Re: [PATCH v7 3/6] stack: Optionally randomize kernel stack offset each syscall

[Thomas Gleixner <tglx@linutronix.de>]

Kees,

On Fri, Mar 19 2021 at 14:28, 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

VLAs are bad, VLAs to the rescue! :)

> + * 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);
> +
> +#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);              \

Not that it matters on x86, but as this has to be called in the
interrupt disabled region of the syscall entry, shouldn't this be a
raw_cpu_read(). The asm-generic version has a preempt_disable/enable
pair around the raw read for native wordsize reads, otherwise a
irqsave/restore pair.

__this_cpu_read() is fine as well, but that has an sanity check before
the raw read when CONFIG_DEBUG_PREEMPT is on, which is harmless but
also pointless in this case.

Probably the same for the counterpart this_cpu_write().

Thanks,

        tglx