[PATCH net v6 1/2] net, bpf: fix null-ptr-deref in xdp_master_redirect() for down master

[Jiayuan Chen <jiayuan.chen@linux.dev>]

syzkaller reported a kernel panic in bond_rr_gen_slave_id() reached via
xdp_master_redirect(). Full decoded trace:

  https://syzkaller.appspot.com/bug?extid=80e046b8da2820b6ba73

bond_rr_gen_slave_id() dereferences bond->rr_tx_counter, a per-CPU
counter that bonding only allocates in bond_open() when the mode is
round-robin. If the bond device was never brought up, rr_tx_counter
stays NULL.

The XDP redirect path can still reach that code on a bond that was
never opened: bpf_master_redirect_enabled_key is a global static key,
so as soon as any bond device has native XDP attached, the
XDP_TX -> xdp_master_redirect() interception is enabled for every
slave system-wide. The path xdp_master_redirect() ->
bond_xdp_get_xmit_slave() -> bond_xdp_xmit_roundrobin_slave_get() ->
bond_rr_gen_slave_id() then runs against a bond that has no
rr_tx_counter and crashes.

Fix this in the generic xdp_master_redirect() by refusing to call into
the master's ->ndo_xdp_get_xmit_slave() when the master device is not
up. IFF_UP is only set after ->ndo_open() has successfully returned,
so this reliably excludes masters whose XDP state has not been fully
initialized. Drop the frame with XDP_ABORTED so the exception is
visible via trace_xdp_exception() rather than silently falling through.
This is not specific to bonding: any current or future master that
defers XDP state allocation to ->ndo_open() is protected.

Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to slave device")
Reported-by: syzbot+80e046b8da2820b6ba73@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/698f84c6.a70a0220.2c38d7.00cc.GAE@google.com/T/
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/core/filter.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index cf2113af4bc9..9ec70c4b7723 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4398,6 +4398,8 @@ 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)))
+		return XDP_ABORTED;
 	slave = master->netdev_ops->ndo_xdp_get_xmit_slave(master, xdp);
 	if (slave && slave != xdp->rxq->dev) {
 		/* The target device is different from the receiving device, so
-- 
2.43.0