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

[Thomas Gleixner]

On Wed, Mar 31 2021 at 14:54, Kees Cook wrote:
> On Wed, Mar 31, 2021 at 09:53:26AM +0200, Thomas Gleixner wrote:
>> 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.
>
> Ah, yes! Fixed in v9.

Hmm, looking at V9 the "ptr" thing got lost ....

> +/*
> + * 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);
> +/*
> + * 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 {					\

> Do you want to take this via -tip (and leave off the arm64 patch until
> it is acked), or would you rather it go via arm64? (I've sent v9 now...)

Either way is fine.

Thanks,

        tglx