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Date:   Mon, 28 Oct 2019 12:57:42 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Laura Abbott <labbott@redhat.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 05/17] add support for Clang's Shadow Call Stack (SCS)
Message-ID: <201910281250.25FBA8533@keescook>
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On Mon, Oct 28, 2019 at 04:35:33PM +0000, Mark Rutland wrote:
> On Fri, Oct 25, 2019 at 01:49:21PM -0700, Sami Tolvanen wrote:
> > To keep the address of the currently active shadow stack out of
> > memory, the arm64 implementation clears this field when it loads x18
> > and saves the current value before a context switch. The generic code
> > doesn't expect the arch code to necessarily do so, but does allow it.
> > This requires us to use __scs_base() when accessing the base pointer
> > and to reset it in idle tasks before they're reused, hence
> > scs_task_reset().
> 
> Ok. That'd be worth a comment somewhere, since it adds a number of
> things which would otherwise be unnecessary.
> 
> IIUC this assumes an adversary who knows the address of a task's
> thread_info, and has an arbitrary-read (to extract the SCS base from
> thead_info) and an arbitrary-write (to modify the SCS area).
> 
> Assuming that's the case, I don't think this buys much. If said
> adversary controls two userspace threads A and B, they only need to wait
> until A is context-switched out or in userspace, and read A's SCS base
> using B.
> 
> Given that, I'd rather always store the SCS base in the thread_info, and
> simplify the rest of the code manipulating it.

I'd like to keep this as-is since it provides a temporal protection.
Having arbitrary kernel read and write at arbitrary time is a very
powerful attack primitive, and is, IMO, not very common. Many attacks
tend to be chains of bugs that give attackers narrow visibility in to the
kernel at specific moments. I would say this design is more about stopping
"current" from dumping thread_info (as there are many more opportunities
for current to see its own thread_info compared to arbitrary addresses
or another task's thread_info). As such, I think it's a reasonable
precaution to take.

-- 
Kees Cook