syscall_user_dispatch: Add kernel.syscall_user_dispatch sysctl

Add a matching sysctl to go with CONFIG_SYSCALL_USER_DISPATCH.

kernel.syscall_user_dispatch (default 1 - allow) controls whether
userspace may arm syscall user dispatch (both via prctl and ptrace).

Disarming is always permitted - same semantics as comparable knobs.

Disabling while a task has armed syscall user dispatch does not
cause it to become inactive - instead it remains active until the
user attempts to disable/re-enable via prctl or ptrace.  On the
next attempt to re-enable, the prctl/ptrace call fails gracefully.

The alternative would cause programs translating non-linux syscalls
to interpret those syscalls as linux syscalls, resulting in undefined
userland behavior.

Signed-off-by: Gregory Price <gourry@gourry.net>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260706140020.873735-3-gourry@gourry.net

Documentation/admin-guide/sysctl/kernel.rst

@@ -1402,6 +1402,23 @@ Note that if you change this from 0 to 1, already created segments
 without users and with a dead originative process will be destroyed.
 
 
+syscall_user_dispatch
+=====================
+
+Controls whether userspace may arm Syscall User Dispatch via
+``prctl(PR_SET_SYSCALL_USER_DISPATCH, ...)`` or the
+``PTRACE_SET_SYSCALL_USER_DISPATCH_CONFIG`` ptrace request:
+
+  ==  ===================================================================
+  0   Arming syscall user dispatch is denied with ``-EPERM``.  Tasks that
+      already armed it keep it, and disabling it is always permitted.
+  1   (default) Arming syscall user dispatch is permitted.
+  ==  ===================================================================
+
+Only present when the kernel is built with ``CONFIG_SYSCALL_USER_DISPATCH``
+and ``CONFIG_PROC_SYSCTL``.
+
+
 sysctl_writes_strict
 ====================
 

kernel/entry/syscall_user_dispatch.c

@@ -2,21 +2,22 @@
 /*
  * Copyright (C) 2020 Collabora Ltd.
  */
-
+#include <linux/elf.h>
 #include <linux/entry-common.h>
-#include <linux/sched.h>
 #include <linux/prctl.h>
 #include <linux/ptrace.h>
-#include <linux/syscall_user_dispatch.h>
-#include <linux/uaccess.h>
-#include <linux/signal.h>
-#include <linux/elf.h>
-
+#include <linux/sched.h>
 #include <linux/sched/signal.h>
 #include <linux/sched/task_stack.h>
+#include <linux/signal.h>
+#include <linux/syscall_user_dispatch.h>
+#include <linux/sysctl.h>
+#include <linux/uaccess.h>
 
 #include <asm/syscall.h>
 
+static bool syscall_user_dispatch_allowed __read_mostly = true;
+
 static void trigger_sigsys(struct pt_regs *regs)
 {
 	struct kernel_siginfo info;
@@ -102,6 +103,10 @@ static int task_set_syscall_user_dispatch(struct task_struct *task, unsigned lon
 		return -EINVAL;
 	}
 
+	/* Arming can be denied at runtime via sysctl, disarming is allowed */
+	if (mode != PR_SYS_DISPATCH_OFF && !syscall_user_dispatch_allowed)
+		return -EPERM;
+
 	/*
 	 * access_ok() will clear memory tags for tagged addresses
 	 * if current has memory tagging enabled.
@@ -172,3 +177,22 @@ int syscall_user_dispatch_set_config(struct task_struct *task, unsigned long siz
 	return task_set_syscall_user_dispatch(task, cfg.mode, cfg.offset, cfg.len,
 					      (char __user *)(uintptr_t)cfg.selector);
 }
+
+#ifdef CONFIG_PROC_SYSCTL
+static const struct ctl_table syscall_user_dispatch_sysctls[] = {
+	{
+		.procname	= "syscall_user_dispatch",
+		.data		= &syscall_user_dispatch_allowed,
+		.maxlen		= sizeof(syscall_user_dispatch_allowed),
+		.mode		= 0644,
+		.proc_handler	= proc_dobool,
+	},
+};
+
+static int __init syscall_user_dispatch_sysctl_init(void)
+{
+	register_sysctl_init("kernel", syscall_user_dispatch_sysctls);
+	return 0;
+}
+late_initcall(syscall_user_dispatch_sysctl_init);
+#endif /* CONFIG_PROC_SYSCTL */