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

From: "Jiayuan Chen" <[email-masked]>
Date: Fri, 06 Mar 2026 12:38:57 +0000

March 6, 2026 at 20:22, "Nikolay Aleksandrov" <[email-masked] mailto:[email-masked]?to=%22Nikolay%20Aleksandrov%22%20%3Crazor%40blackwall.org%3E > wrote:

> 
> 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.

Hi Nikolay,

Apologies for the confusion. When I said "the patch below," I was not referring to the v1 link,
but to the diff snippet appended at the bottom of my email.

That specific snippet (using a local temp pointer) is the optimal approach. It ensures the NULL
check for bond->rr_tx_counter is handled efficiently without introducing the performance
overhead or memory waste you were concerned about.

Best regards,
Jiayuan

> > 
> > -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]
> >
>