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

[Mark Rutland]

On Sun, May 08, 2022 at 09:17:01PM +0300, Alexander Popov wrote:
> 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.

If you feel strongly about it, I can restore the check, but I struggle to
believe that it's worthwhile. The `lowest_stack` value lives in the
task_struct, and if you have the power to corrupt that you have the power to do
much more interesting things.

If we do restore it, I'd like to add a big fat comment explaining the
rationale (i.e. that it only matter if someone could corrupt
`current->lowest_stack`, as otherwise that's guarnateed to be within bounds).

Thanks,
Mark.

> > 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>
> > ---
> >   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)
>