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SubjectRe: [PATCH 02/27] Add a SysRq option to lift kernel lockdown
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Date
On 3/6/19 3:58 PM, Matthew Garrett wrote:
> From: Kyle McMartin <kyle@redhat.com>
>
> 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.

You still need to document this in Documentation/admin-guide/sysrq.rst,
like I mentioned last week.

> 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: Matthew Garrett <matthewgarrett@google.com>
> ---
> arch/x86/include/asm/setup.h | 2 ++
> drivers/input/misc/uinput.c | 1 +
> drivers/tty/sysrq.c | 19 ++++++++++-----
> include/linux/input.h | 5 ++++
> include/linux/sysrq.h | 8 +++++-
> kernel/debug/kdb/kdb_main.c | 2 +-
> security/Kconfig | 9 +++++++
> security/lock_down.c | 47 ++++++++++++++++++++++++++++++++++++
> 8 files changed, 85 insertions(+), 8 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 8ec483e8688b..c2a77dc73fa0 100644
> --- a/drivers/input/misc/uinput.c
> +++ b/drivers/input/misc/uinput.c
> @@ -365,6 +365,7 @@ 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 1f03078ec352..0a05d336008e 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;
> @@ -543,11 +544,15 @@ 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);
> @@ -579,7 +584,7 @@ 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);
>
> @@ -659,7 +664,7 @@ 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,
> @@ -812,8 +817,10 @@ 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;
> }
> @@ -1096,7 +1103,7 @@ 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 7c7516eb7d76..38cd0ea72c37 100644
> --- 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 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 82a3b32a7cfc..efee1abf5e8e 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/Kconfig b/security/Kconfig
> index 47dc3403b5af..8346eb883336 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -244,6 +244,15 @@ config LOCK_DOWN_KERNEL_FORCE
> help
> Enable the kernel lock down functionality automatically at boot.
>
> +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 13a8228c1034..cfbc2c39712b 100644
> --- 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.
> @@ -57,3 +63,44 @@ 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 */
>


--
~Randy

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