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Date:   Thu, 1 Apr 2021 11:15:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Will Deacon' <will@...nel.org>, Kees Cook <keescook@...omium.org>
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>,
        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>, Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "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 v8 3/6] stack: Optionally randomize kernel stack offset
 each syscall

From: Will Deacon
> Sent: 01 April 2021 09:31
...
> > +/*
> > + * These macros must be used during syscall entry when interrupts and
> > + * preempt are disabled, and after user registers have been stored to
> > + * the stack.
> > + */
> > +#define add_random_kstack_offset() do {					\
> > +	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
> > +				&randomize_kstack_offset)) {		\
> > +		u32 offset = __this_cpu_read(kstack_offset);		\
> > +		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
> > +		asm volatile("" : "=m"(*ptr) :: "memory");		\
> 
> Using the "m" constraint here is dangerous if you don't actually evaluate it
> inside the asm. For example, if the compiler decides to generate an
> addressing mode relative to the stack but with writeback (autodecrement), then
> the stack pointer will be off by 8 bytes. Can you use "o" instead?

Is it allowed to use such a mode?
It would have to know that the "m" was substituted exactly once.
I think there are quite a few examples with 'strange' uses of memory
asm arguments.

However, in this case, isn't it enough to ensure the address is 'saved'?
So:
	asm volatile("" : "=r"(ptr) );
should be enough.

	David

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