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Message-ID: <202006221748.DA27A7FFC@keescook>
Date: Mon, 22 Jun 2020 17:56:21 -0700
From: Kees Cook <keescook@...omium.org>
To: Arvind Sankar <nivedita@...m.mit.edu>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Elena Reshetova <elena.reshetova@...el.com>, x86@...nel.org,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
Alexander Potapenko <glider@...gle.com>,
Alexander Popov <alex.popov@...ux.com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Jann Horn <jannh@...gle.com>, kernel-hardening@...ts.openwall.com,
linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
each syscall
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
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