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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Aug 25, 2016 at 10:14:36PM -0400, Kees Cook wrote:
> On Thu, Aug 25, 2016 at 4:47 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > 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@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.
> 
> Ah, yes, I had a total brain failure. This is what I get trying to do
> email between sessions at a conference. :)
> 
> Okay, right. __builtin_object_size() is totally fine, I absolutely
> misspoke: it's the resolution of const value ranges. I wouldn't expect
> gcc to warn here, though, since "copy + 1" isn't a const value...

Look at the code again :-)

__copy_to_user_overflow(), which does the "provably correct" warning, is
"called" when the copy size is non-const (and the object size is const).
So "copy + 1" being non-const is consistent with the warning.

-- 
Josh