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Subject: Re: [PATCH 02/27] Add a SysRq option to lift kernel lockdown
To:     Matthew Garrett <matthewgarrett@google.com>, jmorris@namei.org
Cc:     linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, dhowells@redhat.com
References: <20190306235913.6631-1-matthewgarrett@google.com>
 <20190306235913.6631-3-matthewgarrett@google.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <7bcd6149-332d-15f8-31e1-ef0a6b7f496e@infradead.org>
Date:   Wed, 6 Mar 2019 16:09:57 -0800
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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