[PATCH 2/2] kernel/entry: add kernel.syscall_user_dispatch sysctl

[Gregory Price <gourry@gourry.net>]

Add a matching sysctl to go with CONFIG_SYSCALL_USER_DISPATCH.

kernel.syscall_user_dispatch (default 1) controls whether userspace
may arm SUD (both via prctl and ptrace).

Disarming is always permitted - same semantics as comparable knobs

Signed-off-by: Gregory Price <gourry@gourry.net>
---
 Documentation/admin-guide/sysctl/kernel.rst | 17 +++++++++++++
 kernel/entry/syscall_user_dispatch.c        | 28 +++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index c6994e55d141..4c90caaf1e21 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/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 (SUD) via
+``prctl(PR_SET_SYSCALL_USER_DISPATCH, ...)`` or the
+``PTRACE_SET_SYSCALL_USER_DISPATCH_CONFIG`` ptrace request:
+
+  ==  ===================================================================
+  0   Arming SUD is denied with ``-EPERM``.  Tasks that already armed it
+      keep it, and disabling SUD (``PR_SYS_DISPATCH_OFF``) is always
+      permitted.
+  1   (default) Arming SUD is permitted.
+  ==  ===================================================================
+
+Only present when the kernel is built with ``CONFIG_SYSCALL_USER_DISPATCH``.
+
+
 sysctl_writes_strict
 ====================
 
diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
index d89dffcc2d64..1c39ccd733f5 100644
--- a/kernel/entry/syscall_user_dispatch.c
+++ b/kernel/entry/syscall_user_dispatch.c
@@ -11,12 +11,15 @@
 #include <linux/uaccess.h>
 #include <linux/signal.h>
 #include <linux/elf.h>
+#include <linux/sysctl.h>
 
 #include <linux/sched/signal.h>
 #include <linux/sched/task_stack.h>
 
 #include <asm/syscall.h>
 
+static int syscall_user_dispatch_allowed __read_mostly = 1;
+
 static void trigger_sigsys(struct pt_regs *regs)
 {
 	struct kernel_siginfo info;
@@ -102,6 +105,10 @@ 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 +179,24 @@ 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_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_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
+};
+
+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_SYSCTL */
-- 
2.54.0