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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Will Deacon' <will@kernel.org>, Kees Cook <keescook@chromium.org>
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Subject: RE: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset
 each syscall
Thread-Topic: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset
 each syscall
Thread-Index: AQHXJtFkunedSLxfaUq504kizLaNqaqfgZOw
Date:   Thu, 1 Apr 2021 11:15:43 +0000
Message-ID: <61ae9398a03d4fe7868b68c9026d5998@AcuMS.aculab.com>
References: <20210330205750.428816-1-keescook@chromium.org>
 <20210330205750.428816-4-keescook@chromium.org>
 <20210401083034.GA8554@willie-the-truck>
In-Reply-To: <20210401083034.GA8554@willie-the-truck>
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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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