Re: Crash on armv7-a using KASAN

[Linus Walleij]

On Tue, Oct 15, 2024 at 4:45 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> On Tue, 15 Oct 2024 at 16:35, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >
> > On Tue, Oct 15, 2024 at 04:22:20PM +0200, Ard Biesheuvel wrote:
> > > On Tue, 15 Oct 2024 at 16:00, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > >
> > > > On Tue, Oct 15, 2024 at 03:51:02PM +0200, Linus Walleij wrote:
> > > > > On Tue, Oct 15, 2024 at 12:28 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > > > > On Mon, Oct 14, 2024 at 03:19:49PM +0200, Clement LE GOFFIC wrote:
> > > > >
> > > > > > I think what's happening here is that when switching from prev to next
> > > > > > in the scheduler, we switch to next's mm before we actually switch to
> > > > > > next's register state, and there's a transient window where prev is
> > > > > > executed using next's mm. AFAICT we don't map prev's KASAN stack shadow
> > > > > > into next's mm anywhere, and so inlined KASAN_STACK checks recursively
> > > > > > fault on this until we switch to the overflow stack.
> >
> > [...]
> >
> > > > > Yeah it looks like a spot-on identification of the problem, I can try to
> > > > > think about how we could fix this if I can reproduce it, I keep trying
> > > > > to provoke the crash :/
> > > >
> > > > It's a bit grotty -- AFAICT you'd either need to prefault in the
> > > > specific part of the vmalloc space when switching tasks, or we'd need to
> > > > preallocate all the shared vmalloc tables at the start of time so that
> > > > they're always up-to-date.
> > > >
> > > > While we could disable KASAN_STACK, that's only going to mask the
> > > > problem until this happens for any other vmalloc shadow...
> > >
> > > Is the other vmalloc shadow not covered by the ordinary on-demand faulting?
> >
> > It depends on what the vmalloc memory is used for; if it's anything else
> > used in the fault handling path, that'll fault recursively, and it's
> > possible that'll happen indirectly via other instrumentation.
> >
> > > When I implemented VMAP_STACK for ARM, I added an explicit load from
> > > the new stack while still running from the old one (in __switch_to) so
> > > that the ordinary faulting code can deal with it. Couldn't we do the
> > > same for the vmalloc shadow of the new stack?
> >
> > We could do something similar, but note that it's backwards: we need to
> > ensure that the old/current stack shadow will be mapped in the new mm.
> >
> > So the usual fault handling can't handle that as-is, because you need to
> > fault-in pages for an mm which isn't yet in use. That logic could be
> > factored out and shared, though.
> >
>
> Not sure I follow you here. The crash is in the kernel, no?
>
> So there is only a single vmalloc space where all the mappings should
> reside, but each process has its own copy of the top level page table,
> which needs to be synced up when it goes stale.

That's how it works AFAICT.

The vmalloc/kernel space is mapped using the very same
actual page tables in all per-process MM contexts, so I also
think it's just a matter of syncing the top-level page table (PGD),
so I will try to do that.

Yours,
Linus Walleij