security: introduce kernel_fw_from_file hook

In order to validate the contents of firmware being loaded, there must be
a hook to evaluate any loaded firmware that wasn't built into the kernel
itself. Without this, there is a risk that a root user could load malicious
firmware designed to mount an attack against kernel memory (e.g. via DMA).

Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Takashi Iwai <[email protected]>

include/linux/security.h

@@ -702,6 +702,15 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
  *	@inode points to the inode to use as a reference.
  *	The current task must be the one that nominated @inode.
  *	Return 0 if successful.
+ * @kernel_fw_from_file:
+ *	Load firmware from userspace (not called for built-in firmware).
+ *	@file contains the file structure pointing to the file containing
+ *	the firmware to load. This argument will be NULL if the firmware
+ *	was loaded via the uevent-triggered blob-based interface exposed
+ *	by CONFIG_FW_LOADER_USER_HELPER.
+ *	@buf pointer to buffer containing firmware contents.
+ *	@size length of the firmware contents.
+ *	Return 0 if permission is granted.
  * @kernel_module_request:
  *	Ability to trigger the kernel to automatically upcall to userspace for
  *	userspace to load a kernel module with the given name.
@@ -1568,6 +1577,7 @@ struct security_operations {
 	void (*cred_transfer)(struct cred *new, const struct cred *old);
 	int (*kernel_act_as)(struct cred *new, u32 secid);
 	int (*kernel_create_files_as)(struct cred *new, struct inode *inode);
+	int (*kernel_fw_from_file)(struct file *file, char *buf, size_t size);
 	int (*kernel_module_request)(char *kmod_name);
 	int (*kernel_module_from_file)(struct file *file);
 	int (*task_fix_setuid) (struct cred *new, const struct cred *old,
@@ -1840,6 +1850,7 @@ int security_prepare_creds(struct cred *new, const struct cred *old, gfp_t gfp);
 void security_transfer_creds(struct cred *new, const struct cred *old);
 int security_kernel_act_as(struct cred *new, u32 secid);
 int security_kernel_create_files_as(struct cred *new, struct inode *inode);
+int security_kernel_fw_from_file(struct file *file, char *buf, size_t size);
 int security_kernel_module_request(char *kmod_name);
 int security_kernel_module_from_file(struct file *file);
 int security_task_fix_setuid(struct cred *new, const struct cred *old,
@@ -2366,6 +2377,12 @@ static inline int security_kernel_create_files_as(struct cred *cred,
 	return 0;
 }
 
+static inline int security_kernel_fw_from_file(struct file *file,
+					       char *buf, size_t size)
+{
+	return 0;
+}
+
 static inline int security_kernel_module_request(char *kmod_name)
 {
 	return 0;

security/capability.c

@@ -401,6 +401,11 @@ static int cap_kernel_create_files_as(struct cred *new, struct inode *inode)
 	return 0;
 }
 
+static int cap_kernel_fw_from_file(struct file *file, char *buf, size_t size)
+{
+	return 0;
+}
+
 static int cap_kernel_module_request(char *kmod_name)
 {
 	return 0;
@@ -1015,6 +1020,7 @@ void __init security_fixup_ops(struct security_operations *ops)
 	set_to_cap_if_null(ops, cred_transfer);
 	set_to_cap_if_null(ops, kernel_act_as);
 	set_to_cap_if_null(ops, kernel_create_files_as);
+	set_to_cap_if_null(ops, kernel_fw_from_file);
 	set_to_cap_if_null(ops, kernel_module_request);
 	set_to_cap_if_null(ops, kernel_module_from_file);
 	set_to_cap_if_null(ops, task_fix_setuid);

security/security.c

@@ -845,6 +845,12 @@ int security_kernel_create_files_as(struct cred *new, struct inode *inode)
 	return security_ops->kernel_create_files_as(new, inode);
 }
 
+int security_kernel_fw_from_file(struct file *file, char *buf, size_t size)
+{
+	return security_ops->kernel_fw_from_file(file, buf, size);
+}
+EXPORT_SYMBOL_GPL(security_kernel_fw_from_file);
+
 int security_kernel_module_request(char *kmod_name)
 {
 	return security_ops->kernel_module_request(kmod_name);