From: Josh Poimboeuf

Sent: 22 July 2016 18:46

..

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+/*
+ * 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.

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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.

What ends up in the 'thread_info' area?
If it contains the fp save area then programs like gdb may end up requesting
copy_in/out directly from that area.

Interestingly the avx registers don't need saving on a normal system call
entry (they are all caller-saved) so the kernel stack can safely overwrite
that area.
Syscall entry probably ought to execute the 'zero all avx registers' instruction.
They do need saving on interrupt entry - but the stack used will be less.

	David