LSM: Add "contents" flag to kernel_read_file hook

As with the kernel_load_data LSM hook, add a "contents" flag to the
kernel_read_file LSM hook that indicates whether the LSM can expect
a matching call to the kernel_post_read_file LSM hook with the full
contents of the file. With the coming addition of partial file read
support for kernel_read_file*() API, the LSM will no longer be able
to always see the entire contents of a file during the read calls.

For cases where the LSM must read examine the complete file contents,
it will need to do so on its own every time the kernel_read_file
hook is called with contents=false (or reject such cases). Adjust all
existing LSMs to retain existing behavior.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/r/20201002173828.2099543-12-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/security.h

@@ -391,7 +391,8 @@ int security_kernel_load_data(enum kernel_load_data_id id, bool contents);
 int security_kernel_post_load_data(char *buf, loff_t size,
 				   enum kernel_load_data_id id,
 				   char *description);
-int security_kernel_read_file(struct file *file, enum kernel_read_file_id id);
+int security_kernel_read_file(struct file *file, enum kernel_read_file_id id,
+			      bool contents);
 int security_kernel_post_read_file(struct file *file, char *buf, loff_t size,
 				   enum kernel_read_file_id id);
 int security_task_fix_setuid(struct cred *new, const struct cred *old,
@@ -1030,7 +1031,8 @@ static inline int security_kernel_post_load_data(char *buf, loff_t size,
 }
 
 static inline int security_kernel_read_file(struct file *file,
-					    enum kernel_read_file_id id)
+					    enum kernel_read_file_id id,
+					    bool contents)
 {
 	return 0;
 }