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
[bwh: Forward-ported to 4.15]

Gbp-Pq: Topic features/all/lockdown
Gbp-Pq: Name 0002-Add-a-SysRq-option-to-lift-kernel-lockdown.patch

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index ae13bc974416fd8483f822684165de310933f407..3108e297d87d1bd4332bd18d8dbb1bef0c8ead45 100644 (file)
--- 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 39ddd9a73febb7f4c3d9b70d9422afcc51c42f02..0afeef1672bc2d4e3ec43eddc934206575c21892 100644 (file)
--- a/drivers/input/misc/uinput.c
+++ b/drivers/input/misc/uinput.c
@@ -362,6 +362,7 @@ 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 b674793be478442874a7ca62de1aeccc8ae3c50e..7c06541b422e6f3d12747eae2211794445e526ba 100644 (file)
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -487,6 +487,7 @@ 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 */
@@ -530,7 +531,7 @@ 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;
@@ -550,11 +551,15 @@ void __handle_sysrq(int key, bool check_mask)
 
         op_p = __sysrq_get_key_op(key);
         if (op_p) {
+               /* 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");
                /*
                 * 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)) {
+               if (from == SYSRQ_FROM_KERNEL || sysrq_on_mask(op_p->enable_mask)) {
                        pr_cont("%s\n", op_p->action_msg);
                        console_loglevel = orig_log_level;
                        op_p->handler(key);
@@ -586,7 +591,7 @@ 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);
 
@@ -667,7 +672,7 @@ 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 7c7516eb7d76c64071e51c199fdb0fd0f4e3c934..38cd0ea72c376e0363f05e73683b2bea2d1a99fa 100644 (file)
--- a/include/linux/input.h
+++ b/include/linux/input.h
@@ -42,6 +42,7 @@ struct input_value {
  * @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.)
@@ -124,6 +125,8 @@ 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)];
@@ -190,6 +193,8 @@ 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 8c71874e84852b5cefb9b9bf0a06a877457eaf86..7de1f08b60a913dc1cf06fe86c3132982c6db8a8 100644 (file)
--- 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 c8146d53ca677a9ef28218ba5590fa3e28f530d7..b480cadf9272061306c122748a4fca6c167943dd 100644 (file)
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -1970,7 +1970,7 @@ 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/Kconfig b/security/Kconfig
index 74d9ab21b397078021aaba764a77b8fc3e3fd220..3808a0b2d0a73eac9b7e8ab2c03db2e516a7bb6e 100644 (file)
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -234,6 +234,16 @@ config LOCK_DOWN_KERNEL
          turns off various features that might otherwise allow access to the
          kernel image (eg. setting MSR registers).
 
+config ALLOW_LOCKDOWN_LIFT_BY_SYSRQ
+       bool "Allow the kernel lockdown to be lifted by SysRq"
+       depends on LOCK_DOWN_KERNEL
+       depends on MAGIC_SYSRQ
+       depends on X86
+       help
+         Allow the lockdown on a kernel to be lifted, by pressing a SysRq key
+         combination on a wired keyboard.
+
+
 source security/selinux/Kconfig
 source security/smack/Kconfig
 source security/tomoyo/Kconfig

diff --git a/security/lock_down.c b/security/lock_down.c
index d8595c0e667319dd01dc29106bf4800db8329d4b..2c6b00f0c229e411e093f93dad7b98a1331ef398 100644 (file)
--- a/security/lock_down.c
+++ b/security/lock_down.c
@@ -11,8 +11,14 @@
 
 #include <linux/security.h>
 #include <linux/export.h>
+#include <linux/sysrq.h>
+#include <asm/setup.h>
 
+#ifdef CONFIG_ALLOW_LOCKDOWN_LIFT_BY_SYSRQ
+static __read_mostly bool kernel_locked_down;
+#else
 static __ro_after_init bool kernel_locked_down;
+#endif
 
 /*
  * Put the kernel into lock-down mode.
@@ -58,3 +64,44 @@ bool __kernel_is_locked_down(const char *what, bool first)
        return kernel_locked_down;
 }
 EXPORT_SYMBOL(__kernel_is_locked_down);
+
+#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 = false;
+}
+
+/*
+ * 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 */