ppp: fix refcount underflow on channel unbridge

When setting up a channel bridge, ppp_bridge_channels sets the
pch->bridge field before taking the associated reference on the bridge
file instance.

This opens up a refcount underflow bug if ppp_bridge_channels called
via. iotcl runs concurrently with ppp_unbridge_channels executing via.
file release.

The bug is triggered by ppp_bridge_channels taking the error path
through the 'err_unset' label. In this scenario, pch->bridge is set,
but the reference on the bridged channel will not be taken because
the function errors out. If ppp_unbridge_channels observes pch->bridge
before it is unset by the error path, it will erroneously drop the
reference on the bridged channel and cause a refcount underflow.

To avoid this, ensure that ppp_bridge_channels holds a reference on
each channel in advance of setting the bridge pointers.

Signed-off-by: Tom Parkin <[e-mail]>
Fixes: 4cf476ced45d ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls")
Acked-by: Guillaume Nault <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ppp/ppp_generic.c

···
 		write_unlock_bh(&pch->upl);
 		return -EALREADY;
 	}
+	refcount_inc(&pchb->file.refcnt);
 	rcu_assign_pointer(pch->bridge, pchb);
 	write_unlock_bh(&pch->upl);
 
···
 		write_unlock_bh(&pchb->upl);
 		goto err_unset;
 	}
+	refcount_inc(&pch->file.refcnt);
 	rcu_assign_pointer(pchb->bridge, pch);
 	write_unlock_bh(&pchb->upl);
-
-	refcount_inc(&pch->file.refcnt);
-	refcount_inc(&pchb->file.refcnt);
 
 	return 0;
 
 err_unset:
 	write_lock_bh(&pch->upl);
+	/* Re-read pch->bridge with upl held in case it was modified concurrently */
+	pchb = rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl));
 	RCU_INIT_POINTER(pch->bridge, NULL);
 	write_unlock_bh(&pch->upl);
 	synchronize_rcu();
+
+	if (pchb)
+		if (refcount_dec_and_test(&pchb->file.refcnt))
+			ppp_destroy_channel(pchb);
+
 	return -EALREADY;
 }