[PATCH v12 13/28] prctl: arch-agnostic prctl for indirect branch tracking

[Alexandre Ghiti]

On 14/03/2025 22:39, Deepak Gupta wrote:
> Three architectures (x86, aarch64, riscv) have support for indirect branch
> tracking feature in a very similar fashion. On a very high level, indirect
> branch tracking is a CPU feature where CPU tracks branches which uses
> memory operand to perform control transfer in program. As part of this
> tracking on indirect branches, CPU goes in a state where it expects a
> landing pad instr on target and if not found then CPU raises some fault
> (architecture dependent)
>
> x86 landing pad instr - `ENDBRANCH`
> arch64 landing pad instr - `BTI`
> riscv landing instr - `lpad`
>
> Given that three major arches have support for indirect branch tracking,
> This patch makes `prctl` for indirect branch tracking arch agnostic.
>
> To allow userspace to enable this feature for itself, following prtcls are
> defined:
>   - PR_GET_INDIR_BR_LP_STATUS: Gets current configured status for indirect
>     branch tracking.
>   - PR_SET_INDIR_BR_LP_STATUS: Sets a configuration for indirect branch
>     tracking.
>     Following status options are allowed
>         - PR_INDIR_BR_LP_ENABLE: Enables indirect branch tracking on user
>           thread.
>         - PR_INDIR_BR_LP_DISABLE; Disables indirect branch tracking on user
>           thread.
>   - PR_LOCK_INDIR_BR_LP_STATUS: Locks configured status for indirect branch
>     tracking for user thread.
>
> Signed-off-by: Deepak Gupta <debug at rivosinc.com>
> Reviewed-by: Mark Brown <broonie at kernel.org>
> ---
>   include/linux/cpu.h        |  4 ++++
>   include/uapi/linux/prctl.h | 27 +++++++++++++++++++++++++++
>   kernel/sys.c               | 30 ++++++++++++++++++++++++++++++
>   3 files changed, 61 insertions(+)
>
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index 6a0a8f1c7c90..fb0c394430c6 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -204,4 +204,8 @@ static inline bool cpu_mitigations_auto_nosmt(void)
>   }
>   #endif
>   
> +int arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status);
> +int arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status);
> +int arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long status);
> +
>   #endif /* _LINUX_CPU_H_ */
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 5c6080680cb2..6cd90460cbad 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -353,4 +353,31 @@ struct prctl_mm_map {
>    */
>   #define PR_LOCK_SHADOW_STACK_STATUS      76
>   
> +/*
> + * Get the current indirect branch tracking configuration for the current
> + * thread, this will be the value configured via PR_SET_INDIR_BR_LP_STATUS.
> + */
> +#define PR_GET_INDIR_BR_LP_STATUS      77
> +
> +/*
> + * Set the indirect branch tracking configuration. PR_INDIR_BR_LP_ENABLE will
> + * enable cpu feature for user thread, to track all indirect branches and ensure
> + * they land on arch defined landing pad instruction.
> + * x86 - If enabled, an indirect branch must land on `ENDBRANCH` instruction.
> + * arch64 - If enabled, an indirect branch must land on `BTI` instruction.
> + * riscv - If enabled, an indirect branch must land on `lpad` instruction.
> + * PR_INDIR_BR_LP_DISABLE will disable feature for user thread and indirect
> + * branches will no more be tracked by cpu to land on arch defined landing pad
> + * instruction.
> + */
> +#define PR_SET_INDIR_BR_LP_STATUS      78
> +# define PR_INDIR_BR_LP_ENABLE		   (1UL << 0)


Are we missing PR_INDIR_BR_LP_DISABLE definition here?


> +
> +/*
> + * Prevent further changes to the specified indirect branch tracking
> + * configuration.  All bits may be locked via this call, including
> + * undefined bits.
> + */
> +#define PR_LOCK_INDIR_BR_LP_STATUS      79
> +
>   #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index cb366ff8703a..f347f3518d0b 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2336,6 +2336,21 @@ int __weak arch_lock_shadow_stack_status(struct task_struct *t, unsigned long st
>   	return -EINVAL;
>   }
>   
> +int __weak arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
> +{
> +	return -EINVAL;
> +}
> +
> +int __weak arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status)
> +{
> +	return -EINVAL;
> +}
> +
> +int __weak arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long status)
> +{
> +	return -EINVAL;
> +}
> +
>   #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
>   
>   #ifdef CONFIG_ANON_VMA_NAME
> @@ -2811,6 +2826,21 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>   			return -EINVAL;
>   		error = arch_lock_shadow_stack_status(me, arg2);
>   		break;
> +	case PR_GET_INDIR_BR_LP_STATUS:
> +		if (arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		error = arch_get_indir_br_lp_status(me, (unsigned long __user *)arg2);
> +		break;
> +	case PR_SET_INDIR_BR_LP_STATUS:
> +		if (arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		error = arch_set_indir_br_lp_status(me, arg2);
> +		break;
> +	case PR_LOCK_INDIR_BR_LP_STATUS:
> +		if (arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		error = arch_lock_indir_br_lp_status(me, arg2);
> +		break;
>   	default:
>   		trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5);
>   		error = -EINVAL;
>