Subject: Re: [PATCH net v4 1/2] bonding: fix null-ptr-deref in bond_rr_gen_slave_id()

From: Nikolay Aleksandrov <[email-masked]>
Date: Fri, 6 Mar 2026 14:22:19 +0200

On Fri, Mar 06, 2026 at 02:42:05AM +0000, Jiayuan Chen wrote:
> 2026/3/6 05:03, "Jay Vosburgh" <[email-masked] mailto:[email-masked]?to=%22Jay%20Vosburgh%22%20%3Cjv%40jvosburgh.net%3E > wrote:
> 
> 
> > 
> > Nikolay Aleksandrov <[email-masked]> wrote:
> > 
> > > 
> > > On Wed, Mar 04, 2026 at 09:27:28AM -0800, Jay Vosburgh wrote:
> > > 
> > > > 
> > > > Nikolay Aleksandrov <[email-masked]> wrote:
> > > >  
> > > >  >On Wed, Mar 04, 2026 at 03:42:57PM +0800, Jiayuan Chen wrote:
> > > >  >> From: Jiayuan Chen <[email-masked]>
> > > >  >> 
> > > >  >> bond_rr_gen_slave_id() dereferences bond->rr_tx_counter without a NULL
> > > >  >> check. rr_tx_counter is a per-CPU counter only allocated in bond_open()
> > > >  >> when the bond mode is round-robin. If the bond device was never brought
> > > >  >> up, rr_tx_counter remains NULL, causing a null-ptr-deref.
> > > >  >> 
> > > >  >> The XDP redirect path can reach this code even when the bond is not up:
> > > >  >> bpf_master_redirect_enabled_key is a global static key, so when any bond
> > > >  >> device has native XDP attached, the XDP_TX -> xdp_master_redirect()
> > > >  >> interception is enabled for all bond slaves system-wide. This allows the
> > > >  >> path xdp_master_redirect() -> bond_xdp_get_xmit_slave() ->
> > > >  >> bond_xdp_xmit_roundrobin_slave_get() -> bond_rr_gen_slave_id() to be
> > > >  >> reached on a bond that was never opened.
> > > >  >> 
> > > >  >> Fix this by allocating rr_tx_counter unconditionally in bond_init()
> > > >  >> (ndo_init), which is called by register_netdevice() and covers both
> > > >  >> device creation paths (bond_create() and bond_newlink()). This also
> > > >  >> handles the case where bond mode is changed to round-robin after device
> > > >  >> creation. The conditional allocation in bond_open() is removed. Since
> > > >  >> bond_destructor() already unconditionally calls
> > > >  >> free_percpu(bond->rr_tx_counter), the lifecycle is clean: allocate at
> > > >  >> ndo_init, free at destructor.
> > > >  >> 
> > > >  >> Note: rr_tx_counter is only used by round-robin mode, so this
> > > >  >> deliberately allocates a per-cpu u32 that goes unused for other modes.
> > > >  >> Conditional allocation (e.g., in bond_option_mode_set) was considered
> > > >  >> but rejected: the XDP path can race with mode changes on a downed bond,
> > > >  >> and adding memory barriers to the XDP hot path is not justified for
> > > >  >> saving 4 bytes per CPU.
> > > >  >> 
> > > >  >> Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to slave device")
> > > >  >> Reported-by: [email-masked]
> > > >  >> Closes: https://lore.kernel.org/all/[email-masked]/T/
> > > >  >> Signed-off-by: Jiayuan Chen <[email-masked]>
> > > >  >> ---
> > > >  >> drivers/net/bonding/bond_main.c | 19 +++++++++++++------
> > > >  >> 1 file changed, 13 insertions(+), 6 deletions(-)
> > > >  >> 
> > > >  >
> > > >  >IMO it's not worth it to waste memory in all modes, for an unpopular mode.
> > > >  >I think it'd be better to add a null check in bond_rr_gen_slave_id(),
> > > >  >READ/WRITE_ONCE() should be enough since it is allocated only once, and
> > > >  >freed when the xmit code cannot be reachable anymore (otherwise we'd have
> > > >  >more bugs now). The branch will be successfully predicted practically always,
> > > >  >and you can also mark the ptr being null as unlikely. That way only RR takes
> > > >  >a very minimal hit, if any.
> > > >  
> > > >  Is what you're suggesting different from Jiayuan's proposal[0],
> > > >  in the sense of needing barriers in the XDP hot path to insure ordering?
> > > >  
> > > >  If I understand correctly, your suggestion is something like
> > > >  (totally untested):
> > > > 
> > > Basically yes, that is what I'm proposing + an unlikely() around that
> > > null check since it is really unlikely and will be always predicted
> > > correctly, this way it's only for RR mode.
> > > 
> >  Jiayuan,
> > 
> >  Do you agree that the patch below (including Nikolay's
> > suggestion to add "unlikely") resolves the original issue without memory
> > waste, and without introducing performance issues (barriers) into the
> > XDP path?
> 
> 
> Sure, it's basically similar to what my v1 did, but the patch below can be more generic.
> 
> https://lore.kernel.org/netdev/[email-masked]/T/#m08e3e53a8aa8d837ddc9242f4b14f2651a2b00aa
> 

IMO that is worse, you'll add 2 new tests and potentially 1 more cache line
for everyone in a hot path that is used a lot and must be kept as fast as possible.

> >  -J
> > 
> > > 
> > > > 
> > > > diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> > > >  index eb27cacc26d7..ac2a4fc0aad0 100644
> > > >  --- a/drivers/net/bonding/bond_main.c
> > > >  +++ b/drivers/net/bonding/bond_main.c
> > > >  @@ -4273,13 +4273,17 @@ void bond_work_cancel_all(struct bonding *bond)
> > > >  static int bond_open(struct net_device *bond_dev)
> > > >  {
> > > >  struct bonding *bond = netdev_priv(bond_dev);
> > > >  + u32 __percpu *rr_tx_tmp;
> > > >  struct list_head *iter;
> > > >  struct slave *slave;
> > > >  
> > > >  - if (BOND_MODE(bond) == BOND_MODE_ROUNDROBIN && !bond->rr_tx_counter) {
> > > >  - bond->rr_tx_counter = alloc_percpu(u32);
> > > >  - if (!bond->rr_tx_counter)
> > > >  + if (BOND_MODE(bond) == BOND_MODE_ROUNDROBIN &&
> > > >  + !READ_ONCE(bond->rr_tx_counter)) {
> > > >  + rr_tx_tmp = alloc_percpu(u32);
> > > >  + if (!rr_tx_tmp)
> > > >  return -ENOMEM;
> > > >  + WRITE_ONCE(bond->rr_tx_counter, rr_tx_tmp);
> > > >  +
> > > >  }
> > > >  
> > > >  /* reset slave->backup and slave->inactive */
> > > >  @@ -4866,6 +4870,9 @@ static u32 bond_rr_gen_slave_id(struct bonding *bond)
> > > >  struct reciprocal_value reciprocal_packets_per_slave;
> > > >  int packets_per_slave = bond->params.packets_per_slave;
> > > >  
> > > >  + if (!READ_ONCE(bond->rr_tx_counter))
> > > >  + packets_per_slave = 0;
> > > >  +
> > > >  switch (packets_per_slave) {
> > > >  case 0:
> > > >  slave_id = get_random_u32();
> > > >  
> > > >  -J
> > > >  
> > > >  
> > > >  [0] https://lore.kernel.org/netdev/[email-masked]/
> > > >
> > > 
> > ---
> >  -Jay Vosburgh, [email-masked]
> >