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ppp: fix refcount underflow on channel unbridge

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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: 4cf476ced4 ("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>
This commit is contained in:
Tom Parkin 2021-01-07 18:13:15 +00:00 committed by Jakub Kicinski
parent fd2ddef043
commit c1787ffd0d
1 changed files with 9 additions and 3 deletions
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12
drivers/net/ppp/ppp_generic.c
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@ -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;
}