Re: [PATCH net] net, bpf: check master for NULL in xdp_master_redirect()

[Jakub Kicinski <kuba@kernel.org>]

On Sun, 21 Jun 2026 18:28:09 -0700 Xiang Mei wrote:
> > > diff --git a/net/core/filter.c b/net/core/filter.c
> > > index 40037413dd4e..6037860d5283 100644
> > > --- a/net/core/filter.c
> > > +++ b/net/core/filter.c
> > > @@ -4430,7 +4430,7 @@ u32 xdp_master_redirect(struct xdp_buff *xdp)
> > >       struct net_device *master, *slave;
> > >
> > >       master = netdev_master_upper_dev_get_rcu(xdp->rxq->dev);
> > > -     if (unlikely(!(master->flags & IFF_UP)))
> > > +     if (unlikely(!master || !(master->flags & IFF_UP)))
> > >               return XDP_ABORTED;  
> >
> >
> > I recall that when I previously modified this code, I removed the
> > !master check
> >
> > because this is on the fastpath. However, since this is a triggerable bug,
> > I think adding it here is fine.
>
> Thanks for the review. It's difficult to hit under normal statue, but
> the bug is real.
> We have triggered this bug with a PoC plus GDB to pause one thread (no
> other `cheating').

Can you double-confirm that this triggers on current HEAD
of linux/master ? I thought commit 2674d603a9e6 ("vrf: Fix a potential
NPD when removing a port from a VRF") was supposed to prevent all the
torn master fetches. Adding VRF folks to CC.