From c2eb371cede78df9a47bf3a125aa9a45dd833da7 Mon Sep 17 00:00:00 2001
From: Kyle McMartin <kyle@redhat.com>
Date: Mon, 9 Apr 2018 09:52:45 +0100
Subject: [PATCH] Add a SysRq option to lift kernel lockdown

Make an option to provide a sysrq key that will lift the kernel lockdown,
thereby allowing the running kernel image to be accessed and modified.

On x86 this is triggered with SysRq+x, but this key may not be available on
all arches, so it is set by setting LOCKDOWN_LIFT_KEY in asm/setup.h.
Since this macro must be defined in an arch to be able to use this facility
for that arch, the Kconfig option is restricted to arches that support it.

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: x86@kernel.org
Signed-off-by: Jeremy Cline <jcline@redhat.com>
---
 arch/x86/include/asm/setup.h |  2 ++
 drivers/input/misc/uinput.c  |  1 +
 drivers/tty/sysrq.c          | 27 +++++++++++++---------
 include/linux/input.h        |  5 +++++
 include/linux/sysrq.h        |  8 ++++++-
 kernel/debug/kdb/kdb_main.c  |  2 +-
 security/lockdown/Kconfig    | 11 +++++++++
 security/lockdown/lockdown.c | 43 ++++++++++++++++++++++++++++++++++++
 8 files changed, 87 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index ed8ec011a9fd..8daf633a5347 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -9,6 +9,8 @@
 #include <linux/linkage.h>
 #include <asm/page_types.h>
 
+#define LOCKDOWN_LIFT_KEY 'x'
+
 #ifdef __i386__
 
 #include <linux/pfn.h>
diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
index 84051f20b18a..583ab2bc1916 100644
--- a/drivers/input/misc/uinput.c
+++ b/drivers/input/misc/uinput.c
@@ -353,6 +353,7 @@ static int uinput_create_device(struct uinput_device *udev)
 		dev->flush = uinput_dev_flush;
 	}
 
+	dev->flags |= INPUTDEV_FLAGS_SYNTHETIC;
 	dev->event = uinput_dev_event;
 
 	input_set_drvdata(udev->dev, udev);
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 573b2055173c..99082faafc44 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -480,6 +480,7 @@ static struct sysrq_key_op *sysrq_key_table[36] = {
 	/* x: May be registered on mips for TLB dump */
 	/* x: May be registered on ppc/powerpc for xmon */
 	/* x: May be registered on sparc64 for global PMU dump */
+	/* x: May be registered on x86_64 for disabling secure boot */
 	NULL,				/* x */
 	/* y: May be registered on sparc64 for global register dump */
 	NULL,				/* y */
@@ -523,7 +524,7 @@ static void __sysrq_put_key_op(int key, struct sysrq_key_op *op_p)
                 sysrq_key_table[i] = op_p;
 }
 
-void __handle_sysrq(int key, bool check_mask)
+void __handle_sysrq(int key, unsigned int from)
 {
 	struct sysrq_key_op *op_p;
 	int orig_log_level;
@@ -546,11 +547,15 @@ void __handle_sysrq(int key, bool check_mask)
 
         op_p = __sysrq_get_key_op(key);
         if (op_p) {
-		/*
-		 * Should we check for enabled operations (/proc/sysrq-trigger
-		 * should not) and is the invoked operation enabled?
-		 */
-		if (!check_mask || sysrq_on_mask(op_p->enable_mask)) {
+		/* Ban synthetic events from some sysrq functionality */
+		if ((from == SYSRQ_FROM_PROC || from == SYSRQ_FROM_SYNTHETIC) &&
+		    op_p->enable_mask & SYSRQ_DISABLE_USERSPACE) {
+			printk("This sysrq operation is disabled from userspace.\n");
+		} else if (from == SYSRQ_FROM_KERNEL || sysrq_on_mask(op_p->enable_mask)) {
+			/*
+			 * Should we check for enabled operations (/proc/sysrq-trigger
+			 * should not) and is the invoked operation enabled?
+			 */
 			pr_info("%s\n", op_p->action_msg);
 			console_loglevel = orig_log_level;
 			op_p->handler(key);
@@ -585,7 +590,7 @@ void __handle_sysrq(int key, bool check_mask)
 void handle_sysrq(int key)
 {
 	if (sysrq_on())
-		__handle_sysrq(key, true);
+		__handle_sysrq(key, SYSRQ_FROM_KERNEL);
 }
 EXPORT_SYMBOL(handle_sysrq);
 
@@ -665,7 +670,7 @@ static void sysrq_do_reset(struct timer_list *t)
 static void sysrq_handle_reset_request(struct sysrq_state *state)
 {
 	if (state->reset_requested)
-		__handle_sysrq(sysrq_xlate[KEY_B], false);
+		__handle_sysrq(sysrq_xlate[KEY_B], SYSRQ_FROM_KERNEL);
 
 	if (sysrq_reset_downtime_ms)
 		mod_timer(&state->keyreset_timer,
@@ -818,8 +823,10 @@ static bool sysrq_handle_keypress(struct sysrq_state *sysrq,
 
 	default:
 		if (sysrq->active && value && value != 2) {
+			int from = sysrq->handle.dev->flags & INPUTDEV_FLAGS_SYNTHETIC ?
+					SYSRQ_FROM_SYNTHETIC : 0;
 			sysrq->need_reinject = false;
-			__handle_sysrq(sysrq_xlate[code], true);
+			__handle_sysrq(sysrq_xlate[code], from);
 		}
 		break;
 	}
@@ -1102,7 +1109,7 @@ static ssize_t write_sysrq_trigger(struct file *file, const char __user *buf,
 
 		if (get_user(c, buf))
 			return -EFAULT;
-		__handle_sysrq(c, false);
+		__handle_sysrq(c, SYSRQ_FROM_PROC);
 	}
 
 	return count;
diff --git a/include/linux/input.h b/include/linux/input.h
index 94f277cd806a..8539afa2c001 100644
--- a/include/linux/input.h
+++ b/include/linux/input.h
@@ -48,6 +48,7 @@ enum input_clock_type {
  * @phys: physical path to the device in the system hierarchy
  * @uniq: unique identification code for the device (if device has it)
  * @id: id of the device (struct input_id)
+ * @flags: input device flags (SYNTHETIC, etc.)
  * @propbit: bitmap of device properties and quirks
  * @evbit: bitmap of types of events supported by the device (EV_KEY,
  *	EV_REL, etc.)
@@ -134,6 +135,8 @@ struct input_dev {
 	const char *uniq;
 	struct input_id id;
 
+	unsigned int flags;
+
 	unsigned long propbit[BITS_TO_LONGS(INPUT_PROP_CNT)];
 
 	unsigned long evbit[BITS_TO_LONGS(EV_CNT)];
@@ -204,6 +207,8 @@ struct input_dev {
 };
 #define to_input_dev(d) container_of(d, struct input_dev, dev)
 
+#define	INPUTDEV_FLAGS_SYNTHETIC	0x000000001
+
 /*
  * Verify that we are in sync with input_device_id mod_devicetable.h #defines
  */
diff --git a/include/linux/sysrq.h b/include/linux/sysrq.h
index 8c71874e8485..7de1f08b60a9 100644
--- a/include/linux/sysrq.h
+++ b/include/linux/sysrq.h
@@ -29,6 +29,8 @@
 #define SYSRQ_ENABLE_BOOT	0x0080
 #define SYSRQ_ENABLE_RTNICE	0x0100
 
+#define SYSRQ_DISABLE_USERSPACE	0x00010000
+
 struct sysrq_key_op {
 	void (*handler)(int);
 	char *help_msg;
@@ -43,8 +45,12 @@ struct sysrq_key_op {
  * are available -- else NULL's).
  */
 
+#define SYSRQ_FROM_KERNEL	0x0001
+#define SYSRQ_FROM_PROC		0x0002
+#define SYSRQ_FROM_SYNTHETIC	0x0004
+
 void handle_sysrq(int key);
-void __handle_sysrq(int key, bool check_mask);
+void __handle_sysrq(int key, unsigned int from);
 int register_sysrq_key(int key, struct sysrq_key_op *op);
 int unregister_sysrq_key(int key, struct sysrq_key_op *op);
 struct sysrq_key_op *__sysrq_get_key_op(int key);
diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index 4567fe998c30..d05142ef44c4 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -1981,7 +1981,7 @@ static int kdb_sr(int argc, const char **argv)
 		return KDB_ARGCOUNT;
 
 	kdb_trap_printk++;
-	__handle_sysrq(*argv[1], check_mask);
+	__handle_sysrq(*argv[1], check_mask ? SYSRQ_FROM_KERNEL : 0);
 	kdb_trap_printk--;
 
 	return 0;
diff --git a/security/lockdown/Kconfig b/security/lockdown/Kconfig
index e84ddf484010..20e979178e1c 100644
--- a/security/lockdown/Kconfig
+++ b/security/lockdown/Kconfig
@@ -45,3 +45,14 @@ config LOCK_DOWN_KERNEL_FORCE_CONFIDENTIALITY
 	 disabled.
 
 endchoice
+
+config ALLOW_LOCKDOWN_LIFT_BY_SYSRQ
+	bool "Allow the kernel lockdown to be lifted by SysRq"
+    depends on SECURITY_LOCKDOWN_LSM
+    depends on !LOCK_DOWN_KERNEL_FORCE_CONFIDENTIALITY
+    depends on !LOCK_DOWN_KERNEL_FORCE_INTEGRITY
+    depends on MAGIC_SYSRQ
+    depends on X86
+	help
+      Allow setting the lockdown mode to "none" by pressing a SysRq key
+      combination on a wired keyboard. On x86, this is SysRq+x
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index 8a10b43daf74..df4662257309 100644
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -13,6 +13,8 @@
 #include <linux/security.h>
 #include <linux/export.h>
 #include <linux/lsm_hooks.h>
+#include <linux/sysrq.h>
+#include <asm/setup.h>
 
 static enum lockdown_reason kernel_locked_down;
 
@@ -179,6 +181,47 @@ static int __init lockdown_secfs_init(void)
 	return PTR_ERR_OR_ZERO(dentry);
 }
 
+#ifdef CONFIG_ALLOW_LOCKDOWN_LIFT_BY_SYSRQ
+
+/*
+ * Take the kernel out of lockdown mode.
+ */
+static void lift_kernel_lockdown(void)
+{
+	pr_notice("Lifting lockdown\n");
+	kernel_locked_down = LOCKDOWN_NONE;
+}
+
+/*
+ * Allow lockdown to be lifted by pressing something like SysRq+x (and not by
+ * echoing the appropriate letter into the sysrq-trigger file).
+ */
+static void sysrq_handle_lockdown_lift(int key)
+{
+	if (kernel_locked_down)
+		lift_kernel_lockdown();
+}
+
+static struct sysrq_key_op lockdown_lift_sysrq_op = {
+	.handler	= sysrq_handle_lockdown_lift,
+	.help_msg	= "unSB(x)",
+	.action_msg	= "Disabling Secure Boot restrictions",
+	.enable_mask	= SYSRQ_DISABLE_USERSPACE,
+};
+
+static int __init lockdown_lift_sysrq(void)
+{
+	if (kernel_locked_down) {
+		lockdown_lift_sysrq_op.help_msg[5] = LOCKDOWN_LIFT_KEY;
+		register_sysrq_key(LOCKDOWN_LIFT_KEY, &lockdown_lift_sysrq_op);
+	}
+	return 0;
+}
+
+late_initcall(lockdown_lift_sysrq);
+
+#endif /* CONFIG_ALLOW_LOCKDOWN_LIFT_BY_SYSRQ */
+
 core_initcall(lockdown_secfs_init);
 
 #ifdef CONFIG_SECURITY_LOCKDOWN_LSM_EARLY
-- 
2.21.0