linux-kernel - Re: [PATCH 00/17] ARMv8.3 pointer authentication support

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Message-ID: <CAGXu5jJMfCz4qFY8y1Uf7BdP961ZeBotFVUb5owaksxnz26akg@mail.gmail.com>
Date:   Tue, 13 Nov 2018 17:09:00 -0600
From:   Kees Cook <keescook@...omium.org>
To:     Kristina Martsenko <kristina.martsenko@....com>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Adam Wallis <awallis@...eaurora.org>,
        Amit Kachhap <Amit.Kachhap@....com>,
        Andrew Jones <drjones@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Dave P Martin <Dave.Martin@....com>,
        Jacob Bramley <jacob.bramley@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        "Suzuki K . Poulose" <suzuki.poulose@....com>,
        Will Deacon <will.deacon@....com>,
        kvmarm@...ts.cs.columbia.edu,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/17] ARMv8.3 pointer authentication support

On Tue, Nov 13, 2018 at 10:17 AM, Kristina Martsenko
<kristina.martsenko@....com> wrote:
> When the PAC authentication fails, it doesn't actually generate an
> exception, it just flips a bit in the high-order bits of the pointer,
> making the pointer invalid. Then when the pointer is dereferenced (e.g.
> as a function return address), it generates the usual type of exception
> for an invalid address.

Ah! Okay, thanks. I missed that detail. :)

What area of memory ends up being addressable with such bit flips?
(i.e. is the kernel making sure nothing executable ends up there?)

> So when a function return fails in user mode, the exception is handled
> in __do_user_fault and a forced SIGSEGV is delivered to the task. When a
> function return fails in kernel mode, the exception is handled in
> __do_kernel_fault and the task is killed.
>
> This is different from stack protector as we don't panic the kernel, we
> just kill the task. It would be difficult to panic as we don't have a
> reliable way of knowing that the exception was caused by a PAC
> authentication failure (we just have an invalid pointer with a specific
> bit flipped). We also don't print out any PAC-related warning.

There are other "guesses" in __do_kernel_fault(), I think? Could a
"PAC mismatch?" warning be included in the Oops if execution fails in
the address range that PAC failures would resolve into?

-Kees

-- 
Kees Cook