[PATCH v4] x86/bugs: Enable STIBP for IBPB mitigated RetBleed

[Kim Phillips <kim.phillips@amd.com>]

AMD's "Technical Guidance for Mitigating Branch Type Confusion,
Rev. 1.0 2022-07-12" whitepaper, under section 6.1.2 "IBPB On
Privileged Mode Entry / SMT Safety" says:

"Similar to the Jmp2Ret mitigation, if the code on the sibling thread
cannot be trusted, software should set STIBP to 1 or disable SMT to
ensure SMT safety when using this mitigation."

So, like already being done for retbleed=unret, the also for
retbleed=ibpb, force STIBP on machines that have it, and report
its SMT vulnerability status accordingly.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
Fixes: 3ebc17006888 ("x86/bugs: Add retbleed=ibpb")
Cc: stable@vger.kernel.org # 5.10, 5.15, 5.19
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
v4:  Cc: stable (Greg K-H)
v3:  "unret and ibpb mitigations" -> "UNRET and IBPB mitigations" (Mingo)
v2:  Justify and explain STIBP's role with IBPB (Boris)

 .../admin-guide/kernel-parameters.txt         | 20 ++++++++++++++-----
 arch/x86/kernel/cpu/bugs.c                    | 10 ++++++----
 2 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bab2b0bf5988..ed6a19ae0dd6 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5260,20 +5260,30 @@
 			Speculative Code Execution with Return Instructions)
 			vulnerability.
 
+			AMD-based UNRET and IBPB mitigations alone do not stop
+			sibling threads influencing the predictions of other sibling
+			threads.  For that reason, we use STIBP on processors
+			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
+			ibpb         - [AMD] Mitigate short speculation windows on
 				       basic block boundaries too. Safe, highest
-				       perf impact.
-			unret        - force enable untrained return thunks,
+				       perf impact. It also enables STIBP if
+				       present.
+			ibpb,nosmt   - [AMD] Like ibpb, but will disable SMT when STIBP
+				       is not available. This is the alternative for
+				       systems which do not have STIBP.
+			unret        - [AMD] 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.
+			unret,nosmt  - [AMD] 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.
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6761668100b9..d50686ca5870 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();
@@ -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");
@@ -2320,10 +2321,11 @@ 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