security: Introduce file_release hook

In preparation for moving IMA and EVM to the LSM infrastructure, introduce
the file_release hook.

IMA calculates at file close the new digest of the file content and writes
it to security.ima, so that appraisal at next file access succeeds.

The new hook cannot return an error and cannot cause the operation to be
reverted.
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

fs/file_table.c

@@ -367,6 +367,7 @@ static void __fput(struct file *file)
 	eventpoll_release(file);
 	locks_remove_file(file);
 
+	security_file_release(file);
 	ima_file_free(file);
 	if (unlikely(file->f_flags & FASYNC)) {
 		if (file->f_op->fasync)

include/linux/lsm_hook_defs.h

@@ -173,6 +173,7 @@ LSM_HOOK(int, 0, kernfs_init_security, struct kernfs_node *kn_dir,
 	 struct kernfs_node *kn)
 LSM_HOOK(int, 0, file_permission, struct file *file, int mask)
 LSM_HOOK(int, 0, file_alloc_security, struct file *file)
+LSM_HOOK(void, LSM_RET_VOID, file_release, struct file *file)
 LSM_HOOK(void, LSM_RET_VOID, file_free_security, struct file *file)
 LSM_HOOK(int, 0, file_ioctl, struct file *file, unsigned int cmd,
 	 unsigned long arg)

include/linux/security.h

@@ -395,6 +395,7 @@ int security_kernfs_init_security(struct kernfs_node *kn_dir,
 				  struct kernfs_node *kn);
 int security_file_permission(struct file *file, int mask);
 int security_file_alloc(struct file *file);
+void security_file_release(struct file *file);
 void security_file_free(struct file *file);
 int security_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
 int security_file_ioctl_compat(struct file *file, unsigned int cmd,
@@ -1008,6 +1009,9 @@ static inline int security_file_alloc(struct file *file)
 	return 0;
 }
 
+static inline void security_file_release(struct file *file)
+{ }
+
 static inline void security_file_free(struct file *file)
 { }
 

security/security.c

@@ -2718,6 +2718,17 @@ int security_file_alloc(struct file *file)
 	return rc;
 }
 
+/**
+ * security_file_release() - Perform actions before releasing the file ref
+ * @file: the file
+ *
+ * Perform actions before releasing the last reference to a file.
+ */
+void security_file_release(struct file *file)
+{
+	call_void_hook(file_release, file);
+}
+
 /**
  * security_file_free() - Free a file's LSM blob
  * @file: the file