[PATCH v3 2/2] syscall_user_dispatch: add kernel.syscall_user_dispatch sysctl

[Gregory Price]

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@xxxxxxxxxx>
---
 Documentation/admin-guide/sysctl/kernel.rst | 17 +++++++++
 kernel/entry/syscall_user_dispatch.c        | 38 +++++++++++++++++----
 2 files changed, 48 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index c6994e55d141..b6328cd0f43e 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 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
 ====================
 
diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
index d89dffcc2d64..310c72e6988d 100644
--- a/kernel/entry/syscall_user_dispatch.c
+++ b/kernel/entry/syscall_user_dispatch.c
@@ -3,19 +3,20 @@
  * Copyright (C) 2020 Collabora Ltd.
  */
 
+#include <asm/syscall.h>
+#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)
 {
@@ -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 */
-- 
2.54.0