From c1787ffd0d24eb93eefac2dbba0eac5700da9ff1 Mon Sep 17 00:00:00 2001
From: Tom Parkin <tparkin@katalix.com>
Date: Thu, 7 Jan 2021 18:13:15 +0000
Subject: [PATCH] ppp: fix refcount underflow on channel unbridge
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")
Acked-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20210107181315.3128-1-tparkin@katalix.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
drivers/net/ppp/ppp_generic.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 09c27f7773f95..d445ecb1d0c75 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;
}
--
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