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

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

[Xiang Mei]

xdp_master_redirect() dereferences the result of
netdev_master_upper_dev_get_rcu() without a NULL check, but that helper
returns NULL when the receiving device has no upper-master adjacency.

The reach guard only checks netif_is_bond_slave(). On bond slave release
bond_upper_dev_unlink() drops the upper-master adjacency before clearing
IFF_SLAVE, so an XDP_TX reaching xdp_master_redirect() in that window
still passes netif_is_bond_slave() while master is already NULL, and
faults on master->flags at offset 0xb0:

  BUG: kernel NULL pointer dereference, address: 00000000000000b0
  RIP: 0010:xdp_master_redirect (net/core/filter.c:4432)
  Call Trace:
   xdp_master_redirect (net/core/filter.c:4432)
   bpf_prog_run_generic_xdp (include/net/xdp.h:700)
   do_xdp_generic (net/core/dev.c:5608)
   __netif_receive_skb_one_core (net/core/dev.c:6204)
   process_backlog (net/core/dev.c:6319)
   __napi_poll (net/core/dev.c:7729)
   net_rx_action (net/core/dev.c:7792)
   handle_softirqs (kernel/softirq.c:622)
   __dev_queue_xmit (include/linux/bottom_half.h:33)
   packet_sendmsg (net/packet/af_packet.c:3082)
   __sys_sendto (net/socket.c:2252)
  Kernel panic - not syncing: Fatal exception in interrupt

The missing check dates back to the original code; commit 1921f91298d1
("net, bpf: fix null-ptr-deref in xdp_master_redirect() for down master")
later added the master->flags read where the fault now lands but kept the
unconditional deref. Check master for NULL before use; a NULL master is
treated the same as one that is not up.

Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to slave device")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
 net/core/filter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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;
 	slave = master->netdev_ops->ndo_xdp_get_xmit_slave(master, xdp);
 	if (slave && slave != xdp->rxq->dev) {
-- 
2.43.0

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

[Jiayuan Chen]

On 6/21/26 4:15 AM, Xiang Mei wrote:
> xdp_master_redirect() dereferences the result of
> netdev_master_upper_dev_get_rcu() without a NULL check, but that helper
> returns NULL when the receiving device has no upper-master adjacency.
>
> The reach guard only checks netif_is_bond_slave(). On bond slave release
> bond_upper_dev_unlink() drops the upper-master adjacency before clearing
> IFF_SLAVE, so an XDP_TX reaching xdp_master_redirect() in that window
> still passes netif_is_bond_slave() while master is already NULL, and
> faults on master->flags at offset 0xb0:
>
>    BUG: kernel NULL pointer dereference, address: 00000000000000b0
>    RIP: 0010:xdp_master_redirect (net/core/filter.c:4432)
>    Call Trace:
>     xdp_master_redirect (net/core/filter.c:4432)
>     bpf_prog_run_generic_xdp (include/net/xdp.h:700)
>     do_xdp_generic (net/core/dev.c:5608)
>     __netif_receive_skb_one_core (net/core/dev.c:6204)
>     process_backlog (net/core/dev.c:6319)
>     __napi_poll (net/core/dev.c:7729)
>     net_rx_action (net/core/dev.c:7792)
>     handle_softirqs (kernel/softirq.c:622)
>     __dev_queue_xmit (include/linux/bottom_half.h:33)
>     packet_sendmsg (net/packet/af_packet.c:3082)
>     __sys_sendto (net/socket.c:2252)
>    Kernel panic - not syncing: Fatal exception in interrupt
>
> The missing check dates back to the original code; commit 1921f91298d1
> ("net, bpf: fix null-ptr-deref in xdp_master_redirect() for down master")
> later added the master->flags read where the fault now lands but kept the
> unconditional deref. Check master for NULL before use; a NULL master is
> treated the same as one that is not up.
>
> Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to slave device")
> Reported-by: Weiming Shi <bestswngs@gmail.com>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Xiang Mei <xmei5@asu.edu>
> ---
>   net/core/filter.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> 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.

Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>

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

[Xiang Mei]

On Sun, Jun 21, 2026 at 6:21 PM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>
>
> On 6/21/26 4:15 AM, Xiang Mei wrote:
> > xdp_master_redirect() dereferences the result of
> > netdev_master_upper_dev_get_rcu() without a NULL check, but that helper
> > returns NULL when the receiving device has no upper-master adjacency.
> >
> > The reach guard only checks netif_is_bond_slave(). On bond slave release
> > bond_upper_dev_unlink() drops the upper-master adjacency before clearing
> > IFF_SLAVE, so an XDP_TX reaching xdp_master_redirect() in that window
> > still passes netif_is_bond_slave() while master is already NULL, and
> > faults on master->flags at offset 0xb0:
> >
> >    BUG: kernel NULL pointer dereference, address: 00000000000000b0
> >    RIP: 0010:xdp_master_redirect (net/core/filter.c:4432)
> >    Call Trace:
> >     xdp_master_redirect (net/core/filter.c:4432)
> >     bpf_prog_run_generic_xdp (include/net/xdp.h:700)
> >     do_xdp_generic (net/core/dev.c:5608)
> >     __netif_receive_skb_one_core (net/core/dev.c:6204)
> >     process_backlog (net/core/dev.c:6319)
> >     __napi_poll (net/core/dev.c:7729)
> >     net_rx_action (net/core/dev.c:7792)
> >     handle_softirqs (kernel/softirq.c:622)
> >     __dev_queue_xmit (include/linux/bottom_half.h:33)
> >     packet_sendmsg (net/packet/af_packet.c:3082)
> >     __sys_sendto (net/socket.c:2252)
> >    Kernel panic - not syncing: Fatal exception in interrupt
> >
> > The missing check dates back to the original code; commit 1921f91298d1
> > ("net, bpf: fix null-ptr-deref in xdp_master_redirect() for down master")
> > later added the master->flags read where the fault now lands but kept the
> > unconditional deref. Check master for NULL before use; a NULL master is
> > treated the same as one that is not up.
> >
> > Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to slave device")
> > Reported-by: Weiming Shi <bestswngs@gmail.com>
> > Assisted-by: Claude:claude-opus-4-8
> > Signed-off-by: Xiang Mei <xmei5@asu.edu>
> > ---
> >   net/core/filter.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > 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').

(Theoretically) Race condition exploitation techniques such as expRace
[1] or CardShark [2] may help trigger it.

Xiang

[1]: https://www.usenix.org/conference/usenixsecurity21/presentation/lee-yoochan
[2]: https://www.usenix.org/conference/usenixsecurity24/presentation/han-tianshuo

> Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
>