[PATCH net v2] atm: Fix the cleanup on alloc_mpc failure in atm_mpoa_mpoad_attach

[Deepak Sharma]

Syzbot reported a warning at `add_timer`, which is called from the
`atm_mpoa_mpoad_attach` function

The reason for warning is that in the first call to the ioctl, if
there is no MPOA client created yet (mpcs is the linked list for
these MPOA clients) we do a `mpc_timer_refresh` to arm the timer.
Later on, if the `alloc_mpc` fails (which on success will also
initialize mpcs if it's first MPOA client created) and we didn't
have any MPOA client yet, we return without the timer de-armed

If the same ioctl is called again, since we don't have any MPOA
clients yet we again arm the timer, which might already be left
armed by the previous call to this ioctl in which `alloc_mpc` failed

Hence, de-arm the timer in the event that `alloc_mpc` fails and we
don't have any other MPOA client (that is, `mpcs` is NULL)

Do a `timer_delete_sync` instead of `timer_delete`, since the timer
callback can arm it back again

This does not need to be done at the early return in case of
`mpc->mpoad_vcc`, or a control channel to MPOAD already exists.
The timer should remain there to periodically process caches

Reported-by: syzbot+07b635b9c111c566af8b@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=07b635b9c111c566af8b
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Deepak Sharma <deepak.sharma.472935@xxxxxxxxx>
---
v2:
 - Improved commit message
 - Fix the faulty condition check to disarm the timer
 - Use `timer_delete_sync` instead to avoid re-arming of timer

v1:
 - Disarm the timer using `timer_delete` in case `alloc_mpc`
   fails`

 net/atm/mpc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/atm/mpc.c b/net/atm/mpc.c
index f6b447bba329..4f67ad1d6bef 100644
--- a/net/atm/mpc.c
+++ b/net/atm/mpc.c
@@ -804,7 +804,7 @@ static int atm_mpoa_mpoad_attach(struct atm_vcc *vcc, int arg)
 		/* This lets us now how our LECs are doing */
 		err = register_netdevice_notifier(&mpoa_notifier);
 		if (err < 0) {
-			timer_delete(&mpc_timer);
+			timer_delete_sync(&mpc_timer);
 			return err;
 		}
 	}
@@ -813,8 +813,10 @@ static int atm_mpoa_mpoad_attach(struct atm_vcc *vcc, int arg)
 	if (mpc == NULL) {
 		dprintk("allocating new mpc for itf %d\n", arg);
 		mpc = alloc_mpc();
-		if (mpc == NULL)
+		if (!mpcs) {
+			timer_delete_sync(&mpc_timer);
 			return -ENOMEM;
+		}
 		mpc->dev_num = arg;
 		mpc->dev = find_lec_by_itfnum(arg);
 					/* NULL if there was no lec */
-- 
2.51.0