Re: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset each syscall

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, Mar 30 2021 at 13:57, 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
> + * The asm statement is designed to convince the compiler to keep the
> + * allocation around even after "ptr" goes out of scope.

Nit. That explanation of "ptr" might be better placed right at the
add_random...() macro.

> + */
> +void *__builtin_alloca(size_t size);
> +/*
> + * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
> + * "VLA" from being unbounded (see above). 10 bits leaves enough room for
> + * per-arch offset masks to reduce entropy (by removing higher bits, since
> + * high entropy may overly constrain usable stack space), and for
> + * compiler/arch-specific stack alignment to remove the lower bits.
> + */
> +#define KSTACK_OFFSET_MAX(x)	((x) & 0x3FF)
> +
> +/*
> + * 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");		\
> +	}								\
> +} while (0)

Other than that.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>