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

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 flag survives only in its __setup handler and in
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 branch by the
immediately preceding commit 62e72463ca71 ("arm64: proton-pack: Drop print when !CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY")
and fixed by commit 165eb13e485c ("arm64: proton-pack: Fix hard lockup when !MITIGATE_SPECTRE_BRANCH_HISTORY");
this one 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 branch. The print already lives in
spectre_print_disabled_mitigations(), so only the guard is needed.

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@vger.kernel.org
Signed-off-by: Karl Mehltretter <redacted>
---
 arch/arm64/kernel/proton-pack.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c
index 7bb6553fec08..4eaa523cdeb9 100644
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c

@@ -1036,6 +1036,8 @@ 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 (cpu_mitigations_off() || __nospectre_bhb) {
+		/* Mitigation disabled on the command line */
 	} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
 		state = SPECTRE_MITIGATED;
 		set_bit(BHB_HW, &system_bhb_mitigations);

-- 
2.39.5 (Apple Git-154)