Re: [PATCH 2/2] mm/usercopy: enable usercopy size checking for modern versions of gcc

[Josh Poimboeuf]

On Tue, Aug 23, 2016 at 10:37:43PM -0400, Kees Cook wrote:
> On Tue, Aug 23, 2016 at 3:28 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > This is a revert of:
> >
> >   2fb0815c9ee6 ("gcc4: disable __compiletime_object_size for GCC 4.6+")
> >
> > The goal of that commit was to silence the "provably correct" gcc
> > warnings.  But it went too far: it also disabled the runtime warnings.
> >
> > Now that the pretty much useless gcc warnings have been properly
> > disposed of with the previous patch, re-enable this checking on modern
> > versions of gcc so we can get the runtime warnings again.
> 
> As far as I know, this will still be broken since it's
> __builtin_object_size() that is buggy. Maybe I'm misunderstanding
> which piece is busted, though?

What specifically is buggy with __builtin_object_size()?  Looking at the
generated code for a few of the "provably correct" warning sites, the
values generated by __builtin_object_size() are correct.

I think the problem is really related to the compile-time warning
function attribute used by __copy_to_user_overflow().  The warning is
printed when gcc *can* determine the object size but it *can't*
determine the copy size.  The warning just means that, even though the
object has a const size, gcc isn't able to prove that the overflow won't
happen.

As an example, here's one of the warnings:

  In file included from /home/jpoimboe/git/linux/include/linux/uaccess.h:5:0,
                   from /home/jpoimboe/git/linux/arch/x86/include/asm/stacktrace.h:9,
                   from /home/jpoimboe/git/linux/arch/x86/include/asm/perf_event.h:246,
                   from /home/jpoimboe/git/linux/include/linux/perf_event.h:24,
                   from /home/jpoimboe/git/linux/kernel/sys.c:16:
  In function âcopy_to_user.part.10â,
      inlined from âcopy_to_userâ,
      inlined from âoverride_release.part.11â at /home/jpoimboe/git/linux/kernel/sys.c:1136:9:
  /home/jpoimboe/git/linux/arch/x86/include/asm/uaccess.h:723:46: warning: call to â__copy_to_user_overflowâ declared with attribute warning: copy_to_user() buffer size is not provably correct
   #define __copy_to_user_overflow(size, count) __copy_to_user_overflow()
                                                ^~~~~~~~~~~~~~~~~~~~~~~~~
  /home/jpoimboe/git/linux/arch/x86/include/asm/uaccess.h:791:3: note: in expansion of macro â__copy_to_user_overflowâ
     __copy_to_user_overflow(sz, n);
     ^~~~~~~~~~~~~~~~~~~~~~~

This is from override_release()'s use of copy_to_user().  The object
code shows that __builtin_object_size() correctly reports 65 bytes for
the 'buf' object size.  But the copy size ('copy + 1') isn't known at
compile-time.  Thus the (bogus) warning.

Maybe I'm missing something but I don't even see a gcc bug.  To me it
looks like a mismatch in expectations between the code and the compiler.

-- 
Josh