Re: [PATCH] veth: Fix race with AF_XDP exposing old or uninitialized descriptors

Magnus Karlsson
2023-01-11T16:27:26Z

On Wed, Jan 11, 2023 at 3:21 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Magnus Karlsson <magnus.karlsson@gmail.com> writes:
>
> > On Thu, Dec 22, 2022 at 11:18 AM Paolo Abeni <pabeni@redhat.com> wrote:
> >>
> >> On Tue, 2022-12-20 at 12:59 -0600, Shawn Bohrer wrote:
> >> > When AF_XDP is used on on a veth interface the RX ring is updated in two
> >> > steps. veth_xdp_rcv() removes packet descriptors from the FILL ring
> >> > fills them and places them in the RX ring updating the cached_prod
> >> > pointer. Later xdp_do_flush() syncs the RX ring prod pointer with the
> >> > cached_prod pointer allowing user-space to see the recently filled in
> >> > descriptors. The rings are intended to be SPSC, however the existing
> >> > order in veth_poll allows the xdp_do_flush() to run concurrently with
> >> > another CPU creating a race condition that allows user-space to see old
> >> > or uninitialized descriptors in the RX ring. This bug has been observed
> >> > in production systems.
> >> >
> >> > To summarize, we are expecting this ordering:
> >> >
> >> > CPU 0 __xsk_rcv_zc()
> >> > CPU 0 __xsk_map_flush()
> >> > CPU 2 __xsk_rcv_zc()
> >> > CPU 2 __xsk_map_flush()
> >> >
> >> > But we are seeing this order:
> >> >
> >> > CPU 0 __xsk_rcv_zc()
> >> > CPU 2 __xsk_rcv_zc()
> >> > CPU 0 __xsk_map_flush()
> >> > CPU 2 __xsk_map_flush()
> >> >
> >> > This occurs because we rely on NAPI to ensure that only one napi_poll
> >> > handler is running at a time for the given veth receive queue.
> >> > napi_schedule_prep() will prevent multiple instances from getting
> >> > scheduled. However calling napi_complete_done() signals that this
> >> > napi_poll is complete and allows subsequent calls to
> >> > napi_schedule_prep() and __napi_schedule() to succeed in scheduling a
> >> > concurrent napi_poll before the xdp_do_flush() has been called. For the
> >> > veth driver a concurrent call to napi_schedule_prep() and
> >> > __napi_schedule() can occur on a different CPU because the veth xmit
> >> > path can additionally schedule a napi_poll creating the race.
> >>
> >> The above looks like a generic problem that other drivers could hit.
> >> Perhaps it could be worthy updating the xdp_do_flush() doc text to
> >> explicitly mention it must be called before napi_complete_done().
> >
> > Good observation. I took a quick peek at this and it seems there are
> > at least 5 more drivers that can call napi_complete_done() before
> > xdp_do_flush():
> >
> > drivers/net/ethernet/qlogic/qede/
> > drivers/net/ethernet/freescale/dpaa2
> > drivers/net/ethernet/freescale/dpaa
> > drivers/net/ethernet/microchip/lan966x
> > drivers/net/virtio_net.c
> >
> > The question is then if this race can occur on these five drivers.
> > Dpaa2 has AF_XDP zero-copy support implemented, so it can suffer from
> > this race as the Tx zero-copy path is basically just a napi_schedule()
> > and it can be called/invoked from multiple processes at the same time.
> > In regards to the others, I do not know.
> >
> > Would it be prudent to just switch the order of xdp_do_flush() and
> > napi_complete_done() in all these drivers, or would that be too
> > defensive?
>
> We rely on being inside a single NAPI instance trough to the
> xdp_do_flush() call for RCU protection of all in-kernel data structures
> as well[0]. Not sure if this leads to actual real-world bugs for the
> in-kernel path, but conceptually it's wrong at least. So yeah, I think
> we should definitely swap the order everywhere and document this!

OK, let me take a stab at it. For at least the first four, it will be
compilation tested only from my side since I do not own any of those
SoCs/cards. Will need the help of those maintainers for sure.

> -Toke
>
> [0] See https://lore.kernel.org/r/20210624160609.292325-1-toke@redhat.com
>