| Message ID | 20210107181315.3128-1-tparkin@katalix.com |
|---|---|
| State | New |
| Headers | show |
| Series | [net,v3] ppp: fix refcount underflow on channel unbridge | expand |
On Thu, Jan 07, 2021 at 06:13:15PM +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. Thanks for following up on this! Acked-by: Guillaume Nault <gnault@redhat.com>
On Fri, 8 Jan 2021 21:57:50 +0100 Guillaume Nault wrote: > On Thu, Jan 07, 2021 at 06:13:15PM +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. > > Thanks for following up on this! > > Acked-by: Guillaume Nault <gnault@redhat.com> Applied, thanks!
On Fri, Jan 08, 2021 at 21:57:50 +0100, Guillaume Nault wrote: > On Thu, Jan 07, 2021 at 06:13:15PM +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. > > Thanks for following up on this! > > Acked-by: Guillaume Nault <gnault@redhat.com> Thanks again for reviewing, Guillaume.
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c index 09c27f7773f9..d445ecb1d0c7 100644 --- a/drivers/net/ppp/ppp_generic.c +++ b/drivers/net/ppp/ppp_generic.c @@ -623,6 +623,7 @@ static int ppp_bridge_channels(struct channel *pch, struct channel *pchb) 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 +633,24 @@ 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); + /* 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; }
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 <tparkin@katalix.com> Fixes: 4cf476ced45d ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls") --- v3: * remove bool tracking variable in ppp_bridge_channels and re-read pch->bridge instead * add missing tags 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 | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)