[net] bonding: fix slave stuck in BOND_LINK_FAIL state

The bonding miimon logic has a flaw, in that a failure of the
rtnl_trylock can cause a slave to become permanently stuck in
BOND_LINK_FAIL state.

	The sequence of events to cause this is as follows:

	1) bond_miimon_inspect finds that a slave's link is down, and so
calls bond_propose_link_state, setting slave->new_link_state to
BOND_LINK_FAIL, then sets slave->new_link to BOND_LINK_DOWN and returns
non-zero.

	2) In bond_mii_monitor, the rtnl_trylock fails, and the timer is
rescheduled.  No change is committed.

	3) bond_miimon_inspect is called again, but this time the slave
from step 1 has recovered.  slave->new_link is reset to NOCHANGE, and, as
slave->link was never changed, the switch enters the BOND_LINK_UP case,
and does nothing.  The pending BOND_LINK_FAIL state from step 1 remains
pending, as new_link_state is not reset.

	4) The state from step 3 persists until another slave changes link
state and causes bond_miimon_inspect to return non-zero.  At this point,
the BOND_LINK_FAIL state change on the slave from steps 1-3 is committed,
and the slave will remain stuck in BOND_LINK_FAIL state even though it
is actually link up.

	The remedy for this is to initialize new_link_state on each entry
to bond_miimon_inspect, as is already done with new_link.

Reported-by: Alex Sidorenko <alexandre.sidorenko@hpe.com>
Reviewed-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Fixes: fb9eb899a6dc ("bonding: handle link transition from FAIL to UP correctly")
---
 drivers/net/bonding/bond_main.c | 1 +
 1 file changed, 1 insertion(+)

On Tue, Nov 7, 2017 at 2:50 AM, Jay Vosburgh <jay.vosburgh@canonical.com> wrote:
>         The bonding miimon logic has a flaw, in that a failure of the
> rtnl_trylock can cause a slave to become permanently stuck in
> BOND_LINK_FAIL state.
>
>         The sequence of events to cause this is as follows:
>
>         1) bond_miimon_inspect finds that a slave's link is down, and so
> calls bond_propose_link_state, setting slave->new_link_state to
> BOND_LINK_FAIL, then sets slave->new_link to BOND_LINK_DOWN and returns
> non-zero.
>
>         2) In bond_mii_monitor, the rtnl_trylock fails, and the timer is
> rescheduled.  No change is committed.
>
>         3) bond_miimon_inspect is called again, but this time the slave
> from step 1 has recovered.  slave->new_link is reset to NOCHANGE, and, as
> slave->link was never changed, the switch enters the BOND_LINK_UP case,
> and does nothing.  The pending BOND_LINK_FAIL state from step 1 remains
> pending, as new_link_state is not reset.
>
>         4) The state from step 3 persists until another slave changes link
> state and causes bond_miimon_inspect to return non-zero.  At this point,
> the BOND_LINK_FAIL state change on the slave from steps 1-3 is committed,
> and the slave will remain stuck in BOND_LINK_FAIL state even though it
> is actually link up.
>
>         The remedy for this is to initialize new_link_state on each entry
> to bond_miimon_inspect, as is already done with new_link.
>
> Reported-by: Alex Sidorenko <alexandre.sidorenko@hpe.com>
> Reviewed-by: Jarod Wilson <jarod@redhat.com>
> Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
> Fixes: fb9eb899a6dc ("bonding: handle link transition from FAIL to UP correctly")
Acked-by: Mahesh Bandewar <maheshb@google.com>
> ---
>  drivers/net/bonding/bond_main.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index c99dc59d729b..167434e952da 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -2042,6 +2042,7 @@ static int bond_miimon_inspect(struct bonding *bond)
>
>         bond_for_each_slave_rcu(bond, slave, iter) {
>                 slave->new_link = BOND_LINK_NOCHANGE;
> +               slave->link_new_state = slave->link;
>
>                 link_state = bond_check_dev_link(bond, slave->dev, 0);
>
> --
> 2.14.1
>

From: Jay Vosburgh <jay.vosburgh@canonical.com>
Date: Tue, 07 Nov 2017 19:50:07 +0900

> 	The bonding miimon logic has a flaw, in that a failure of the
> rtnl_trylock can cause a slave to become permanently stuck in
> BOND_LINK_FAIL state.
> 
> 	The sequence of events to cause this is as follows:
> 
> 	1) bond_miimon_inspect finds that a slave's link is down, and so
> calls bond_propose_link_state, setting slave->new_link_state to
> BOND_LINK_FAIL, then sets slave->new_link to BOND_LINK_DOWN and returns
> non-zero.
> 
> 	2) In bond_mii_monitor, the rtnl_trylock fails, and the timer is
> rescheduled.  No change is committed.
> 
> 	3) bond_miimon_inspect is called again, but this time the slave
> from step 1 has recovered.  slave->new_link is reset to NOCHANGE, and, as
> slave->link was never changed, the switch enters the BOND_LINK_UP case,
> and does nothing.  The pending BOND_LINK_FAIL state from step 1 remains
> pending, as new_link_state is not reset.
> 
> 	4) The state from step 3 persists until another slave changes link
> state and causes bond_miimon_inspect to return non-zero.  At this point,
> the BOND_LINK_FAIL state change on the slave from steps 1-3 is committed,
> and the slave will remain stuck in BOND_LINK_FAIL state even though it
> is actually link up.
> 
> 	The remedy for this is to initialize new_link_state on each entry
> to bond_miimon_inspect, as is already done with new_link.
> 
> Reported-by: Alex Sidorenko <alexandre.sidorenko@hpe.com>
> Reviewed-by: Jarod Wilson <jarod@redhat.com>
> Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
> Fixes: fb9eb899a6dc ("bonding: handle link transition from FAIL to UP correctly")

Applied and queued up for -stable.

As discussed with some others here at netdev... rtnl_trylock() really needs
to be re-evaluated if not removed completely.