Channels are bridged using the PPPIOCBRIDGECHAN ioctl, which executes
with the ppp_mutex held.
Unbridging may occur in two code paths: firstly an explicit
PPPIOCUNBRIDGECHAN ioctl, and secondly on channel unregister. The
latter may occur when closing the /dev/ppp instance or on teardown of
the channel itself.
This opens up a refcount underflow bug if ppp_bridge_channels called via.
ioctl races with ppp_unbridge_channels called via. file release.
The race is triggered by ppp_unbridge_channels taking the error path
through the 'err_unset' label. In this scenario, pch->bridge has been
set, but no reference will be taken on pch->file because the function
errors out. Therefore, if ppp_unbridge_channels is called in the window
between pch->bridge being set and pch->bridge being unset, it will
erroneously drop the reference on pch->file and cause a refcount
underflow.
To avoid this, hold the ppp_mutex while calling ppp_unbridge_channels in
the shutdown path. This serialises the unbridge operation with any
concurrently executing ioctl.
Signed-off-by: Tom Parkin <tpar...@katalix.com>
---
drivers/net/ppp/ppp_generic.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 09c27f7773f9..e87a05fee2db 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -2938,7 +2938,9 @@ ppp_unregister_channel(struct ppp_channel *chan)
list_del(&pch->list);
spin_unlock_bh(&pn->all_channels_lock);
+ mutex_lock(&ppp_mutex);
ppp_unbridge_channels(pch);
+ mutex_unlock(&ppp_mutex);
pch->file.dead = 1;
wake_up_interruptible(&pch->file.rwait);
--
2.17.1
