Commit c1787ffd authored by Tom Parkin's avatar Tom Parkin Committed by Jakub Kicinski
Browse files

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: default avatarTom Parkin <tparkin@katalix.com>
Fixes: 4cf476ce ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls")
Acked-by: default avatarGuillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20210107181315.3128-1-tparkin@katalix.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent fd2ddef0

drivers/net/ppp/ppp_generic.c

+9 −3
Original line number Diff line number Diff line
@@ -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;

}