[PATCH v2 0/3] kexec: Add new parameter to limit the access to kexec

[PATCH v2 0/3] kexec: Add new parameter to limit the access to kexec

[Ricardo Ribalda]

Add two parameter to specify how many times a kexec kernel can be loaded.

These parameter allow hardening the system.

While we are at it, fix a documentation issue and refactor some code.

To: Jonathan Corbet <corbet@lwn.net>
To: Eric Biederman <ebiederm@xmission.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kexec@lists.infradead.org
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ross Zwisler <zwisler@kernel.org>
To: Philipp Rudo <prudo@redhat.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>

---
Changes in v2:
- Instead of kexec_reboot_disabled, add two new counters (Thanks Philipp!)
- Link to v1: https://lore.kernel.org/r/20221114-disable-kexec-reset-v1-0-fb51d20cf871@chromium.org

---
Ricardo Ribalda (3):
      Documentation: sysctl: Correct kexec_load_disabled
      kexec: Factor out kexec_load_permited
      kexec: Introduce paramters load_limit_reboot and load_limit_panic

 Documentation/admin-guide/kernel-parameters.txt | 14 ++++
 Documentation/admin-guide/sysctl/kernel.rst     |  7 +-
 include/linux/kexec.h                           |  3 +-
 kernel/kexec.c                                  |  2 +-
 kernel/kexec_core.c                             | 98 ++++++++++++++++++++++++-
 kernel/kexec_file.c                             |  2 +-
 6 files changed, 119 insertions(+), 7 deletions(-)
---
base-commit: 479174d402bcf60789106eedc4def3957c060bad
change-id: 20221114-disable-kexec-reset-19b7e117338f

Best regards,
-- 
Ricardo Ribalda <ribalda@chromium.org>

[PATCH v2 1/3] Documentation: sysctl: Correct kexec_load_disabled

[Ricardo Ribalda]

kexec_load_disabled affects both ``kexec_load`` and ``kexec_file_load``
syscalls. Make it explicit.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
---
 Documentation/admin-guide/sysctl/kernel.rst | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 98d1b198b2b4..97394bd9d065 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -450,9 +450,10 @@ this allows system administrators to override the
 kexec_load_disabled
 ===================
 
-A toggle indicating if the ``kexec_load`` syscall has been disabled.
-This value defaults to 0 (false: ``kexec_load`` enabled), but can be
-set to 1 (true: ``kexec_load`` disabled).
+A toggle indicating if the syscalls ``kexec_load`` and
+``kexec_file_load`` have been disabled.
+This value defaults to 0 (false: ``kexec_*load`` enabled), but can be
+set to 1 (true: ``kexec_*load`` disabled).
 Once true, kexec can no longer be used, and the toggle cannot be set
 back to false.
 This allows a kexec image to be loaded before disabling the syscall,

-- 
2.39.0.rc0.267.gcb52ba06e7-goog-b4-0.11.0-dev-696ae

[PATCH v2 2/3] kexec: Factor out kexec_load_permited

[Ricardo Ribalda]

Both syscalls (kexec and kexec_file) do the same check, lets factor it
out.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
---
 include/linux/kexec.h |  3 ++-
 kernel/kexec.c        |  2 +-
 kernel/kexec_core.c   | 11 ++++++++++-
 kernel/kexec_file.c   |  2 +-
 4 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index 41a686996aaa..e9e1ab5e8006 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -406,7 +406,8 @@ extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
 
 extern struct kimage *kexec_image;
 extern struct kimage *kexec_crash_image;
-extern int kexec_load_disabled;
+
+bool kexec_load_permited(void);
 
 #ifndef kexec_flush_icache_page
 #define kexec_flush_icache_page(page)
diff --git a/kernel/kexec.c b/kernel/kexec.c
index cb8e6e6f983c..d83fc9093aff 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 (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
+	if (!kexec_load_permited())
 		return -EPERM;
 
 	/* Permit LSMs and IMA to fail the kexec */
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index ca2743f9c634..18bd90ca9c99 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -928,7 +928,7 @@ int kimage_load_segment(struct kimage *image,
 
 struct kimage *kexec_image;
 struct kimage *kexec_crash_image;
-int kexec_load_disabled;
+static int kexec_load_disabled;
 #ifdef CONFIG_SYSCTL
 static struct ctl_table kexec_core_sysctls[] = {
 	{
@@ -952,6 +952,15 @@ static int __init kexec_core_sysctl_init(void)
 late_initcall(kexec_core_sysctl_init);
 #endif
 
+bool kexec_load_permited(void)
+{
+	/*
+	 * Only the superuser can use the kexec syscall and if it has not
+	 * been disabled.
+	 */
+	return capable(CAP_SYS_BOOT) && !kexec_load_disabled;
+}
+
 /*
  * No panic_cpu check version of crash_kexec().  This function is called
  * only when panic_cpu holds the current CPU number; this is the only CPU
diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 45637511e0de..701147c118d4 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 (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
+	if (!kexec_load_permited())
 		return -EPERM;
 
 	/* Make sure we have a legal set of flags */

-- 
2.39.0.rc0.267.gcb52ba06e7-goog-b4-0.11.0-dev-696ae

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

[Ricardo Ribalda]

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))
 		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;
+};
+
+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)
+{
+	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;
 }
 
 /*
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

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

[Joel Fernandes]

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

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

[Guilherme G. Piccoli]

On 08/12/2022 13:38, 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>
> ---

Thanks for your patches Ricardo!

Small nit in the subject: s/paramters/parameters. Just observed that
after Joel's review anyway, so kudos to him heh

Cheers,


Guilherme

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

[Ricardo Ribalda]

Hi Joel

Thanks for looking into this

On Thu, 15 Dec 2022 at 20:16, Joel Fernandes <joel@joelfernandes.org> wrote:
>
> 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.

I could not find a way to use atomic_t. The operations are not just
counters, but maybe I am missing a better way to do it :)

The current operations:

- permitted():

if (param==-1) {
  return -1;
}
if (param =0)
 return -1;
param = param -1;



- paramter_set()

new_param = min(-1, new_param);

if (param == -1) {
   param = new_param;
   return
}

if (new_param == -1) {
   return -EINVAL;
}

param = min(new_param, param);

>
> > +};
> > +
> > +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



-- 
Ricardo Ribalda