On Mon, Jul 25, 2016 at 7:03 PM, Michael Ellerman [off-list ref] wrote:

Josh Poimboeuf [off-list ref] writes:

quoted

On Thu, Jul 21, 2016 at 11:34:25AM -0700, Kees Cook wrote:

quoted

On Wed, Jul 20, 2016 at 11:52 PM, Michael Ellerman [off-list ref] wrote:

quoted

Kees Cook [off-list ref] writes:

quoted

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 @@

...

quoted

+
+/*
+ * 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.

quoted

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

Yeah, I agree: this should be in the arch code. If the arch can
actually do frame checking, the thread_info (if it exists on the
stack) would already be excluded. But it'd be a nice tightening of the
check.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security

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