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

Alexandre Ghiti
2025-04-09T08:13:05Z

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@rivosinc.com>
> Reviewed-by: Mark Brown <broonie@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;