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Date: Wed, 12 Oct 2016 15:35:37 -0700
From: Kees Cook <keescook@...omium.org>
To: Colin Vidal <colin@...dal.org>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
AKASHI Takahiro <takahiro.akashi@...aro.org>, "Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Re: self introduction
On Wed, Oct 12, 2016 at 1:25 AM, Colin Vidal <colin@...dal.org> wrote:
>> > So, I will try to start to port PAX_REFCOUNT arm-specific features
>> > to hardened_atomic_on_next, and keep you in touch. Is there a
>> > deadline? (4.10 / 5.0 merge window?)
>>
>> You may want to compare notes with Takahiro (CCed) who may have
>> started to look at arm64 (and maybe arm too).
>
> Thanks, I would be grateful!
Here's a thread Takahiro replied to:
http://www.openwall.com/lists/kernel-hardening/2016/10/12/2
>> As for a deadline, as Elena says, we have no specific target. ("As
>> soon as possible.") The only thing around timing that I like to see
>> is persistent progress: if a patch series goes up for review,
>> getting people to take a look at it, ask questions, make comments,
>> and then hopefully within a week or so, the next version comes
>> up. Momentum is easier to maintain than to build. ;)
>
> OK, good! I will have more time to work on it this WE, still I began to
> familiarize myself with atomic_t internals (and PaX patch).
>
> I noticed the build is broken for non-x86 architecture (at least
> arm/arm64), since basically each arch needs to provide atomic_*_wrap()
> functions. Is there plans to have (probably dummies) functions in case
> the architecture does not implements this functionality? I noticed the
> list of define atomic_*_wrap at the end of atomic-long.h, but it does
> not seems to works (they are defined after the call sites in the
> expansion of previous macros).
Yeah, I think this has already come up in the other thread and folks
are fixing (have fixed?) it. I expect to see a v2 RFC soon.
> Apart from that, in case of over-/underflow, hardened_atomic_overflow()
> is called to hang the system if CONFIG_HARDENED_ATOMIC is set. I don't
No, not hang the system, but rather stop further kernel execution and
kill the userspace process.
> get why the test in arch/x86/include/kernel/traps.c
>
> if (trapnr == X86_TRAP_OF)
> hardened_atomic_overflow(regs);
>
> is not guarded by CONFIG_HARDENED_ATOMIC: the trap cannot occurs if
> CONFIG_HARDENED_ATOMIC is unset (since "int" instructions in
> arch/x86/include/asm/atomic.h are guarded by it), and it would avoid
> the other implementation of hardened_atomic_overflow in
> include/asm-generic/bug.h.
The implementation is a static inline with no body, so the compiler
will automatically eliminate the entire "if" statement, since it's a
no-op. This is standard kernel coding style, to make the .c code as
readable as possible. Having it littered with #ifdefs makes things
harder to read.
>> > [1] https://pax.grsecurity.net/pax-linux-3.6-201210022100.patch
>>
>> This is a quite old version of PaX. (Note the date.) If you want to
>> examine PaX separately from Grsecurity (noting differences can be
>> enlightening), check here:
>>
>> https://www.grsecurity.net/~paxguy1/?C=M;O=D
>
> Thanks!
Sure thing! I'm glad to have another set of eyes on this -- it's a
pretty big set of changes. :)
-Kees
--
Kees Cook
Nexus Security
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