Re: UBSAN: object-size-mismatch in wg_xmit

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Thu, Jan 7, 2021 at 1:22 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, Dec 21, 2020 at 12:23 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > Hi Dmitry,
> >
> > On Mon, Dec 21, 2020 at 10:14 AM Dmitry Vyukov <dvyukov@google.com> wrote:
> > > Hi Jason,
> > >
> > > Thanks for looking into this.
> > >
> > > Reading clang docs for ubsan:
> > >
> > > https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html
> > > -fsanitize=object-size: An attempt to potentially use bytes which the
> > > optimizer can determine are not part of the object being accessed.
> > > This will also detect some types of undefined behavior that may not
> > > directly access memory, but are provably incorrect given the size of
> > > the objects involved, such as invalid downcasts and calling methods on
> > > invalid pointers. These checks are made in terms of
> > > __builtin_object_size, and consequently may be able to detect more
> > > problems at higher optimization levels.
> > >
> > > From skimming though your description this seems to fall into
> > > "provably incorrect given the size of the objects involved".
> > > I guess it's one of these cases which trigger undefined behavior and
> > > compiler can e.g. remove all of this code assuming it will be never
> > > called at runtime and any branches leading to it will always branch in
> > > other directions, or something.
> >
> > Right that sort of makes sense, and I can imagine that in more general
> > cases the struct casting could lead to UB. But what has me scratching
> > my head is that syzbot couldn't reproduce. The cast happens every
> > time. What about that one time was special? Did the address happen to
> > fall on the border of a mapping? Is UBSAN non-deterministic as an
> > optimization? Or is there actually some mysterious UaF happening with
> > my usage of skbs that I shouldn't overlook?
>
> These UBSAN checks were just enabled recently.
> It's indeed super easy to trigger: 133083 VMs were crashed on this already:
> https://syzkaller.appspot.com/bug?extid=8f90d005ab2d22342b6d
> So it's one of the top crashers by now.

Ahh, makes sense. So it is easily reproducible after all.

You're still of the opinion that it's a false positive, right? I
shouldn't spend more cycles on this?


Jason