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From:   Jann Horn <jannh@google.com>
Date:   Tue, 24 Mar 2020 22:28:35 +0100
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Subject: Re: [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall
To:     Kees Cook <keescook@chromium.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Potapenko <glider@google.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        "Perla, Enrico" <enrico.perla@intel.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arm-kernel@lists.infradead.org,
        Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>
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On Tue, Mar 24, 2020 at 9:32 PM Kees Cook <keescook@chromium.org> wrote:
> This is a continuation and refactoring of Elena's earlier effort to add
> kernel stack base offset randomization. In the time since the previous
> discussions, two attacks[1][2] were made public that depended on stack
> determinism, so we're no longer in the position of "this is a good idea
> but we have no examples of attacks". :)
[...]
> [1] https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html

This one only starts using the stack's location after having parsed
it out of dmesg (which in any environment that wants to provide a
reasonable level of security really ought to be restricted to root),
right? If you give people read access to dmesg, they can leak all
sorts of pointers; not just the stack pointer, but also whatever else
happens to be in the registers at that point - which is likely to give
the attacker more ways to place controlled data at a known location.
See e.g. <https://googleprojectzero.blogspot.com/2018/09/a-cache-invalidation-bug-in-linux.html>,
which leaks the pointer to a BPF map out of dmesg.

Also, are you sure that it isn't possible to make the syscall that
leaked its stack pointer never return to userspace (via ptrace or
SIGSTOP or something like that), and therefore never realign its
stack, while keeping some controlled data present on the syscall's
stack?

> [2] https://repositorio-aberto.up.pt/bitstream/10216/125357/2/374717.pdf

That's a moderately large document; which specific part are you referencing?