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Date:   Thu, 23 Apr 2020 11:28:40 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        clang-built-linux@googlegroups.com,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v11 01/12] add support for Clang's Shadow Call Stack (SCS)
Message-ID: <202004231121.A13FDA100@keescook>
References: <20191018161033.261971-1-samitolvanen@google.com>
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 <20200416161245.148813-2-samitolvanen@google.com>
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 <20200422173938.GA3069@willie-the-truck>
 <20200422235134.GA211149@google.com>
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On Wed, Apr 22, 2020 at 04:51:34PM -0700, Sami Tolvanen wrote:
> On Wed, Apr 22, 2020 at 06:39:47PM +0100, Will Deacon wrote:
> > On Mon, Apr 20, 2020 at 02:18:30PM -0700, Sami Tolvanen wrote:
> > > On Mon, Apr 20, 2020 at 06:17:28PM +0100, Will Deacon wrote:
> > > > > +	 * The shadow call stack is aligned to SCS_SIZE, and grows
> > > > > +	 * upwards, so we can mask out the low bits to extract the base
> > > > > +	 * when the task is not running.
> > > > > +	 */
> > > > > +	return (void *)((unsigned long)task_scs(tsk) & ~(SCS_SIZE - 1));
> > > > 
> > > > Could we avoid forcing this alignment it we stored the SCS pointer as a
> > > > (base,offset) pair instead? That might be friendlier on the allocations
> > > > later on.
> > > 
> > > The idea is to avoid storing the current task's shadow stack address in
> > > memory, which is why I would rather not store the base address either.
> > 
> > What I mean is that, instead of storing the current shadow stack pointer,
> > we instead store a base and an offset. We can still clear the base, as you
> > do with the pointer today, and I don't see that the offset is useful to
> > an attacker on its own.
> 
> I see what you mean. However, even if we store the base address +
> the offset, we still need aligned allocation if we want to clear
> the address. This would basically just move __scs_base() logic to
> cpu_switch_to() / scs_save().

Okay, so, I feel like this has gotten off into the weeds, or I'm really
dense (or both). :) Going back to the original comment:

> > > > Could we avoid forcing this alignment it we stored the SCS
> > > > pointer as a (base,offset) pair instead? That might be friendlier
> > > > on the allocations later on.

I think there was some confusion about mixing the "we want to be able to
wipe the value" combined with the masking in __scs_base(). These are
unrelated, as was correctly observed with "We can still clear the base".

What I don't understand here is the suggestion to store two values:

Why is two better than storing one? With one, we only need a single access.

Why would storing the base be "friendlier on the allocations later on"?
This is coming out of a single kmem cache, in 1K chunks. They will be
naturally aligned to 1K (unless redzoing has been turned on for some
slab debugging reason). The base masking is a way to avoid needing to
store two values, and only happens at task death.

Storing two values eats memory for all tasks for seemingly no meaningful
common benefit. What am I missing here?

-- 
Kees Cook