Messages in this thread | ![/](/images/icornerl.gif) | | From | Kees Cook <> | Date | Thu, 21 Jul 2016 11:34:25 -0700 | Subject | Re: [PATCH v3 02/11] mm: Hardened usercopy |
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On Wed, Jul 20, 2016 at 11:52 PM, Michael Ellerman <mpe@ellerman.id.au> wrote: > Kees Cook <keescook@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.
-Kees
-- Kees Cook Chrome OS & Brillo Security
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