[PATCH v3] x86/bugs: Default retbleed to =stuff when retpoline is 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. Even if IBRS
is enumerated, a user can still select spectre_v2=retpoline.

As with other mitigations, mitigate retbleed by default. On Intel
systems when retpoline is enabled, and retbleed mitigation is set to
auto, enable 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>
Cc: stable@xxxxxxxxxx
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
v3:
- Rebased to v6.9-rc2

v2: https://lore.kernel.org/r/20240212-retbleed-auto-stuff-v2-1-89401649341a@xxxxxxxxxxxxxxx
- Mitigate retbleed by default for spectre_v2=retpoline. (Josh)
- Add the missing ',' in the comment. (Josh)
- Rebased to v6.8-rc4

v1: https://lore.kernel.org/r/20240208-retbleed-auto-stuff-v1-1-6f12e513868f@xxxxxxxxxxxxxxx
---
 arch/x86/kernel/cpu/bugs.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index e7ba936d798b..69d8ce58f244 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1092,11 +1092,17 @@ static void __init retbleed_select_mitigation(void)
 			else if (IS_ENABLED(CONFIG_MITIGATION_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_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
-		 * spectre_v2_select_mitigation().  'retbleed_mitigation' will
+		 * If Intel mitigation (IBRS or eIBRS) was already selected in
+		 * spectre_v2_select_mitigation(),  'retbleed_mitigation' will
 		 * be set accordingly below.
 		 */
 

---
base-commit: 39cd87c4eb2b893354f3b850f916353f2658ae6f
change-id: 20240208-retbleed-auto-stuff-53e0fa91305e