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Date: Tue, 23 Jun 2020 13:42:30 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Kees Cook' <keescook@...omium.org>, Arvind Sankar <nivedita@...m.mit.edu>
CC: Thomas Gleixner <tglx@...utronix.de>, Elena Reshetova
<elena.reshetova@...el.com>, "x86@...nel.org" <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" <kernel-hardening@...ts.openwall.com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>, "linux-mm@...ck.org"
<linux-mm@...ck.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
each syscall
From: Kees Cook
> Sent: 23 June 2020 01:56
> 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).
Do you need another asm volatile("" : : "r" (ptr) : "memory");
(or similar) at the bottom of the function - that the compiler thinks
might access the memory whose address it thought got saved earlier?
I wonder if it would be easier to allocate the stack space
in the asm wrapper? At least as an architecture option.
David
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