On Tue, Aug 23, 2016 at 10:37:43PM -0400, Kees Cook wrote:
> On Tue, Aug 23, 2016 at 3:28 PM, Josh Poimboeuf <jpoim...@redhat.com> 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 ‘copy_to_user.part.10’,
inlined from ‘copy_to_user’,
inlined from ‘override_release.part.11’ 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 ‘__copy_to_user_overflow’ 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 ‘__copy_to_user_overflow’
__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
