[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 at 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