[PATCH v2 06/13] stackleak: rework stack high bound handling

[Alexander Popov]

On 27.04.2022 20:31, Mark Rutland wrote:
> Prior to returning to userpace, we reset current->lowest_stack to a
> reasonable high bound. Currently we do this by subtracting the arbitrary
> value `THREAD_SIZE/64` from the top of the stack, for reasons lost to
> history.
> 
> Looking at configurations today:
> 
> * On i386 where THREAD_SIZE is 8K, the bound will be 128 bytes. The
>    pt_regs at the top of the stack is 68 bytes (with 0 to 16 bytes of
>    padding above), and so this covers an additional portion of 44 to 60
>    bytes.
> 
> * On x86_64 where THREAD_SIZE is at least 16K (up to 32K with KASAN) the
>    bound will be at least 256 bytes (up to 512 with KASAN). The pt_regs
>    at the top of the stack is 168 bytes, and so this cover an additional
>    88 bytes of stack (up to 344 with KASAN).
> 
> * On arm64 where THREAD_SIZE is at least 16K (up to 64K with 64K pages
>    and VMAP_STACK), the bound will be at least 256 bytes (up to 1024 with
>    KASAN). The pt_regs at the top of the stack is 336 bytes, so this can
>    fall within the pt_regs, or can cover an additional 688 bytes of
>    stack.
> 
> Clearly the `THREAD_SIZE/64` value doesn't make much sense -- in the
> worst case, this will cause more than 600 bytes of stack to be erased
> for every syscall, even if actual stack usage were substantially
> smaller.
> 
> This patches makes this slightly less nonsensical by consistently
> resetting current->lowest_stack to the base of the task pt_regs. For
> clarity and for consistency with the handling of the low bound, the
> generation of the high bound is split into a helper with commentary
> explaining why.
> 
> Since the pt_regs at the top of the stack will be clobbered upon the
> next exception entry, we don't need to poison these at exception exit.
> By using task_pt_regs() as the high stack boundary instead of
> current_top_of_stack() we avoid some redundant poisoning, and the
> compiler can share the address generation between the poisoning and
> restting of `current->lowest_stack`, making the generated code more
> optimal.
> 
> It's not clear to me whether the existing `THREAD_SIZE/64` offset was a
> dodgy heuristic to skip the pt_regs, or whether it was attempting to
> minimize the number of times stackleak_check_stack() would have to
> update `current->lowest_stack` when stack usage was shallow at the cost
> of unconditionally poisoning a small portion of the stack for every exit
> to userspace.

I inherited this 'THREAD_SIZE/64' logic is from the original grsecurity patch.
As I mentioned, originally this was written in asm.

For x86_64:
	mov	TASK_thread_sp0(%r11), %rdi
	sub	$256, %rdi
	mov	%rdi, TASK_lowest_stack(%r11)

For x86_32:
	mov TASK_thread_sp0(%ebp), %edi
	sub $128, %edi
	mov %edi, TASK_lowest_stack(%ebp)

256 bytes for x86_64 and 128 bytes for x86_32 are exactly THREAD_SIZE/64.

I think this value was chosen as optimal for minimizing poison scanning.
It's possible that stackleak_track_stack() is not called during the syscall 
because all the called functions have small stack frames.

> For now I've simply removed the offset, and if we need/want to minimize
> updates for shallow stack usage it should be easy to add a better
> heuristic atop, with appropriate commentary so we know what's going on.

I like your idea to erase the thread stack up to pt_regs if we call the 
stackleak erasing from the trampoline stack.

But here I don't understand where task_pt_regs() points to...

> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Alexander Popov <alex.popov at linux.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Andy Lutomirski <luto at kernel.org>
> Cc: Kees Cook <keescook at chromium.org>
> ---
>   include/linux/stackleak.h | 14 ++++++++++++++
>   kernel/stackleak.c        | 19 ++++++++++++++-----
>   2 files changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h
> index 67430faa5c518..467661aeb4136 100644
> --- a/include/linux/stackleak.h
> +++ b/include/linux/stackleak.h
> @@ -28,6 +28,20 @@ stackleak_task_low_bound(const struct task_struct *tsk)
>   	return (unsigned long)end_of_stack(tsk) + sizeof(unsigned long);
>   }
>   
> +/*
> + * The address immediately after the highest address on tsk's stack which we
> + * can plausibly erase.
> + */
> +static __always_inline unsigned long
> +stackleak_task_high_bound(const struct task_struct *tsk)
> +{
> +	/*
> +	 * The task's pt_regs lives at the top of the task stack and will be
> +	 * overwritten by exception entry, so there's no need to erase them.
> +	 */
> +	return (unsigned long)task_pt_regs(tsk);
> +}
> +
>   static inline void stackleak_task_init(struct task_struct *t)
>   {
>   	t->lowest_stack = stackleak_task_low_bound(t);
> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> index d5f684dc0a2d9..ba346d46218f5 100644
> --- a/kernel/stackleak.c
> +++ b/kernel/stackleak.c
> @@ -73,6 +73,7 @@ late_initcall(stackleak_sysctls_init);
>   static __always_inline void __stackleak_erase(void)
>   {
>   	const unsigned long task_stack_low = stackleak_task_low_bound(current);
> +	const unsigned long task_stack_high = stackleak_task_high_bound(current);
>   	unsigned long erase_low = current->lowest_stack;
>   	unsigned long erase_high;
>   	unsigned int poison_count = 0;
> @@ -93,14 +94,22 @@ static __always_inline void __stackleak_erase(void)
>   #endif
>   
>   	/*
> -	 * 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.
> +	 * Write poison to the task's stack between 'erase_low' and
> +	 * 'erase_high'.
> +	 *
> +	 * If we're running on a different stack (e.g. an entry trampoline
> +	 * stack) we can erase everything below the pt_regs at the top of the
> +	 * task stack.
> +	 *
> +	 * If we're running on the task stack itself, we must not clobber any
> +	 * stack used by this function and its caller. We assume that this
> +	 * function has a fixed-size stack frame, and the current stack pointer
> +	 * doesn't change while we write poison.
>   	 */
>   	if (on_thread_stack())
>   		erase_high = current_stack_pointer;
>   	else
> -		erase_high = current_top_of_stack();
> +		erase_high = task_stack_high;
>   
>   	while (erase_low < erase_high) {
>   		*(unsigned long *)erase_low = STACKLEAK_POISON;
> @@ -108,7 +117,7 @@ static __always_inline void __stackleak_erase(void)
>   	}
>   
>   	/* Reset the 'lowest_stack' value for the next syscall */
> -	current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
> +	current->lowest_stack = task_stack_high;
>   }
>   
>   asmlinkage void noinstr stackleak_erase(void)