On Thu, Jul 21, 2016 at 11:34:25AM -0700, Kees Cook wrote: > On Wed, Jul 20, 2016 at 11:52 PM, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote: > > Kees Cook <keescook@xxxxxxxxxxxx> writes: > > > >> diff --git a/mm/usercopy.c b/mm/usercopy.c > >> new file mode 100644 > >> index 000000000000..e4bf4e7ccdf6 > >> --- /dev/null > >> +++ b/mm/usercopy.c > >> @@ -0,0 +1,234 @@ > > ... > >> + > >> +/* > >> + * Checks if a given pointer and length is contained by the current > >> + * stack frame (if possible). > >> + * > >> + * 0: not at all on the stack > >> + * 1: fully within a valid stack frame > >> + * 2: fully on the stack (when can't do frame-checking) > >> + * -1: error condition (invalid stack position or bad stack frame) > >> + */ > >> +static noinline int check_stack_object(const void *obj, unsigned long len) > >> +{ > >> + const void * const stack = task_stack_page(current); > >> + const void * const stackend = stack + THREAD_SIZE; > > > > That allows access to the entire stack, including the struct thread_info, > > is that what we want - it seems dangerous? Or did I miss a check > > somewhere else? > > That seems like a nice improvement to make, yeah. > > > We have end_of_stack() which computes the end of the stack taking > > thread_info into account (end being the opposite of your end above). > > Amusingly, the object_is_on_stack() check in sched.h doesn't take > thread_info into account either. :P Regardless, I think using > end_of_stack() may not be best. To tighten the check, I think we could > add this after checking that the object is on the stack: > > #ifdef CONFIG_STACK_GROWSUP > stackend -= sizeof(struct thread_info); > #else > stack += sizeof(struct thread_info); > #endif > > e.g. then if the pointer was in the thread_info, the second test would > fail, triggering the protection. FWIW, this won't work right on x86 after Andy's CONFIG_THREAD_INFO_IN_TASK patches get merged. -- Josh -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html