[PATCH] x86/bugs: Default retbleed to =stuff when retpoline is auto enabled

[Pawan Gupta]

On Intel systems when retpoline mitigation is enabled for spectre-v2,
retbleed=auto does not enable RSB stuffing. This may make the system
vulnerable to retbleed. Retpoline is not the default mitigation when
IBRS is present, but in virtualized cases a VMM can hide IBRS from
guests, resulting in guest deploying retpoline by default.

On Intel systems, when spectre_v2 and retbleed mitigations are in auto
mode, and retpoline is enabled, deploy Call Depth Tracking and RSB
stuffing i.e. retbleed=stuff mitigation. For AMD/Hygon auto mode already
selects the appropriate mitigation.

Reported-by: Alyssa Milburn <alyssa.milburn@xxxxxxxxx>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/bugs.c | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index bb0ab8466b91..55d94b71af18 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -50,6 +50,8 @@ static void __init l1d_flush_select_mitigation(void);
 static void __init srso_select_mitigation(void);
 static void __init gds_select_mitigation(void);
 
+static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void);
+
 /* The base value of the SPEC_CTRL MSR without task-specific bits set */
 u64 x86_spec_ctrl_base;
 EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
@@ -892,6 +894,20 @@ static int __init nospectre_v1_cmdline(char *str)
 }
 early_param("nospectre_v1", nospectre_v1_cmdline);
 
+/* The kernel command line selection for spectre v2 */
+enum spectre_v2_mitigation_cmd {
+	SPECTRE_V2_CMD_NONE,
+	SPECTRE_V2_CMD_AUTO,
+	SPECTRE_V2_CMD_FORCE,
+	SPECTRE_V2_CMD_RETPOLINE,
+	SPECTRE_V2_CMD_RETPOLINE_GENERIC,
+	SPECTRE_V2_CMD_RETPOLINE_LFENCE,
+	SPECTRE_V2_CMD_EIBRS,
+	SPECTRE_V2_CMD_EIBRS_RETPOLINE,
+	SPECTRE_V2_CMD_EIBRS_LFENCE,
+	SPECTRE_V2_CMD_IBRS,
+};
+
 enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init = SPECTRE_V2_NONE;
 
 #undef pr_fmt
@@ -1025,10 +1041,17 @@ static void __init retbleed_select_mitigation(void)
 				retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
 			else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY) && boot_cpu_has(X86_FEATURE_IBPB))
 				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+		} else if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
+			   spectre_v2_parse_cmdline() == SPECTRE_V2_CMD_AUTO &&
+			   spectre_v2_enabled == SPECTRE_V2_RETPOLINE) {
+			if (IS_ENABLED(CONFIG_CALL_DEPTH_TRACKING))
+				retbleed_mitigation = RETBLEED_MITIGATION_STUFF;
+			else
+				pr_err("WARNING: Retpoline enabled, but kernel not compiled with CALL_DEPTH_TRACKING.\n");
 		}
 
 		/*
-		 * The Intel mitigation (IBRS or eIBRS) was already selected in
+		 * If Intel mitigation (IBRS or eIBRS) was already selected in
 		 * spectre_v2_select_mitigation().  'retbleed_mitigation' will
 		 * be set accordingly below.
 		 */
@@ -1157,20 +1180,6 @@ static inline bool match_option(const char *arg, int arglen, const char *opt)
 	return len == arglen && !strncmp(arg, opt, len);
 }
 
-/* The kernel command line selection for spectre v2 */
-enum spectre_v2_mitigation_cmd {
-	SPECTRE_V2_CMD_NONE,
-	SPECTRE_V2_CMD_AUTO,
-	SPECTRE_V2_CMD_FORCE,
-	SPECTRE_V2_CMD_RETPOLINE,
-	SPECTRE_V2_CMD_RETPOLINE_GENERIC,
-	SPECTRE_V2_CMD_RETPOLINE_LFENCE,
-	SPECTRE_V2_CMD_EIBRS,
-	SPECTRE_V2_CMD_EIBRS_RETPOLINE,
-	SPECTRE_V2_CMD_EIBRS_LFENCE,
-	SPECTRE_V2_CMD_IBRS,
-};
-
 enum spectre_v2_user_cmd {
 	SPECTRE_V2_USER_CMD_NONE,
 	SPECTRE_V2_USER_CMD_AUTO,

---
base-commit: 54be6c6c5ae8e0d93a6c4641cb7528eb0b6ba478
change-id: 20240208-retbleed-auto-stuff-53e0fa91305e

Best regards,
-- 
Thanks,
Pawan