r.kernel.org>, <thomas.lendacky@....com>
Subject: [PATCH] x86/bugs: Enable STIBP for IBPB mitigated RetBleed

For retbleed=ibpb, force STIBP on machines that have it,
and report its SMT vulnerability status accordingly.

Fixes: 3ebc17006888 ("x86/bugs: Add retbleed=ibpb")
Signed-off-by: Kim Phillips <kim.phillips@....com>
---
 Documentation/admin-guide/kernel-parameters.txt |  4 +++-
 arch/x86/kernel/cpu/bugs.c                      | 10 ++++++----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 597ac77b541c..127fa4328360 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5212,10 +5212,12 @@
 			ibpb	     - mitigate short speculation windows on
 				       basic block boundaries too. Safe, highest
 				       perf impact.
+			ibpb,nosmt   - like ibpb, but will disable SMT when STIBP
+			               is not available.
 			unret        - force enable untrained return thunks,
 				       only effective on AMD f15h-f17h
 				       based systems.
-			unret,nosmt  - like unret, will disable SMT when STIBP
+			unret,nosmt  - like unret, but will disable SMT when STIBP
 			               is not available.
 
 			Selecting 'auto' will choose a mitigation method at run
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index fd5464ff714d..f710c012f1eb 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/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();
@@ -1181,7 +1181,8 @@ 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");
@@ -2346,10 +2347,11 @@ 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],
-- 
2.34.1