Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall

[Kees Cook]

On Mon, Jun 22, 2020 at 06:56:15PM -0400, Arvind Sankar wrote:
> On Mon, Jun 22, 2020 at 12:31:44PM -0700, Kees Cook wrote:
> > +
> > +#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(offset & 0x3FF);		\
> > +		asm volatile("" : "=m"(*ptr));				\
> > +	}								\
> > +} while (0)
> 
> This feels a little fragile. ptr doesn't escape the block, so the
> compiler is free to restore the stack immediately after this block. In
> fact, given that all you've said is that the asm modifies *ptr, but
> nothing uses that output, the compiler could eliminate the whole thing,
> no?
> 
> https://godbolt.org/z/HT43F5
> 
> gcc restores the stack immediately, if no function calls come after it.
> 
> clang completely eliminates the code if no function calls come after.

nothing uses the stack in your example. And adding a barrier (which is
what the "=m" is, doesn't change it.

> I'm not sure why function calls should affect it, though, given that
> those functions can't possibly access ptr or the memory it points to.

It seems to work correctly:
https://godbolt.org/z/c3W5BW

-- 
Kees Cook