On Fri, 2014-05-02 at 23:28 +0200, Bjørn Mork wrote:
> Fixes this warning introduced by commit 5b8f15f78e6f
> ("net: cdc_mbim: handle IPv6 Neigbor Solicitations"):
>
> ===============================
> [ INFO: suspicious RCU usage. ]
> 3.15.0-rc3 #213 Tainted: G W O
> -------------------------------
> net/8021q/vlan_core.c:69 suspicious rcu_dereference_check() usage!
>
> other info that might help us debug this:
>
> rcu_scheduler_active = 1, debug_locks = 1
> no locks held by ksoftirqd/0/3.
>
> stack backtrace:
> CPU: 0 PID: 3 Comm: ksoftirqd/0 Tainted: G W O 3.15.0-rc3 #213
> Hardware name: LENOVO 2776LEG/2776LEG, BIOS 6EET55WW (3.15 ) 12/19/2011
> 0000000000000001 ffff880232533bf0 ffffffff813a5ee6 0000000000000006
> ffff880232530090 ffff880232533c20 ffffffff81076b94 0000000000000081
> 0000000000000000 ffff8802085ac000 ffff88007fc8ea00 ffff880232533c50
> Call Trace:
> [<ffffffff813a5ee6>] dump_stack+0x4e/0x68
> [<ffffffff81076b94>] lockdep_rcu_suspicious+0xfa/0x103
> [<ffffffff813978a6>] __vlan_find_dev_deep+0x54/0x94
> [<ffffffffa04a1938>] cdc_mbim_rx_fixup+0x379/0x66a [cdc_mbim]
> [<ffffffff813ab76f>] ? _raw_spin_unlock_irqrestore+0x3a/0x49
> [<ffffffff81079671>] ? trace_hardirqs_on_caller+0x192/0x1a1
> [<ffffffffa059bd10>] usbnet_bh+0x59/0x287 [usbnet]
> [<ffffffff8104067d>] tasklet_action+0xbb/0xcd
> [<ffffffff81040057>] __do_softirq+0x14c/0x30d
> [<ffffffff81040237>] run_ksoftirqd+0x1f/0x50
> [<ffffffff8105f13e>] smpboot_thread_fn+0x172/0x18e
> [<ffffffff8105efcc>] ? SyS_setgroups+0xdf/0xdf
> [<ffffffff810594b0>] kthread+0xb5/0xbd
> [<ffffffff813a84b1>] ? __wait_for_common+0x13b/0x170
> [<ffffffff810593fb>] ? __kthread_parkme+0x5c/0x5c
> [<ffffffff813b147c>] ret_from_fork+0x7c/0xb0
> [<ffffffff810593fb>] ? __kthread_parkme+0x5c/0x5c
>
> Fixes: 5b8f15f78e6f ("net: cdc_mbim: handle IPv6 Neigbor Solicitations")
> Signed-off-by: Bjørn Mork <bjorn@xxxxxxx>
> ---
> Please add this to the stable v3.13 and v3.14 queues as well. Thanks.
>
>
> drivers/net/usb/cdc_mbim.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/usb/cdc_mbim.c b/drivers/net/usb/cdc_mbim.c
> index c9f3281506af..34b5c5cc27ed 100644
> --- a/drivers/net/usb/cdc_mbim.c
> +++ b/drivers/net/usb/cdc_mbim.c
> @@ -204,13 +204,16 @@ static void do_neigh_solicit(struct usbnet *dev, u8 *buf, u16 tci)
> return;
>
> /* need to send the NA on the VLAN dev, if any */
> - if (tci)
> + if (tci) {
> + rcu_read_lock();
> netdev = __vlan_find_dev_deep(dev->net, htons(ETH_P_8021Q),
> - tci);
> - else
> + tci & VLAN_VID_MASK);
> + rcu_read_unlock();
> + if (!netdev)
> + return;
> + } else {
> netdev = dev->net;
> - if (!netdev)
> - return;
> + }
>
> in6_dev = in6_dev_get(netdev);
> if (!in6_dev)
While this 'removes' the warning, this doesn't solve the fundamental
problem.
If you write :
rcu_read_lock();
netdev = __vlan_find_dev_deep(...)
rcu_read_unlock();
Then you cannot dereference netdev safely after the unlock.
In order to do so, you need to take a reference on netdev (aka
dev_hold()) before doing rcu_read_unlock();
And of course, release it later (aka dev_put()) when you are done with
netdev.
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