[PATCH v2] arm64: proton-pack: Restore the nospectre_bhb command-line option

[Karl Mehltretter]

Commit 7f1635737823 ("arm64: proton-pack: Fix hard lockup due to print in
scheduler context") moved the "mitigation disabled" printks into
spectre_print_disabled_mitigations(). For spectre-v2 and spectre-v4 only
the pr_info_once() calls were removed, but for spectre-bhb the whole
branch went with the print:

  -   } else if (cpu_mitigations_off() || __nospectre_bhb) {
  -           pr_info_once("spectre-bhb mitigation disabled ...\n");

spectre_bhb_enable_mitigation() therefore no longer tests __nospectre_bhb
or cpu_mitigations_off() and the mitigation is enabled regardless of the
command line. The parameter is still parsed and its flag is still checked
by spectre_print_disabled_mitigations(), so the kernel prints "spectre-bhb
mitigation disabled by command-line option" while
/sys/devices/system/cpu/vulnerabilities/spectre_v2 reports "Mitigation:
CSV2, BHB" and the vectors are switched to EL1_VECTOR_BHB_LOOP.

The only remaining escape is the SPECTRE_VULNERABLE arm at the top of the
chain, which a CSV2 core never reaches, so from Cortex-A76 and Neoverse N1
onwards both nospectre_bhb and mitigations=off are ignored. Both are
documented in Documentation/admin-guide/kernel-parameters.txt.

The identical mistake was made on the neighbouring compile-time-option
branch immediately before this regression and fixed shortly afterwards;
this command-line branch was missed.
build_bhb_mitigation() in arch/arm64/net/bpf_jit_comp.c still tests both
flags, so nospectre_bhb currently keeps the exception-vector loop while
dropping the cBPF epilogue mitigation.

Restore the check, folded into a spectre_bhb_mitigations_off() helper
alongside its spectre_v2/v4 counterparts, and use it for the boot-time
print in spectre_print_disabled_mitigations() as well. The print itself
already lives there and does not need restoring.

Tested under QEMU with -cpu neoverse-n1 (CSV2, no ECBHB, no CLRBHB).
Before, spectre_v2 read "Mitigation: CSV2, BHB" with and without the
option; after, nospectre_bhb and mitigations=off both give "Mitigation:
CSV2, but not BHB" and a boot without either is unchanged.

Fixes: 7f1635737823 ("arm64: proton-pack: Fix hard lockup due to print in scheduler context")
Assisted-by: Claude:claude-opus-5
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
v2: fold the check into a spectre_bhb_mitigations_off() helper, matching
    spectre_v2_mitigations_off()/spectre_v4_mitigations_off(), and use it
    for the boot-time print too, as requested by Will. All three QEMU
    boots re-run on v2; results unchanged.
v1: https://lore.kernel.org/r/20260725122959.98361-1-kmehltretter@xxxxxxxxx/

 arch/arm64/kernel/proton-pack.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c
index 7bb6553fec08..3bcf86154d95 100644
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c
@@ -1023,6 +1023,11 @@ static int __init parse_spectre_bhb_param(char *str)
 }
 early_param("nospectre_bhb", parse_spectre_bhb_param);

+static bool spectre_bhb_mitigations_off(void)
+{
+	return __nospectre_bhb || cpu_mitigations_off();
+}
+
 void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
 {
 	bp_hardening_cb_t cpu_cb;
@@ -1036,6 +1041,8 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
 		/* No point mitigating Spectre-BHB alone. */
 	} else if (!IS_ENABLED(CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY)) {
 		/* Do nothing */
+	} else if (spectre_bhb_mitigations_off()) {
+		/* Mitigation disabled on the command line */
 	} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
 		state = SPECTRE_MITIGATED;
 		set_bit(BHB_HW, &system_bhb_mitigations);
@@ -1201,6 +1208,6 @@ void spectre_print_disabled_mitigations(void)
 	if (spectre_v4_mitigations_off())
 		pr_info("spectre-v4 %s", spectre_disabled_suffix);

-	if (__nospectre_bhb || cpu_mitigations_off())
+	if (spectre_bhb_mitigations_off())
 		pr_info("spectre-bhb %s", spectre_disabled_suffix);
 }
-- 
2.51.0