Re: [PATCH v2 03/13] stackleak: remove redundant check

[Alexander Popov <alex.popov@linux.com>]

On 27.04.2022 20:31, Mark Rutland wrote:
> In __stackleak_erase() we check that the `erase_low` value derived from
> `current->lowest_stack` is above the lowest legitimate stack pointer
> value, but this is already enforced by stackleak_track_stack() when
> recording the lowest stack value.
> 
> Remove the redundant check.
> 
> There should be no functional change as a result of this patch.

Mark, I can't agree here. I think this check is important.
The performance profit from dropping it is less than the confidence decrease :)

With this check, if the 'lowest_stack' value is corrupted, stackleak doesn't 
overwrite some wrong kernel memory, but simply clears the whole thread stack, 
which is safe behavior.

> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Popov <alex.popov@linux.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> ---
>   kernel/stackleak.c | 4 ----
>   1 file changed, 4 deletions(-)
> 
> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> index 753eab797a04d..f7a0f8cf73c37 100644
> --- a/kernel/stackleak.c
> +++ b/kernel/stackleak.c
> @@ -78,10 +78,6 @@ static __always_inline void __stackleak_erase(void)
>   	unsigned int poison_count = 0;
>   	const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
>   
> -	/* Check that 'lowest_stack' value is sane */
> -	if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
> -		kstack_ptr = boundary;
> -
>   	/* Search for the poison value in the kernel stack */
>   	while (kstack_ptr > boundary && poison_count <= depth) {
>   		if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)