[PATCH v22 13/28] prctl: arch-agnostic prctl for indirect branch tracking

[Deepak Gupta via B4 Relay <devnull+debug.rivosinc.com@kernel.org>]

From: Deepak Gupta <debug@rivosinc.com>

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 <zong.li@sifive.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 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;

-- 
2.43.0