Re: [PATCH net v5 1/2] bonding: fix null-ptr-deref in bond_rr_gen_slave_id()

[Paolo Abeni <pabeni@redhat.com>]

On 3/10/26 1:39 PM, Nikolay Aleksandrov wrote:
> On Tue, Mar 10, 2026 at 01:07:15PM +0100, Eric Dumazet wrote:
>> On Tue, Mar 10, 2026 at 1:00 PM Eric Dumazet <edumazet@google.com> wrote:
>>>
>>> On Tue, Mar 10, 2026 at 12:49 PM Nikolay Aleksandrov
>>> <razor@blackwall.org> wrote:
>>>>
>>>> On Mon, Mar 09, 2026 at 11:06:58AM +0800, Jiayuan Chen wrote:
>>>>> From: Jiayuan Chen <jiayuan.chen@shopee.com>
>>>>>
>>>>> 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 adding a NULL check with unlikely() in bond_rr_gen_slave_id()
>>>>> before dereferencing rr_tx_counter. When rr_tx_counter is NULL (bond was
>>>>> never opened), fall back to get_random_u32() for slave selection. The
>>>>> allocation in bond_open() is kept, with WRITE_ONCE() added to safely
>>>>> publish the pointer to the XDP read side. A plain read suffices for the
>>>>> !bond->rr_tx_counter guard in bond_open() itself, as bond_open() runs
>>>>> under RTNL lock and is the only writer of rr_tx_counter.
>>>>>
>>>>> 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/
>>>>> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
>>>>> ---
>>>>>  drivers/net/bonding/bond_main.c | 9 +++++++--
>>>>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>>>>
>>>>
>>>> This is Jay's patch + the unlikely change, looks good to me.
>>>> Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
>>>
>>> Orthogonal to this patch  :
>>>
>>>  get_random_u32() typical cost is around 10 to 20 ns, I really wonder
>>> if this makes sense
>>> for the packets_per_slave == 0 or 1 case to haves this kind of
>>> randomness in the first place.
>>>
>>> Perhaps we could use a
>>>
>>> static DEFINE_PER_CPU(u32, rr_tx_counter)
>>>
>>> And :
>>>  slave_id = this_cpu_inc_return(rr_tx_counter);
>>
>> I also have mixed feelings about this patch.
>>
>> We probably should detect that the device is not ready before hitting
>> something deeper in the stack.
>>
>> Sure, a NULL deref is avoided, bu what happens next ?
>>
>> We send a packet while the device is not UP, I am pretty sure this
>> violates at least some RCU rules in device dismantling.
> 
> IIRC when the redirect continues, the packet should get dropped if the device is
> not up (checks at a few places), but that's outside of bond's jurisdiction and
> after the slave id is needed in xdp master redirect's path unfortunately.
> I'm not sure it can reach much further, it just has the master dev's slave id
> generation in its path.
> 
> In any case we shouldn't crash in the slave id generation in the bonding,
> that ndo's only job is to return a slave id.

I'm sorry for the back and forth, but I share Eric's concern. I think
the approach suggested by Daniel:

https://lore.kernel.org/netdev/4d15be93-b497-4499-996d-9f3a67a2abc6@iogearbox.net/

or the initial patch form:

https://lore.kernel.org/netdev/20260224112545.37888-1-jiayuan.chen@linux.dev/T/#m7c67bb12f85bc88d583788fb6e41113c46208ae7

would be better. To respond to old concerns raised there: the check is
IMHO bond-specific, as control moves from the lower interface to the
upper bonding device, and the code is under an RCU critical section, the
device can't go away before the xmit is completed.

/P