Commit 5ca243f6 authored by Deepak Gupta's avatar Deepak Gupta Committed by Paul Walmsley
Browse files

prctl: add arch-agnostic prctl()s for indirect branch tracking 



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 use a memory operand to transfer control. As part of
this tracking, during an indirect branch, the CPU expects a landing
pad instruction on the target PC, and if not found, the CPU raises
some fault (architecture-dependent).

x86 landing pad instr - 'ENDBRANCH'
arch64 landing pad instr - 'BTI'
riscv landing instr - 'lpad'

Given that three major architectures have support for indirect branch
tracking, this patch creates architecture-agnostic 'prctls' to allow
userspace to control this feature.  They are:
 - PR_GET_INDIR_BR_LP_STATUS: Get the current configured status for indirect
   branch tracking.
 - PR_SET_INDIR_BR_LP_STATUS: Set the configuration for indirect branch
   tracking.
   The 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.

Reviewed-by: default avatarMark Brown <broonie@kernel.org>
Reviewed-by: default avatarZong Li <zong.li@sifive.com>
Signed-off-by: default avatarDeepak Gupta <debug@rivosinc.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de> # QEMU, custom CVA6
Tested-by: default avatarValentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-13-b55691eacf4f@rivosinc.com


[pjw@kernel.org: cleaned up patch description, code comments]
Signed-off-by: default avatarPaul Walmsley <pjw@kernel.org>
parent 61a02002

include/linux/cpu.h

+4 −0
Original line number Diff line number Diff line
@@ -229,4 +229,8 @@ static inline bool cpu_attack_vector_mitigated(enum cpu_attack_vectors v) 
#define smt_mitigations SMT_MITIGATIONS_OFF

#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_ */

include/uapi/linux/prctl.h

+27 −0
Original line number Diff line number Diff line
@@ -386,4 +386,31 @@ struct prctl_mm_map { 
# define PR_FUTEX_HASH_SET_SLOTS	1

# define PR_FUTEX_HASH_GET_SLOTS	2



/*

 * 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      79



/*

 * 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 an ENDBRANCH instruction.

 * arch64 - If enabled, an indirect branch must land on a BTI instruction.

 * riscv - If enabled, an indirect branch must land on an 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      80

# 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      81



#endif /* _LINUX_PRCTL_H */

kernel/sys.c

+30 −0
Original line number Diff line number Diff line
@@ -2388,6 +2388,21 @@ 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)



static int prctl_set_vma(unsigned long opt, unsigned long addr,
@@ -2868,6 +2883,21 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, 
	case PR_FUTEX_HASH:

		error = futex_hash_prctl(arg2, arg3, arg4);

		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;