[PATCH] x86/bugs: Fix retbleed reporting "Vulnerable" when spectre_v2=ibrs

[Pawan Gupta]

With cmdline "spectre_v2=ibrs retbleed=off" sysfs reports vulnerable to
retbleed:

  $ cat /sys/devices/system/cpu/vulnerabilities/retbleed
  Vulnerable

On Intel CPUs when IBRS or Enhanced IBRS is enabled, system is not
vulnerable to retbleed.

Even if a user has passed retbleed=off, mitigation for retbleed could be
deployed as part of spectre_v2 mitigation. Fix retbleed reporting for
such a case.

Fixes: 6ad0ad2bf8a6 ("x86/bugs: Report Intel retbleed vulnerability")
Reported-by: Antonio Gomez Iglesias <antonio.gomez.iglesias@xxxxxxxxxxxxxxx>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 .../admin-guide/kernel-parameters.txt         |  4 +++-
 arch/x86/kernel/cpu/bugs.c                    | 19 +++++++++++--------
 2 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 426fa892d311..70447979111c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5280,7 +5280,9 @@
 			cessors that support it, and mitigate SMT on processors
 			that don't.
 
-			off          - no mitigation
+			off          - no mitigation. Overridden when
+				       spectre_v2 mitigation already mitigates
+				       retbleed.
 			auto         - automatically select a migitation
 			auto,nosmt   - automatically select a mitigation,
 				       disabling SMT if necessary for
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index da7c361f47e0..02f4e0a2f725 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -778,6 +778,14 @@ early_param("nospectre_v1", nospectre_v1_cmdline);
 static enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init =
 	SPECTRE_V2_NONE;
 
+static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
+{
+	return mode == SPECTRE_V2_IBRS ||
+	       mode == SPECTRE_V2_EIBRS ||
+	       mode == SPECTRE_V2_EIBRS_RETPOLINE ||
+	       mode == SPECTRE_V2_EIBRS_LFENCE;
+}
+
 #undef pr_fmt
 #define pr_fmt(fmt)     "RETBleed: " fmt
 
@@ -856,6 +864,9 @@ static void __init retbleed_select_mitigation(void)
 
 	switch (retbleed_cmd) {
 	case RETBLEED_CMD_OFF:
+		if (spectre_v2_in_ibrs_mode(spectre_v2_enabled) &&
+		    boot_cpu_data.x86_vendor == X86_VENDOR_INTEL)
+			goto do_cmd_auto;
 		return;
 
 	case RETBLEED_CMD_UNRET:
@@ -1095,14 +1106,6 @@ spectre_v2_parse_user_cmdline(void)
 	return SPECTRE_V2_USER_CMD_AUTO;
 }
 
-static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
-{
-	return mode == SPECTRE_V2_IBRS ||
-	       mode == SPECTRE_V2_EIBRS ||
-	       mode == SPECTRE_V2_EIBRS_RETPOLINE ||
-	       mode == SPECTRE_V2_EIBRS_LFENCE;
-}
-
 static void __init
 spectre_v2_user_select_mitigation(void)
 {

base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5
-- 
2.37.2