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Date:   Mon, 22 Jun 2020 17:56:21 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Elena Reshetova <elena.reshetova@intel.com>, 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>,
        Alexander Popov <alex.popov@linux.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jann Horn <jannh@google.com>,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
 each syscall
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On Mon, Jun 22, 2020 at 08:05:10PM -0400, Arvind Sankar wrote:
> But I still don't see anything _stopping_ the compiler from optimizing
> this better in the future. The "=m" is not a barrier: it just informs
> the compiler that the asm produces an output value in *ptr (and no other
> outputs). If nothing can consume that output, it doesn't stop the
> compiler from freeing the allocation immediately after the asm instead
> of at the end of the function.

Ah, yeah, I get what you mean.

> I'm talking about something like
> 	asm volatile("" : : "r" (ptr) : "memory");
> which tells the compiler that the asm may change memory arbitrarily.

Yeah, I will adjust it.

> Here, we don't use it really as a barrier, but to tell the compiler that
> the asm may have stashed the value of ptr somewhere in memory, so it's
> not free to reuse the space that it pointed to until the function
> returns (unless it can prove that nothing accesses memory, not just that
> nothing accesses ptr).

-- 
Kees Cook