[PATCH] ppp: fix refcount underflow on channel unbridge

[Tom Parkin]

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.
---
v2:
 * rework ppp_bridge_channels code to avoid the race condition in
   preference to holding ppp_mutex while calling ppp_unbridge_channels
---
 drivers/net/ppp/ppp_generic.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 09c27f7773f9..d019fa40b494 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -617,12 +617,15 @@ static struct bpf_prog *compat_ppp_get_filter(struct 
sock_fprog32 __user *p)
  */
 static int ppp_bridge_channels(struct channel *pch, struct channel *pchb)
 {
+       bool drop_ref = false;
+
        write_lock_bh(&pch->upl);
        if (pch->ppp ||
            rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) 
{
                write_unlock_bh(&pch->upl);
                return -EALREADY;
        }
+       refcount_inc(&pchb->file.refcnt);
        rcu_assign_pointer(pch->bridge, pchb);
        write_unlock_bh(&pch->upl);
 
@@ -632,19 +635,28 @@ static int ppp_bridge_channels(struct channel *pch, 
struct channel *pchb)
                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);
-       RCU_INIT_POINTER(pch->bridge, NULL);
+       /* Re-check pch->bridge with upl held since a racing unbridge might 
already
+        * have cleared it and dropped the reference on pch->bridge.
+        */
+       if (rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) 
{
+               RCU_INIT_POINTER(pch->bridge, NULL);
+               drop_ref = true;
+       }
        write_unlock_bh(&pch->upl);
        synchronize_rcu();
+
+       if (drop_ref)
+               if (refcount_dec_and_test(&pchb->file.refcnt))
+                       ppp_destroy_channel(pchb);
+
        return -EALREADY;
 }
 
-- 
2.17.1