Re: [PATCH] kasan:fix access invalid shadow address when input is illegal

[Jann Horn <jannh@google.com>]

On Thu, Sep 14, 2023 at 10:35 PM Andrey Konovalov <andreyknvl@gmail.com> wrote:
> On Thu, Sep 14, 2023 at 8:29 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > > --- a/mm/kasan/kasan.h
> > > +++ b/mm/kasan/kasan.h
> > > @@ -304,8 +304,17 @@ static __always_inline bool addr_has_metadata(const void *addr)
> > >  #ifdef __HAVE_ARCH_SHADOW_MAP
> > >       return (kasan_mem_to_shadow((void *)addr) != NULL);
> > >  #else
> > > -     return (kasan_reset_tag(addr) >=
> > > -             kasan_shadow_to_mem((void *)KASAN_SHADOW_START));
> > > +     u8 *shadow, shadow_val;
> > > +
> > > +     if (kasan_reset_tag(addr) <
> > > +             kasan_shadow_to_mem((void *)KASAN_SHADOW_START))
> > > +             return false;
> > > +     /* use read with nofault to check whether the shadow is accessible */
> > > +     shadow = kasan_mem_to_shadow((void *)addr);
> > > +     __get_kernel_nofault(&shadow_val, shadow, u8, fault);
> > > +     return true;
> > > +fault:
> > > +     return false;
> > >  #endif
> > >  }
> >
> > Are we able to identify a Fixes: target for this?
> > 9d7b7dd946924de43021f57a8bee122ff0744d93 ("kasan: split out
> > print_report from __kasan_report") altered the code but I expect the
> > bug was present before that commit.
> >
> > Seems this bug has been there for over a year.  Can you suggest why it
> > has been discovered after such a lengthy time?
>
> Accessing unmapped memory with KASAN always led to a crash when
> checking shadow memory. This was reported/discussed before. To improve
> crash reporting for this case, Jann added kasan_non_canonical_hook and
> Mark integrated it into arm64. But AFAIU, for some reason, it stopped
> working.
>
> Instead of this patch, we need to figure out why
> kasan_non_canonical_hook stopped working and fix it.
>
> This approach taken by this patch won't work for shadow checks added
> by compiler instrumentation. It only covers explicitly checked
> accesses, such as via memcpy, etc.

FWIW, AFAICS kasan_non_canonical_hook() currently only does anything
under CONFIG_KASAN_INLINE; I think the idea when I added that was that
it assumes that when KASAN checks an access in out-of-line
instrumentation or a slowpath, it will do the required checks to avoid
this kind of fault?