[02/30] Add a SysRq option to lift kernel lockdown

Origin: https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit?id=a420ebff9e6d0153119096706918e3925ea860fc

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 upstream lockdown:
 - Change config dependency to SECURITY_LOCKDOWN_LSM
 - Compare kernel_locked_down with LOCKDOWN_NONE instead of treating it
   as a bool
 - Adjust filenames, context]

Gbp-Pq: Topic features/all/lockdown
Gbp-Pq: Name 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 ed8ec011a9fdc6d3af834b67ed63d5a30d8f0adb..8daf633a5347ea84de2ad858d6b8122da8db421c 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 002654ec704040608e001fb9d453a189c276ddce..5a50e13be4e27fb4c41d84319b7bd192ef3c6e70 100644 (file)
--- a/drivers/input/misc/uinput.c
+++ b/drivers/input/misc/uinput.c
@@ -357,6 +357,7 @@ 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 573b2055173c177c9def565967027da99a341237..7cc95a8bdf8d3138e302228de2c3a5265ed28fae 100644 (file)
--- 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) {
+               /* 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_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 94f277cd806a81c2148b380b1756951871edd019..8539afa2c00191c1add040f9eb57119d95a70e18 100644 (file)
--- 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 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 4567fe998c306730c76c0b3be245f6dba145d3a6..d05142ef44c4eb8c0d29470f21cc0a90ff4e6aac 100644 (file)
--- 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 628a87dd1b6b5a55b7273281147a6f85d81d42e7..0f4b88913e46cfdc5899a59c4b5a7f4d459c034b 100644 (file)
--- a/security/lockdown/Kconfig
+++ b/security/lockdown/Kconfig
@@ -60,3 +60,12 @@ config LOCK_DOWN_IN_EFI_SECURE_BOOT
 
          Enabling this option results in kernel lockdown being
          triggered in confidentiality mode if EFI Secure Boot is set.
+
+config ALLOW_LOCKDOWN_LIFT_BY_SYSRQ
+       bool "Allow the kernel lockdown to be lifted by SysRq"
+       depends on SECURITY_LOCKDOWN_LSM
+       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.

diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index 01b60402320fdd7c55fcb109852a4a15812d1836..3b6a764e7783262b1c601120266ad72da23e13e0 100644 (file)
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -13,8 +13,14 @@
 #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;
+#ifdef CONFIG_ALLOW_LOCKDOWN_LIFT_BY_SYSRQ
+static __read_mostly enum lockdown_reason kernel_locked_down;
+#else
+static __ro_after_init enum lockdown_reason kernel_locked_down;
+#endif
 
 static const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
        [LOCKDOWN_NONE] = "none",
@@ -190,3 +196,44 @@ DEFINE_LSM(lockdown) = {
        .name = "lockdown",
        .init = lockdown_lsm_init,
 };
+
+#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 != LOCKDOWN_NONE)
+               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_NONE) {
+               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 */