Re: [PATCH v2 3/3] kexec: Introduce paramters load_limit_reboot and load_limit_panic

[Joel Fernandes <joel@joelfernandes.org>]

Hi Ricardo,

On Thu, Dec 08, 2022 at 05:38:02PM +0100, Ricardo Ribalda wrote:
> Add two parameter to specify how many times a kexec kernel can be loaded.
> 
> The sysadmin can set different limits for kexec panic and kexec reboot
> kernels.
> 
> The value can be modified at runtime via sysfs, but only with a value
> smaller than the current one (except -1).
> 
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 14 ++++
>  include/linux/kexec.h                           |  2 +-
>  kernel/kexec.c                                  |  2 +-
>  kernel/kexec_core.c                             | 91 ++++++++++++++++++++++++-
>  kernel/kexec_file.c                             |  2 +-
>  5 files changed, 106 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 42af9ca0127e..2b37d6a20747 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2374,6 +2374,20 @@
>  			for Movable pages.  "nn[KMGTPE]", "nn%", and "mirror"
>  			are exclusive, so you cannot specify multiple forms.
>  
> +	kexec_core.load_limit_reboot=
> +	kexec_core.load_limit_panic=
> +			[KNL]
> +			This parameter specifies a limit to the number of times
> +			a kexec kernel can be loaded.
> +			Format: <int>
> +			-1  = Unlimited.
> +			int = Number of times kexec can be called.
> +
> +			During runtime, this parameter can be modified with a
> +			value smaller than the current one (but not -1).
> +
> +			Default: -1
> +
>  	kgdbdbgp=	[KGDB,HW] kgdb over EHCI usb debug port.
>  			Format: <Controller#>[,poll interval]
>  			The controller # is the number of the ehci usb debug
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index e9e1ab5e8006..3d7d10f7187a 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -407,7 +407,7 @@ extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
>  extern struct kimage *kexec_image;
>  extern struct kimage *kexec_crash_image;
>  
> -bool kexec_load_permited(void);
> +bool kexec_load_permited(bool crash_image);
>  
>  #ifndef kexec_flush_icache_page
>  #define kexec_flush_icache_page(page)
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index d83fc9093aff..2b0856e83fe1 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -193,7 +193,7 @@ static inline int kexec_load_check(unsigned long nr_segments,
>  	int result;
>  
>  	/* We only trust the superuser with rebooting the system. */
> -	if (!kexec_load_permited())
> +	if (!kexec_load_permited(flags & KEXEC_ON_CRASH))

nit: permitted.

>  		return -EPERM;
>  
>  	/* Permit LSMs and IMA to fail the kexec */
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 18bd90ca9c99..7f9d5288b24b 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -952,13 +952,100 @@ static int __init kexec_core_sysctl_init(void)
>  late_initcall(kexec_core_sysctl_init);
>  #endif
>  
> -bool kexec_load_permited(void)
> +struct kexec_load_limit {
> +	/* Mutex protects the limit count. */
> +	struct mutex mutex;
> +	int limit;

Can you not just use atomic ops for limit, and get rid of the mutex?

That will simplify the code as well.

> +};
> +
> +struct kexec_load_limit load_limit_reboot = {
> +	.mutex = __MUTEX_INITIALIZER(load_limit_reboot.mutex),
> +	.limit = -1,
> +};
> +
> +struct kexec_load_limit load_limit_panic = {
> +	.mutex = __MUTEX_INITIALIZER(load_limit_panic.mutex),
> +	.limit = -1,
> +};
> +
> +static int param_get_limit(char *buffer, const struct kernel_param *kp)
>  {
> +	int ret;
> +	struct kexec_load_limit *limit = kp->arg;
> +
> +	mutex_lock(&limit->mutex);
> +	ret = scnprintf(buffer, PAGE_SIZE, "%i\n", limit->limit);
> +	mutex_unlock(&limit->mutex);
> +
> +	return ret;
> +}
> +
> +static int param_set_limit(const char *buffer, const struct kernel_param *kp)
> +{
> +	int ret;
> +	struct kexec_load_limit *limit = kp->arg;
> +	int new_val;
> +
> +	ret = kstrtoint(buffer, 0, &new_val);
> +	if (ret)
> +		return ret;
> +
> +	new_val = max(-1, new_val);
> +
> +	mutex_lock(&limit->mutex);
> +
> +	if (new_val == -1 && limit->limit != -1) {
> +		ret = -EINVAL;
> +		goto done;
> +	}
> +
> +	if (limit->limit != -1 && new_val > limit->limit) {
> +		ret = -EINVAL;
> +		goto done;
> +	}
> +
> +	limit->limit = new_val;
> +
> +done:
> +	mutex_unlock(&limit->mutex);
> +
> +	return ret;
> +}
> +
> +static const struct kernel_param_ops load_limit_ops = {
> +	.get = param_get_limit,
> +	.set = param_set_limit,
> +};
> +
> +module_param_cb(load_limit_reboot, &load_limit_ops, &load_limit_reboot, 0644);
> +MODULE_PARM_DESC(load_limit_reboot, "Maximum attempts to load a kexec reboot kernel");
> +
> +module_param_cb(load_limit_panic, &load_limit_ops, &load_limit_panic, 0644);
> +MODULE_PARM_DESC(load_limit_reboot, "Maximum attempts to load a kexec panic kernel");
> +
> +bool kexec_load_permited(bool crash_image)

nit: permitted.

> +{
> +	struct kexec_load_limit *limit;
> +
>  	/*
>  	 * Only the superuser can use the kexec syscall and if it has not
>  	 * been disabled.
>  	 */
> -	return capable(CAP_SYS_BOOT) && !kexec_load_disabled;
> +	if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
> +		return false;
> +
> +	/* Check limit counter and decrease it.*/
> +	limit = crash_image ? &load_limit_panic : &load_limit_reboot;
> +	mutex_lock(&limit->mutex);
> +	if (!limit->limit) {
> +		mutex_unlock(&limit->mutex);
> +		return false;
> +	}
> +	if (limit->limit != -1)
> +		limit->limit--;
> +	mutex_unlock(&limit->mutex);
> +
> +	return true;

IMO, patch 2/3 can be squashed with 3/3 but no strong preference.

thanks,

 - Joel



>  }
>  
>  /*
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 701147c118d4..61212a9252a6 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -330,7 +330,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
>  	struct kimage **dest_image, *image;
>  
>  	/* We only trust the superuser with rebooting the system. */
> -	if (!kexec_load_permited())
> +	if (!kexec_load_permited(flags & KEXEC_FILE_FLAGS))
>  		return -EPERM;
>  
>  	/* Make sure we have a legal set of flags */
> 
> -- 
> 2.39.0.rc0.267.gcb52ba06e7-goog-b4-0.11.0-dev-696ae