netdev - [PATCH] net: tipc: prevent possible null deref of link

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Message-Id: <20201207081423.67313-1-cengiz@kernel.wtf>
Date:   Mon,  7 Dec 2020 11:14:24 +0300
From:   Cengiz Can <cengiz@...nel.wtf>
To:     Jon Maloy <jmaloy@...hat.com>, Ying Xue <ying.xue@...driver.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, tipc-discussion@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, Cengiz Can <cengiz@...nel.wtf>
Subject: [PATCH] net: tipc: prevent possible null deref of link

`tipc_node_apply_property` does a null check on a `tipc_link_entry`
pointer but also accesses the same pointer out of the null check block.

This triggers a warning on Coverity Static Analyzer because we're
implying that `e->link` can BE null.

Move "Update MTU for node link entry" line into if block to make sure
that we're not in a state that `e->link` is null.

Signed-off-by: Cengiz Can <cengiz@...nel.wtf>
---
 net/tipc/node.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/tipc/node.c b/net/tipc/node.c
index c95d037fde51..83978d5dae59 100644
--- a/net/tipc/node.c
+++ b/net/tipc/node.c
@@ -2181,9 +2181,11 @@ void tipc_node_apply_property(struct net *net, struct tipc_bearer *b,
 							&xmitq);
 			else if (prop == TIPC_NLA_PROP_MTU)
 				tipc_link_set_mtu(e->link, b->mtu);
+
+			/* Update MTU for node link entry */
+			e->mtu = tipc_link_mss(e->link);
 		}
-		/* Update MTU for node link entry */
-		e->mtu = tipc_link_mss(e->link);
+
 		tipc_node_write_unlock(n);
 		tipc_bearer_xmit(net, bearer_id, &xmitq, &e->maddr, NULL);
 	}
-- 
2.29.2