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From:   Kees Cook <keescook@chromium.org>
Date:   Fri, 19 Oct 2018 09:05:57 -0700
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Subject: Re: [PATCH v5 07/17] arm64: add basic pointer authentication support
To:     Will Deacon <will.deacon@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Kristina Martsenko <kristina.martsenko@arm.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
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        Andrew Jones <drjones@redhat.com>,
        Jacob Bramley <jacob.bramley@arm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
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        Adam Wallis <awallis@codeaurora.org>,
        "Suzuki K . Poulose" <suzuki.poulose@arm.com>,
        Christoffer Dall <christoffer.dall@arm.com>,
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        Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
        Amit Kachhap <Amit.Kachhap@arm.com>,
        Dave P Martin <Dave.Martin@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Cyrill Gorcunov <gorcunov@openvz.org>
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On Fri, Oct 19, 2018 at 8:49 AM, Will Deacon <will.deacon@arm.com> wrote:
> On Fri, Oct 19, 2018 at 08:36:45AM -0700, Kees Cook wrote:
>> On Fri, Oct 19, 2018 at 4:24 AM, Will Deacon <will.deacon@arm.com> wrote:
>> > FWIW: I think we should be entertaining a prctl() interface to use a new
>> > key on a per-thread basis. Obviously, this would need to be used with care
>> > (e.g. you'd fork(); use the prctl() and then you'd better not return from
>> > the calling function!).
>> >
>> > Assuming we want this (Kees -- I was under the impression that everything in
>> > Android would end up with the same key otherwise?), then the question is
>> > do we want:
>> >
>> >   - prctl() get/set operations for the key, or
>> >   - prctl() set_random_key operation, or
>> >   - both of the above?
>> >
>> > Part of the answer to that may lie in the requirements of CRIU, where I
>> > strongly suspect they need explicit get/set operations, although these
>> > could be gated on CONFIG_CHECKPOINT_RESTORE=y.
>>
>> Oh CRIU. Yikes. I'd like the get/set to be gated by the CONFIG, yes.
>> No reason to allow explicit access to the key (and selected algo) if
>> we don't have to.
>
> Makes sense.
>
>> As for per-thread or not, having a "pick a new key now" prctl() sounds
>> good, but I'd like to have an eye toward having it just be "automatic"
>> on clone().
>
> I thought about that too, but we're out of clone() flags afaict and there's
> no arch hook in there. We could add yet another clone syscall, but yuck (and
> I reckon viro would kill us).
>
> Or are you saying that we could infer the behaviour from the existing set
> of flags?

I mean if it's starting a new thread, it should get a new key
automatically, just like the ssp canary happens in dup_task_struct().

(Or did I miss some context for why that's not possible?)

-Kees

-- 
Kees Cook
Pixel Security