Date: Fri, 10 May 2024 16:29:19 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Deepak Gupta <debug@...osinc.com>
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Subject: Re: [PATCH v3 17/29] prctl: arch-agnostic prctl for indirect branch
 tracking

On Wed, Apr 03, 2024 at 04:35:05PM -0700, 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`
> aarch64 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@...osinc.com>
> ---
>  include/uapi/linux/prctl.h | 27 +++++++++++++++++++++++++++
>  kernel/sys.c               | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 57 insertions(+)
> 
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 3c66ed8f46d8..b7a8212a068e 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -328,4 +328,31 @@ struct prctl_mm_map {
>   */
>  #define PR_LOCK_SHADOW_STACK_STATUS      73
>  
> +/*
> + * 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      74
> +
> +/*
> + * 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      75
> +# define PR_INDIR_BR_LP_ENABLE		   (1UL << 0)
> +
> +/*
> + * 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      76
> +
>  #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 242e9f147791..c770060c3f06 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2330,6 +2330,21 @@ 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 __user *status)
> +{
> +	return -EINVAL;
> +}
> +
> +int __weak arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
> +{
> +	return -EINVAL;
> +}
> +

These weak references each cause a warning:

kernel/sys.c:2333:12: warning: no previous prototype for 'arch_get_indir_br_lp_status' [-Wmissing-prototypes]
 2333 | int __weak arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
kernel/sys.c:2338:12: warning: no previous prototype for 'arch_set_indir_br_lp_status' [-Wmissing-prototypes]
 2338 | int __weak arch_set_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
kernel/sys.c:2343:12: warning: no previous prototype for 'arch_lock_indir_br_lp_status' [-Wmissing-prototypes]
 2343 | int __weak arch_lock_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)

Can the definitions be added to include/linux/mm.h alongside the
*_shadow_stack_status() definitions?

- Charlie

>  #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
>  
>  #ifdef CONFIG_ANON_VMA_NAME
> @@ -2787,6 +2802,21 @@ 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, (unsigned long __user *) arg2);
> +		break;
> +	case PR_LOCK_INDIR_BR_LP_STATUS:
> +		if (arg3 || arg4 || arg5)
> +			return -EINVAL;
> +		error = arch_lock_indir_br_lp_status(me, (unsigned long __user *) arg2);
> +		break;
>  	default:
>  		error = -EINVAL;
>  		break;
> -- 
> 2.43.2
> 

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