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

Magnus Karlsson
2023-01-11T14:02:47Z

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?

> (in a separate, net-next patch)
>
> Thanks!
>
> Paolo
>
> >
> > The fix as suggested by Magnus Karlsson, is to simply move the
> > xdp_do_flush() call before napi_complete_done(). This syncs the
> > producer ring pointers before another instance of napi_poll can be
> > scheduled on another CPU. It will also slightly improve performance by
> > moving the flush closer to when the descriptors were placed in the
> > RX ring.
> >
> > Fixes: d1396004dd86 ("veth: Add XDP TX and REDIRECT")
> > Suggested-by: Magnus Karlsson <magnus.karlsson@gmail.com>
> > Signed-off-by: Shawn Bohrer <sbohrer@cloudflare.com>
> > ---
> > drivers/net/veth.c | 5 +++--
> > 1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > index ac7c0653695f..dfc7d87fad59 100644
> > --- a/drivers/net/veth.c
> > +++ b/drivers/net/veth.c
> > @@ -974,6 +974,9 @@ static int veth_poll(struct napi_struct *napi, int budget)
> > xdp_set_return_frame_no_direct();
> > done = veth_xdp_rcv(rq, budget, &bq, &stats);
> >
> > + if (stats.xdp_redirect > 0)
> > + xdp_do_flush();
> > +
> > if (done < budget && napi_complete_done(napi, done)) {
> > /* Write rx_notify_masked before reading ptr_ring */
> > smp_store_mb(rq->rx_notify_masked, false);
> > @@ -987,8 +990,6 @@ static int veth_poll(struct napi_struct *napi, int budget)
> >
> > if (stats.xdp_tx > 0)
> > veth_xdp_flush(rq, &bq);
> > - if (stats.xdp_redirect > 0)
> > - xdp_do_flush();
> > xdp_clear_return_frame_no_direct();
> >
> > return done;
>