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Date: Tue, 25 Oct 2016 13:51:06 -0400
From: David Windsor <dave@...gbits.org>
To: kernel-hardening@...ts.openwall.com
Cc: Kees Cook <keescook@...omium.org>, Elena Reshetova <elena.reshetova@...el.com>
Subject: Re: [RFC v2 PATCH 00/13] HARDENED_ATOMIC
On Tue, Oct 25, 2016 at 1:18 PM, Colin Vidal <colin@...dal.org> wrote:
> Hi Kees, Hans,
>
>> > > > This series brings the PaX/Grsecurity PAX_REFCOUNT
>> > > > feature support to the upstream kernel. All credit for the
>> > > > feature goes to the feature authors.
>> > > >
>> > > > The name of the upstream feature is HARDENED_ATOMIC
>> > > > and it is configured using CONFIG_HARDENED_ATOMIC and
>> > > > HAVE_ARCH_HARDENED_ATOMIC.
>> > > >
>> > > > This series only adds x86 support; other architectures are expected
>> > > > to add similar support gradually.
>> > >
>> > > I have some worries on the generic arch independent implementation of
>> > > atomic64_t/atomic64_wrap_t (include/asm-generic/atomic64.h). We provide _wrap
>> > > versions for atomic64, but protection is dependant on arch implementation and
>> > > config. That is, one could possibly implement HARDENED_ATOMIC support while
>> > > leaving atomic64_t unprotected depending on specific configs, for instance by
>> > > then defaulting to CONFIG_GENERIC_ATOMIC64 (in linuc/hardened/atomic.h:676). Or
>> > > maybe I'm just under-/overthinking this?
>> >
>> > IIUC, ARMv6 builds could have GENERIC_ATOMIC64 and (once implemented)
>> > HARDENED_ATOMIC, so I think that combination is worth spending time
>> > on.
>>
>> I'm not completely sure what you mean? Our current patchset doesn't implement
>> any protections for the generic atomic64, but rather relies on HARDENED_ATOMIC
>> enabled archs to provide a protected implementation. So currently any
>> HARDENED_ATOMIC archs cannot depend on GENERIC_ATOMIC64. Does this sound
>> reasonable?
>
> In the actual situation, you can use a architecture with
> GENERIC_ATOMIC64 (imx_v6_v7_defconfig on arm for instance), and set
> CONFIG_HARDENED_ATOMIC=y. That will broke the build. Therefore, we
> should put a negative dependency between GENERIC_ATOMIC64 and
> HAVE_ARCH_HARDENED_ATOMIC, in order to be sure that HARDENED_ATOMIC
> cannot be set when GENERIC_ATOMIC64 is set.
>
This is starting to get out of hand. I'm reviewing the situation now
as it relates to local_wrap_t getting defined in certain
circumstances, but not others, and have found that the dependency
resolution scheme in HARDENED_ATOMIC is confusing. I think we need to
document this somewhere. If not in-tree, then on this mailing list,
at least.
If you have a solid understanding of what types get defined when
architectural support for CONFIG_HARDENED_ATOMIC is enabled, where
those types get defined (arch-specific vs. arch-independent code),
would you mind writing something up here for all of us? Also, it very
well could be the case that this is easier than I'm making it out to
be. If so, just tell me and I'll go away.
> But it seems wired, or a pity, that HARDENED_ATOMIC is disabled on some
> architecture just because code implementation issues, no?
>
>> > > My concern is that this is a very easy place to include errors and
>> > > inconsistencies. We've been trying to cleanly fix this, but haven't really found
>> > > a satisfactory solution (e.g. one that actually works on different configs/arcs
>> > > and isn't a horrible mess). I recall that the hardened_atomic ARM implementation
>> > > already faced issues with atomic64, so this seems to be a real cause for
>> > > problems. Any suggestions on how to do this more cleanly?
>> >
>> > I haven't looked too closely yet, though maybe Colin will have some
>> > thoughts as he looks at the ARM port.
>>
>> Ok, that would probably be helpful. It would be good to get this cleanly done
>> from the start so it doesn't grow increasingly messy with every arch needing to
>> do incremental fixes/hacks as they get implemented.
>
> Since GENERIC_ATOMIC64 is only on few architecture (arm, metatag,
> microblaze, sparc, and perhaps mips?), I wonder if it would not be a
> better idea to drop asm-generic/atomic64.h: it will induces a code
> duplication, for sure, but avoid the wired situation above.
>
> That said, I don't really understand how asm-generic/atomic64.h works:
> it defines lot of extern functions (atomic64_add, for instance) and a
> can't find the implementation in the arch directory (in sparc, for
> instance)... Some ideas? It could be an interesting workaround: define
> atomic64_*_wrap prototypes in asm-generic/atomic64.h, and each
> architecture with GENERIC_ATOMIC64 must implement them.
>
I don't think anyone else understands, either. It would be great to
have some documentation of how this all works.
> Thanks,
>
> Colin
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