[PATCH v24 13/28] prctl: arch-agnostic prctl for indirect branch tracking
From: Deepak Gupta <hidden>
Date: 2025-12-04 20:04:28
Also in:
linux-arch, linux-devicetree, linux-doc, linux-fsdevel, linux-kselftest, linux-riscv, lkml, rust-for-linux
Subsystem:
cpu hotplug, the rest · Maintainers:
Thomas Gleixner, Peter Zijlstra, Linus Torvalds
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.
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Zong Li <redacted>
Tested-by: Andreas Korb <redacted>
Tested-by: Valentin Haudiquet <redacted>
Signed-off-by: Deepak Gupta <redacted>
---
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 487b3bf2e1ea..8239cd95a005 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h@@ -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_ */
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 51c4e8c82b1e..9b4afdc85099 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h@@ -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 `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 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 */
diff --git a/kernel/sys.c b/kernel/sys.c
index 8b58eece4e58..9071422c1609 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c@@ -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;
--
2.45.0