Re: [PATCH v3 50/51] x86/mm: move arch_within_stack_frames() to usercopy.c

[Kees Cook]

On Fri, Aug 12, 2016 at 1:36 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Fri, Aug 12, 2016 at 01:06:41PM -0700, Kees Cook wrote:
>> On Fri, Aug 12, 2016 at 12:12 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> > On Fri, Aug 12, 2016 at 10:36:21AM -0700, Kees Cook wrote:
>> >> On Fri, Aug 12, 2016 at 7:29 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> >> > When I tried to port arch_within_stack_frames() to use the new unwinder,
>> >> > I got a nightmare include file "header soup" scenario when unwind.h was
>> >> > included from thread_info.h.  And anyway, I think thread_info.h isn't
>> >> > really an appropriate place for this function.  So move it to usercopy.c
>> >> > instead.
>> >> >
>> >> > Since it relies on its parent's stack pointer, and the function is no
>> >> > longer inlined, the arguments to the __builtin_frame_address() calls
>> >> > have been incremented.
>> >>
>> >> Cool, looks good (minor change noted below). This patch might be a
>> >> good place to drop this from mm/Makefile too:
>> >>
>> >> # Since __builtin_frame_address does work as used, disable the warning.
>> >> CFLAGS_usercopy.o += $(call cc-disable-warning, frame-address)
>> >>
>> >> Since frame-address warnings have been disabled globally now since
>> >> commit 124a3d88fa20 ("Disable "frame-address" warning").
>> >
>> > Ok, I'll do that with the next patch (51/51) which removes the
>> > __builtin_frame_address() calls.
>> >
>> >> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>> >> > ---
>> >> >  arch/x86/include/asm/thread_info.h | 46 ++++++++------------------------------
>> >> >  arch/x86/lib/usercopy.c            | 43 +++++++++++++++++++++++++++++++++++
>> >> >  2 files changed, 52 insertions(+), 37 deletions(-)
>> >> >
>> >> > diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
>> >> > index 8b7c8d8e..fd849e6 100644
>> >> > --- a/arch/x86/include/asm/thread_info.h
>> >> > +++ b/arch/x86/include/asm/thread_info.h
>> >> > @@ -176,49 +176,21 @@ static inline unsigned long current_stack_pointer(void)
>> >> >         return sp;
>> >> >  }
>> >> >
>> >> > -/*
>> >> > - * Walks up the stack frames to make sure that the specified object is
>> >> > - * entirely contained by a single stack frame.
>> >> > - *
>> >> > - * Returns:
>> >> > - *              1 if within a frame
>> >> > - *             -1 if placed across a frame boundary (or outside stack)
>> >> > - *              0 unable to determine (no frame pointers, etc)
>> >> > - */
>> >> > +#ifdef CONFIG_HARDENED_USERCOPY
>> >>
>> >> This ifdef shouldn't be needed: the arch_within_stack_frames wasn't
>> >> designed to depend on it.
>> >>
>> >> > +#ifdef CONFIG_FRAME_POINTER
>> >> > +int arch_within_stack_frames(const void * const stack,
>> >> > +                            const void * const stackend,
>> >> > +                            const void *obj, unsigned long len);
>> >> > +#else
>> >> >  static inline int arch_within_stack_frames(const void * const stack,
>> >> >                                            const void * const stackend,
>> >> >                                            const void *obj, unsigned long len)
>> >> >  {
>> >> > -#if defined(CONFIG_FRAME_POINTER)
>> >> > -       const void *frame = NULL;
>> >> > -       const void *oldframe;
>> >> > -
>> >> > -       oldframe = __builtin_frame_address(1);
>> >> > -       if (oldframe)
>> >> > -               frame = __builtin_frame_address(2);
>> >> > -       /*
>> >> > -        * low ----------------------------------------------> high
>> >> > -        * [saved bp][saved ip][args][local vars][saved bp][saved ip]
>> >> > -        *                     ^----------------^
>> >> > -        *               allow copies only within here
>> >> > -        */
>> >> > -       while (stack <= frame && frame < stackend) {
>> >> > -               /*
>> >> > -                * If obj + len extends past the last frame, this
>> >> > -                * check won't pass and the next frame will be 0,
>> >> > -                * causing us to bail out and correctly report
>> >> > -                * the copy as invalid.
>> >> > -                */
>> >> > -               if (obj + len <= frame)
>> >> > -                       return obj >= oldframe + 2 * sizeof(void *) ? 1 : -1;
>> >> > -               oldframe = frame;
>> >> > -               frame = *(const void * const *)frame;
>> >> > -       }
>> >> > -       return -1;
>> >> > -#else
>> >> >         return 0;
>> >> > -#endif
>> >> >  }
>> >> > +#endif /* CONFIG_FRAME_POINTER */
>> >> > +#endif /* CONFIG_HARDENED_USERCOPY */
>> >> > +
>> >> >
>> >> >  #else /* !__ASSEMBLY__ */
>> >> >
>> >> > diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c
>> >> > index b490878..96ce151 100644
>> >> > --- a/arch/x86/lib/usercopy.c
>> >> > +++ b/arch/x86/lib/usercopy.c
>> >> > @@ -9,6 +9,7 @@
>> >> >
>> >> >  #include <asm/word-at-a-time.h>
>> >> >  #include <linux/sched.h>
>> >> > +#include <asm/unwind.h>
>> >> >
>> >> >  /*
>> >> >   * We rely on the nested NMI work to allow atomic faults from the NMI path; the
>> >> > @@ -34,3 +35,45 @@ copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
>> >> >         return ret;
>> >> >  }
>> >> >  EXPORT_SYMBOL_GPL(copy_from_user_nmi);
>> >> > +
>> >> > +#ifdef CONFIG_HARDENED_USERCOPY
>> >>
>> >> Same thing: no need to check CONFIG_HARDENED_USERCOPY here: it should
>> >> be checking CONFIG_FRAME_POINTER instead.
>> >
>> > Now that this function is no longer inlined and is instead compiled in
>> > its own .c file, I was thinking that the tinyconfig folks would
>> > appreciate not growing the text size if there's no reason to do so.
>> > Keeping this #ifdef won't break anything, right?
>>
>> Hrm, well, I guess not, but it means if anyone else wants to use it
>> they have to remove the ifdef. I guess I don't object that much. :P
>
> Ah.  Do you expect other uses for it?

None that I'm aware of. :)

-Kees

>
>> > Also I moved the CONFIG_FRAME_POINTER check to the header file so it
>> > doesn't pollute the .c code.
>>
>> Right, but if FRAME_POINTER=n and HARDENED_USERCOPY=y you'll get a
>> build error about it being both in the .h and the .c file, if I'm
>> reading that correctly.
>
> Oh, right.
>
> --
> Josh



-- 
Kees Cook
Nexus Security