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

[Will Deacon]

On Tue, Mar 30, 2021 at 01:57:47PM -0700, Kees Cook wrote:
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> new file mode 100644
> index 000000000000..351520803006
> --- /dev/null
> +++ b/include/linux/randomize_kstack.h
> @@ -0,0 +1,55 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#ifndef _LINUX_RANDOMIZE_KSTACK_H
> +#define _LINUX_RANDOMIZE_KSTACK_H
> +
> +#include <linux/kernel.h>
> +#include <linux/jump_label.h>
> +#include <linux/percpu-defs.h>
> +
> +DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
> +			 randomize_kstack_offset);
> +DECLARE_PER_CPU(u32, kstack_offset);
> +
> +/*
> + * 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.
> + */
> +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");		\

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?

Will