Re: Crash on armv7-a using KASAN

[Mark Rutland]

On Tue, Oct 15, 2024 at 06:07:00PM +0200, Ard Biesheuvel wrote:
> On Tue, 15 Oct 2024 at 17:26, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > Looking some more, I don't see how VMAP_STACK guarantees that the
> > old/active stack is mapped in the new mm when switching from the old mm
> > to the new mm (which happens before __switch_to()).
> >
> > Either I'm missing something, or we have a latent bug. Maybe we have
> > some explicit copying/prefaulting elsewhere I'm missing?
> 
> We bump the vmalloc_seq counter for that. Given that the top-level
> page table can only gain entries covering the kernel space, this
> should be sufficient for the old task's stack to be mapped in the new
> task's page tables.

Ah, yep -- I had missed that. Thanks for the pointer!

>From a superficial look, it sounds like it should be possible to extend
that to also handle the KASAN shadow of the vmalloc area (which
__check_vmalloc_seq() currently doesn't copy), but I'm not sure of
exactly when we initialise the shadow for a vmalloc allocation relative
to updating vmalloc_seq.

Mark.