Messages in this thread | | | Date | Tue, 10 May 2022 12:46:48 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2 03/13] stackleak: remove redundant check |
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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@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) >
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