[PATCH] ppp: fix refcount underflow on channel unbridge

[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

Re: [PATCH] ppp: fix refcount underflow on channel unbridge

[Guillaume Nault]

On Tue, Jan 05, 2021 at 09:17:43PM +0000, Tom Parkin wrote:
>  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);
> +

I think this works because ppp_mutex prevents pch->bridge from being
reassigned to another channel. However, this isn't obvious when reading
the code, and well, I prefer to not introduce new dependencies on
ppp_mutex (otherwise we'd better go with your original patch).

I think we could just save pch->bridge while we're under ->upl
protection, and then drop the reference of that channel (if non-NULL):

 err_unset:
 	write_lock_bh(&pch->upl);
+	/* Re-read pch->bridge 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;
 }

This way we know that pchb is the channel we were pointing to when we
cleared pch->bridge. And this is also a bit simpler than using an extra
boolean.

Re: [PATCH] ppp: fix refcount underflow on channel unbridge

[Jakub Kicinski]

On Tue,  5 Jan 2021 21:17:43 +0000 Tom Parkin wrote:
> 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.

Please prefix the subject with [PATCH net v3] for v3, add a fixes tag,
and make sure to include you sign-off.