netdev - [PATCH net v2 17/18] net/sched: don't dereference a->goto

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Message-Id: <1f64c8daa2a98f0808d7f834013cb36fe2eb2bff.1553085664.git.dcaratti@redhat.com>
Date:   Wed, 20 Mar 2019 15:00:15 +0100
From:   Davide Caratti <dcaratti@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Vlad Buslov <vladbu@...lanox.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Cc:     Paolo Abeni <pabeni@...hat.com>
Subject: [PATCH net v2 17/18] net/sched: don't dereference a->goto_chain to read the chain index

callers of tcf_gact_goto_chain_index() can potentially read an old value
of the chain index, or even dereference a NULL 'goto_chain' pointer,
because 'goto_chain' and 'tcfa_action' are read in the traffic path
without caring of concurrent write in the control path. The most recent
value of chain index can be read also from a->tcfa_action (it's encoded
there together with TC_ACT_GOTO_CHAIN bits), so we don't really need to
dereference 'goto_chain': just read the chain id from the control action.

Fixes: e457d86ada27 ("net: sched: add couple of goto_chain helpers")
Signed-off-by: Davide Caratti <dcaratti@...hat.com>
---
 include/net/tc_act/tc_gact.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/tc_act/tc_gact.h b/include/net/tc_act/tc_gact.h
index ee8d005f56fc..eb8f01c819e6 100644
--- a/include/net/tc_act/tc_gact.h
+++ b/include/net/tc_act/tc_gact.h
@@ -56,7 +56,7 @@ static inline bool is_tcf_gact_goto_chain(const struct tc_action *a)
 
 static inline u32 tcf_gact_goto_chain_index(const struct tc_action *a)
 {
-	return a->goto_chain->index;
+	return READ_ONCE(a->tcfa_action) & TC_ACT_EXT_VAL_MASK;
 }
 
 #endif /* __NET_TC_GACT_H */
-- 
2.20.1