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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Kees Cook' <keescook@chromium.org>,
        Arvind Sankar <nivedita@alum.mit.edu>
CC:     Thomas Gleixner <tglx@linutronix.de>,
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        "Catalin Marinas" <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        "Mark Rutland" <mark.rutland@arm.com>,
        Alexander Potapenko <glider@google.com>,
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Subject: RE: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
 each syscall
Thread-Topic: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
 each syscall
Thread-Index: AQHWSPkh/p0bc5D3iUelrjbiR4wWCajmNFDA
Date:   Tue, 23 Jun 2020 13:42:30 +0000
Message-ID: <917458f43d194385a760d75292d4eb47@AcuMS.aculab.com>
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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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