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: Mark Brown <[e-mail]>
Reviewed-by: Zong Li <[e-mail]>
Signed-off-by: Deepak Gupta <[e-mail]>
Tested-by: Andreas Korb <[e-mail]> # QEMU, custom CVA6
Tested-by: Valentin Haudiquet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
[[e-mail]: cleaned up patch description, code comments]
Signed-off-by: Paul Walmsley <[e-mail]>

include/linux/cpu.h

···
 #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

···
 # 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

···
 	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,
···
 		break;
 	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);