[PATCH v3 02/11] mm: Hardened usercopy

[Michael Ellerman]

Josh Poimboeuf <jpoimboe at redhat.com> writes:

> 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 at ellerman.id.au> wrote:
>> > Kees Cook <keescook at chromium.org> 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.

Yeah. I wonder if it's better for the arch helper to just take the obj and len,
and work out it's own bounds for the stack using current and whatever makes
sense on that arch.

It would avoid too much ifdefery in the generic code, and also avoid any
confusion about whether stackend is the high or low address.

eg. on powerpc we could do:

int noinline arch_within_stack_frames(const void *obj, unsigned long len)
{
	void *stack_low  = end_of_stack(current);
	void *stack_high = task_stack_page(current) + THREAD_SIZE;


Whereas arches with STACK_GROWSUP=y could do roughly the reverse, and x86 can do
whatever it needs to depending on whether the thread_info is on or off stack.

cheers