Subject: Re: [PATCH v2 3/5] stack: Optionally randomize kernel stack offset each syscall

From: Kees Cook <[email-masked]>
Date: Mon, 30 Mar 2020 11:18:40 -0700

On Mon, Mar 30, 2020 at 12:25:36PM +0100, Mark Rutland wrote:
> On Tue, Mar 24, 2020 at 01:32:29PM -0700, Kees Cook wrote:
> > +/*
> > + * Do not use this anywhere else in the kernel. This is used here because
> > + * it provides an arch-agnostic way to grow the stack with correct
> > + * alignment. Also, since this use is being explicitly masked to a max of
> > + * 10 bits, stack-clash style attacks are unlikely. For more details see
> > + * "VLAs" in Documentation/process/deprecated.rst
> > + */
> > +void *__builtin_alloca(size_t size);
> > +
> > +#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);		\
> > +		char *ptr = __builtin_alloca(offset & 0x3FF);		\
> > +		asm volatile("" : "=m"(*ptr));				\
> 
> Is this asm() a homebrew OPTIMIZER_HIDE_VAR(*ptr)? If the asm
> constraints generate metter code, could we add those as alternative
> constraints in OPTIMIZER_HIDE_VAR() ?

Hah, yes, it is. And this produces identical asm, so I've replaced it
with OPTIMIZER_HIDE_VAR() now. Now if I could figure out how to hide it
from stack protector. :(

-- 
Kees Cook

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