[PATCH net-next v1 1/2] netpoll: fix use after free

[David Decotigny]

After a bonding master reclaims the netpoll info struct, slaves could
still hold a pointer to the reclaimed data. This patch fixes it: as
soon as netpoll_async_cleanup is called for a slave (eg. when
un-enslaved), we make sure that this slave doesn't point to the data.

The style of this patch is not nice, but it represents the simplest
fix. The function is restructured by the next patch anyway ("netpoll:
avoid reference leaks").

Tested:
  One way to reproduce the panic is with netconsole: with the script
  below run in a loop without this patch. Same procedure with this
  patch can run without panic.
    CONFIG_NETCONSOLE=m
    CONFIG_NETCONSOLE_DYNAMIC=y
  Then run in a loop (dual port NIC makes it easier to crash):
    ifconfig eth0 192.168.42.4 up
    ifconfig eth1 192.168.56.4 up
    sleep 10
    modprobe -r netconsole
    modprobe -r bonding
    modprobe netconsole
    modprobe bonding mode=4
    echo +bond0 > /sys/class/net/bonding_masters
    ifconfig bond0 192.168.56.3 up
    mkdir /sys/kernel/config/netconsole/blah
    echo 0 > /sys/kernel/config/netconsole/blah/enabled
    echo bond0 > /sys/kernel/config/netconsole/blah/dev_name
    echo 192.168.56.42 > /sys/kernel/config/netconsole/blah/remote_ip
    echo 1 > /sys/kernel/config/netconsole/blah/enabled
    # npinfo refcnt ->1
    ifenslave bond0 eth1
    # npinfo refcnt ->2
    ifenslave bond0 eth0
    # (this should be optional, preventing ndo_cleanup_nepoll below)
    # npinfo refcnt ->3
    sleep 3
    ifenslave -d bond0 eth1
    # npinfo refcnt ->2, eth1 keeps ptr to npinfo reclaimed later => garbage
    sleep 1
    echo -bond0 > /sys/class/net/bonding_masters
    # netpoll_cleanup(bond0) + dec(refcnt)
    # (should be optional: becomes -> 1 (aka. != 0)
    #                      ==> do not call ndo_cleanup_nepoll)
    # try to increase chance of writing garbage onto npinfo:
    ls -lR / 2>&1 | tail -30
    echo +bond0 > /sys/class/net/bonding_masters
    ifconfig bond0 192.168.56.3 up
    ifenslave bond0 eth1
    # dev_open() called --> netpoll_rx_disable() + npinfo is garbage
    #    -> down(&ni->dev_lock);

Signed-off-by: David Decotigny <decot@xxxxxxxxxxxx>
---
 net/core/netpoll.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index e33937f..907fb5e 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -822,7 +822,8 @@ void __netpoll_cleanup(struct netpoll *np)
 
 		RCU_INIT_POINTER(np->dev->npinfo, NULL);
 		call_rcu_bh(&npinfo->rcu, rcu_cleanup_netpoll_info);
-	}
+	} else
+		RCU_INIT_POINTER(np->dev->npinfo, NULL);
 }
 EXPORT_SYMBOL_GPL(__netpoll_cleanup);
 
-- 
2.0.0.526.g5318336

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