efi/runtime-wrappers: Keep track of the efi_runtime_lock owner

The EFI runtime wrappers use a file local semaphore to serialize access
to the EFI runtime services. This means that any calls to the arch
wrappers around the runtime services will also be serialized, removing
the need for redundant locking.

For robustness, add a facility that allows those arch wrappers to assert
that the semaphore was taken by the current task.

Signed-off-by: Ard Biesheuvel <[e-mail]>
Acked-by: Catalin Marinas <[e-mail]>
Signed-off-by: Catalin Marinas <[e-mail]>

drivers/firmware/efi/runtime-wrappers.c

···
  */
 static DEFINE_SEMAPHORE(efi_runtime_lock, 1);
 
+static struct task_struct *efi_runtime_lock_owner;
+
 /*
  * Expose the EFI runtime lock to the UV platform
  */
···
 	const union efi_rts_args *args = efi_rts_work.args;
 	efi_status_t status = EFI_NOT_FOUND;
 	unsigned long flags;
+
+	efi_runtime_lock_owner = current;
 
 	arch_efi_call_virt_setup();
 	flags = efi_call_virt_save_flags();
···
 
 	efi_rts_work.status = status;
 	complete(&efi_rts_work.efi_rts_comp);
+	efi_runtime_lock_owner = NULL;
 }
 
 static efi_status_t __efi_queue_work(enum efi_rts_ids id,
···
 	if (down_trylock(&efi_runtime_lock))
 		return EFI_NOT_READY;
 
+	efi_runtime_lock_owner = current;
 	status = efi_call_virt_pointer(efi.runtime, set_variable, name, vendor,
 				       attr, data_size, data);
+	efi_runtime_lock_owner = NULL;
 	up(&efi_runtime_lock);
 	return status;
 }
···
 	if (down_trylock(&efi_runtime_lock))
 		return EFI_NOT_READY;
 
+	efi_runtime_lock_owner = current;
 	status = efi_call_virt_pointer(efi.runtime, query_variable_info, attr,
 				       storage_space, remaining_space,
 				       max_variable_size);
+	efi_runtime_lock_owner = NULL;
 	up(&efi_runtime_lock);
 	return status;
 }
···
 		return;
 	}
 
+	efi_runtime_lock_owner = current;
 	arch_efi_call_virt_setup();
 	efi_rts_work.efi_rts_id = EFI_RESET_SYSTEM;
 	arch_efi_call_virt(efi.runtime, reset_system, reset_type, status,
 			   data_size, data);
 	arch_efi_call_virt_teardown();
-
+	efi_runtime_lock_owner = NULL;
 	up(&efi_runtime_lock);
 }
 
···
 }
 
 #endif
+
+void efi_runtime_assert_lock_held(void)
+{
+	WARN_ON(efi_runtime_lock_owner != current);
+}

include/linux/efi.h

···
 extern void efi_call_virt_check_flags(unsigned long flags, const void *caller);
 extern unsigned long efi_call_virt_save_flags(void);
 
+void efi_runtime_assert_lock_held(void);
+
 enum efi_secureboot_mode {
 	efi_secureboot_mode_unset,
 	efi_secureboot_mode_unknown,