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Date:   Wed, 23 Dec 2020 18:47:30 +0000
From:   Tom Parkin <tparkin@...alix.com>
To:     netdev@...r.kernel.org
Cc:     gnault@...hat.com, jchapman@...alix.com,
        Tom Parkin <tparkin@...alix.com>
Subject: [PATCH net] ppp: hold mutex when unbridging channels in unregister path

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 <tparkin@...alix.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

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