From 686fef928bba6be13cabe639f154af7d72b63120 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 28 Sep 2017 06:38:17 -0700 Subject: [PATCH] timer: Prepare to change timer callback argument type Modern kernel callback systems pass the structure associated with a given callback to the callback function. The timer callback remains one of the legacy cases where an arbitrary unsigned long argument continues to be passed as the callback argument. This has several problems: - This bloats the timer_list structure with a normally redundant .data field. - No type checking is being performed, forcing callbacks to do explicit type casts of the unsigned long argument into the object that was passed, rather than using container_of(), as done in most of the other callback infrastructure. - Neighboring buffer overflows can overwrite both the .function and the .data field, providing attackers with a way to elevate from a buffer overflow into a simplistic ROP-like mechanism that allows calling arbitrary functions with a controlled first argument. - For future Control Flow Integrity work, this creates a unique function prototype for timer callbacks, instead of allowing them to continue to be clustered with other void functions that take a single unsigned long argument. This adds a new timer initialization API, which will ultimately replace the existing setup_timer(), setup_{deferrable,pinned,etc}_timer() family, named timer_setup() (to mirror hrtimer_setup(), making instances of its use much easier to grep for). In order to support the migration of existing timers into the new callback arguments, timer_setup() casts its arguments to the existing legacy types, and explicitly passes the timer pointer as the legacy data argument. Once all setup_*timer() callers have been replaced with timer_setup(), the casts can be removed, and the data argument can be dropped with the timer expiration code changed to just pass the timer to the callback directly. Since the regular pattern of using container_of() during local variable declaration repeats the need for the variable type declaration to be included, this adds a helper modeled after other from_*() helpers that wrap container_of(), named from_timer(). This helper uses typeof(*variable), removing the type redundancy and minimizing the need for line wraps in forthcoming conversions from "unsigned data long" to "struct timer_list *" in the timer callbacks: -void callback(unsigned long data) +void callback(struct timer_list *t) { - struct some_data_structure *local = (struct some_data_structure *)data; + struct some_data_structure *local = from_timer(local, t, timer); Finally, in order to support the handful of timer users that perform open-coded assignments of the .function (and .data) fields, provide cast macros (TIMER_FUNC_TYPE and TIMER_DATA_TYPE) that can be used temporarily. Once conversion has been completed, these can be globally trivially removed. Signed-off-by: Kees Cook Signed-off-by: Thomas Gleixner Link: https://lkml.kernel.org/r/20170928133817.GA113410@beast --- include/linux/timer.h | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/include/linux/timer.h b/include/linux/timer.h index e6789b8757d5..6383c528b148 100644 --- a/include/linux/timer.h +++ b/include/linux/timer.h @@ -168,6 +168,20 @@ static inline void init_timer_on_stack_key(struct timer_list *timer, #define setup_pinned_deferrable_timer_on_stack(timer, fn, data) \ __setup_timer_on_stack((timer), (fn), (data), TIMER_DEFERRABLE | TIMER_PINNED) +#define TIMER_DATA_TYPE unsigned long +#define TIMER_FUNC_TYPE void (*)(TIMER_DATA_TYPE) + +static inline void timer_setup(struct timer_list *timer, + void (*callback)(struct timer_list *), + unsigned int flags) +{ + __setup_timer(timer, (TIMER_FUNC_TYPE)callback, + (TIMER_DATA_TYPE)timer, flags); +} + +#define from_timer(var, callback_timer, timer_fieldname) \ + container_of(callback_timer, typeof(*var), timer_fieldname) + /** * timer_pending - is a timer pending? * @timer: the timer in question -- 2.20.1