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    SubjectRe: [PATCH v3 02/11] mm: Hardened usercopy
    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@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.

    FWIW, this won't work right on x86 after Andy's
    CONFIG_THREAD_INFO_IN_TASK patches get merged.

    --
    Josh

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