linux-kernel - Re: [PATCH v2 05/13] stackleak: clarify variable names

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Date:   Sun, 8 May 2022 23:49:46 +0300
From:   Alexander Popov <alex.popov@...ux.com>
To:     Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     akpm@...ux-foundation.org, catalin.marinas@....com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        luto@...nel.org, will@...nel.org
Subject: Re: [PATCH v2 05/13] stackleak: clarify variable names

On 27.04.2022 20:31, Mark Rutland wrote:
> The logic within __stackleak_erase() can be a little hard to follow, as
> `boundary` switches from being the low bound to the high bound mid way
> through the function, and `kstack_ptr` is used to represent the start of
> the region to erase while `boundary` represents the end of the region to
> erase.
> 
> Make this a little clearer by consistently using clearer variable names.
> The `boundary` variable is removed, the bounds of the region to erase
> are described by `erase_low` and `erase_high`, and bounds of the task
> stack are described by `task_stack_low` and `task_stck_high`.

A typo here in `task_stck_high`.

> As the same time, remove the comment above the variables, since it is
> unclear whether it's intended as rationale, a complaint, or a TODO, and
> is more confusing than helpful.

Yes, this comment is a bit confusing :) I can elaborate.

In the original grsecurity patch, the stackleak erasing was written in asm.
When I adopted it and proposed for the upstream, Linus strongly opposed this.
So I developed stackleak erasing in C.

And I wrote this comment to remember that having 'kstack_ptr' and 'boundary' 
variables on the stack (which we are clearing) would not be good.

That was also the main reason why I reused the 'boundary' variable: I wanted the 
compiler to allocate it in the register and I avoided creating many local variables.

Mark, did your refactoring make the compiler allocate local variables on the 
stack instead of the registers?

> There should be no functional change as a result of this patch.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: Alexander Popov <alex.popov@...ux.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Kees Cook <keescook@...omium.org>
> ---
>   kernel/stackleak.c | 30 ++++++++++++++----------------
>   1 file changed, 14 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> index 24b7cf01b2972..d5f684dc0a2d9 100644
> --- a/kernel/stackleak.c
> +++ b/kernel/stackleak.c
> @@ -73,40 +73,38 @@ late_initcall(stackleak_sysctls_init);
>   static __always_inline void __stackleak_erase(void)
>   {
>   	const unsigned long task_stack_low = stackleak_task_low_bound(current);
> -
> -	/* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
> -	unsigned long kstack_ptr = current->lowest_stack;
> -	unsigned long boundary = task_stack_low;
> +	unsigned long erase_low = current->lowest_stack;
> +	unsigned long erase_high;
>   	unsigned int poison_count = 0;
>   	const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
>   
>   	/* Search for the poison value in the kernel stack */
> -	while (kstack_ptr > boundary && poison_count <= depth) {
> -		if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)
> +	while (erase_low > task_stack_low && poison_count <= depth) {
> +		if (*(unsigned long *)erase_low == STACKLEAK_POISON)
>   			poison_count++;
>   		else
>   			poison_count = 0;
>   
> -		kstack_ptr -= sizeof(unsigned long);
> +		erase_low -= sizeof(unsigned long);
>   	}
>   
>   #ifdef CONFIG_STACKLEAK_METRICS
> -	current->prev_lowest_stack = kstack_ptr;
> +	current->prev_lowest_stack = erase_low;
>   #endif
>   
>   	/*
> -	 * Now write the poison value to the kernel stack. Start from
> -	 * 'kstack_ptr' and move up till the new 'boundary'. We assume that
> -	 * the stack pointer doesn't change when we write poison.
> +	 * Now write the poison value to the kernel stack between 'erase_low'
> +	 * and 'erase_high'. We assume that the stack pointer doesn't change
> +	 * when we write poison.
>   	 */
>   	if (on_thread_stack())
> -		boundary = current_stack_pointer;
> +		erase_high = current_stack_pointer;
>   	else
> -		boundary = current_top_of_stack();
> +		erase_high = current_top_of_stack();
>   
> -	while (kstack_ptr < boundary) {
> -		*(unsigned long *)kstack_ptr = STACKLEAK_POISON;
> -		kstack_ptr += sizeof(unsigned long);
> +	while (erase_low < erase_high) {
> +		*(unsigned long *)erase_low = STACKLEAK_POISON;
> +		erase_low += sizeof(unsigned long);
>   	}
>   
>   	/* Reset the 'lowest_stack' value for the next syscall */