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Date: Mon, 24 Oct 2016 15:38:23 -0700
From: Kees Cook <keescook@...omium.org>
To: Hans Liljestrand <ishkamiel@...il.com>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Elena Reshetova <elena.reshetova@...el.com>
Subject: Re: [RFC v2 PATCH 00/13] HARDENED_ATOMIC
On Thu, Oct 20, 2016 at 6:13 AM, Hans Liljestrand <ishkamiel@...il.com> wrote:
> On Thu, Oct 20, 2016 at 01:25:18PM +0300, Elena Reshetova wrote:
>> Changes since RFC v1:
>>
>> - documentation added: Documentation/security/hardened-atomic.txt
>> - percpu-refcount diversion from PaX/Grsecurity explained better
>> - arch. independent base has full functional coverage for atomic,
>> atomic-long and atomic64 types.
>> - arch. independent base is better structured and organized
>> - lkdtm: tests are now defined using macros
>> - x86 implementation added for missing functions
>> - fixed trap handling on x86 and overall reporting
>> - many small polishing and fixes
>>
>> Open items:
>>
>> - performance measurements: we are still waiting for numbers
>> - arch. independent implementation doesn't have coverage for
>> local_wrap_t type in cases when include/asm-generic/local.h
>> is not used (meaning architecture does provide its implementation
>> but does not yet provide *_wrap functions). We haven't yet
>> find a nice way of doing it in arch. independent definitions,
>> since some kernel code includes asm/local.h directly and we
>> are not sure where to place new definitions (new file under
>> inlcude/linux/local_wrap.h (to be inline with include/linux/
>> atomic.h) + definition of local_wrap_t to include/linux/types.h?)
>> Ideas and suggestions on this are very warlmy welcomed!
>>
>> Compilation and testing results:
>>
>> - CONFIG_HARDENED_ATOMIC=y, arch=x86_64 or x86_32, full x86 coverage implementation: compiles, lkdtm atomic tests PASS
>> - CONFIG_HARDENED_ATOMIC=n, arch=x86_64 or x86_32, full x86 coverage implementation: compiles, feature not enabled, so tests not run
>> - CONFIG_HARDENED_ATOMIC=n, arch=x86_64 or x86_32, with x86 hardening implementation removed
>> (simulate not implemented for arch. case): compile does not yet pass due to issues with local_wrap_t decribed above
>
> As noted our current implementation fails on local_t without arch support (at
> least in kernel/trace/ring_buffer.c where local_wrap_t is used). It seems that
> local_t is almost never used, which is also what the related documentation
> recommends (at Documentation/local_ops.txt). I would be inclined to drop local_t
> support and switch the generic implementation to use atomic_long_wrap_t instead
> of atomic_long_t.
>
> So my question is then, do we actually want to provide a protected version of
> local_t, or can we just drop this support?
What is the combination of arch/CONFIG that causes local_t to fail?
I'd prefer to retain coverage, but I don't quite understand where the
problem is. It sounds like this is a header file issue? Should local.h
get moved under asm-generic?
>> 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.
> 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.
> In contrast to local_t issue, atomic64_t is in fact directly used in several
> places, including some that we patch to use atomic64_wrap_t. The atomic_(long)_t
> implementation is also possibly intertwined with atomic64_t, so I doubt just
> dropping bare atomic64_t protections is a viable solution.
Agreed.
> On that note, our lkdtm test are still lacking atomic64 tests, which would
> probably be good idea to add.
Agreed, I'd like as much coverage as possible (especially if we have
some fragile combinations).
-Kees
--
Kees Cook
Nexus Security
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