[PATCH] arm64: proton-pack: Fix hard lockup when !MITIGATE_SPECTRE_BRANCH_HISTORY

[PATCH] arm64: proton-pack: Fix hard lockup when !MITIGATE_SPECTRE_BRANCH_HISTORY

[Jonathan Marek]

The "drop print" commit removed the whole branch and not just the print.
For some ARM64 cpus, this leads to hard lockup when
CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY is not enabled.

Fixes: 62e72463ca71 ("arm64: proton-pack: Drop print when !CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY")
---
 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 c7d70d04c164c..80a580e019c50 100644
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c
@@ -1032,6 +1032,8 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
 
 	if (arm64_get_spectre_v2_state() == SPECTRE_VULNERABLE) {
 		/* No point mitigating Spectre-BHB alone. */
+	} else if (!IS_ENABLED(CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY)) {
+		/* Do nothing */
 	} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
 		state = SPECTRE_MITIGATED;
 		set_bit(BHB_HW, &system_bhb_mitigations);
-- 
2.51.0

Re: [PATCH] arm64: proton-pack: Fix hard lockup when !MITIGATE_SPECTRE_BRANCH_HISTORY

[Jonathan Marek]

Can you pick this up? Its an urgent fix..

And looks like I forgot it, you can add:

Signed-off-by: Jonathan Marek <jonathan@marek.ca>

On 11/17/25 12:12 AM, Jonathan Marek wrote:
> The "drop print" commit removed the whole branch and not just the print.
> For some ARM64 cpus, this leads to hard lockup when
> CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY is not enabled.
> 
> Fixes: 62e72463ca71 ("arm64: proton-pack: Drop print when !CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY")
> ---
>   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 c7d70d04c164c..80a580e019c50 100644
> --- a/arch/arm64/kernel/proton-pack.c
> +++ b/arch/arm64/kernel/proton-pack.c
> @@ -1032,6 +1032,8 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
>   
>   	if (arm64_get_spectre_v2_state() == SPECTRE_VULNERABLE) {
>   		/* No point mitigating Spectre-BHB alone. */
> +	} else if (!IS_ENABLED(CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY)) {
> +		/* Do nothing */
>   	} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
>   		state = SPECTRE_MITIGATED;
>   		set_bit(BHB_HW, &system_bhb_mitigations);
> 

Re: [PATCH] arm64: proton-pack: Fix hard lockup when !MITIGATE_SPECTRE_BRANCH_HISTORY

[Will Deacon]

On Mon, 17 Nov 2025 00:12:08 -0500, Jonathan Marek wrote:
> The "drop print" commit removed the whole branch and not just the print.
> For some ARM64 cpus, this leads to hard lockup when
> CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY is not enabled.
> 
> 

Applied (with your SoB) to arm64 (for-next/fixes), thanks!

[1/1] arm64: proton-pack: Fix hard lockup when !MITIGATE_SPECTRE_BRANCH_HISTORY
      https://git.kernel.org/arm64/c/165eb13e485c

I'd be inclined to remove CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
entirely (i.e. make it unconditional) given the number of CPUs
affected and the dynamic controls we already have for the mitigation.

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

Re: [PATCH] arm64: proton-pack: Fix hard lockup when !MITIGATE_SPECTRE_BRANCH_HISTORY

[Vladimir Zapolskiy]

On 11/17/25 07:12, Jonathan Marek wrote:
> The "drop print" commit removed the whole branch and not just the print.
> For some ARM64 cpus, this leads to hard lockup when
> CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY is not enabled.
> 
> Fixes: 62e72463ca71 ("arm64: proton-pack: Drop print when !CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY")
> ---
>   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 c7d70d04c164c..80a580e019c50 100644
> --- a/arch/arm64/kernel/proton-pack.c
> +++ b/arch/arm64/kernel/proton-pack.c
> @@ -1032,6 +1032,8 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
>   
>   	if (arm64_get_spectre_v2_state() == SPECTRE_VULNERABLE) {
>   		/* No point mitigating Spectre-BHB alone. */
> +	} else if (!IS_ENABLED(CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY)) {
> +		/* Do nothing */
>   	} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
>   		state = SPECTRE_MITIGATED;
>   		set_bit(BHB_HW, &system_bhb_mitigations);


If CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY is unset, the fix is needed
to boot Linux on Lantronix SM8450-HDK and SM8550-HDK boards, which are
otherwise broken since v6.18-rc6.

Reviewed-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Tested-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>

-- 
Best wishes,
Vladimir