netdev - [PATCH 25/29] l2tp: don't BUG_ON session magic checks in l2tp

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Message-Id: <20200721173221.4681-26-tparkin@katalix.com>
Date:   Tue, 21 Jul 2020 18:32:17 +0100
From:   Tom Parkin <tparkin@...alix.com>
To:     netdev@...r.kernel.org
Cc:     Tom Parkin <tparkin@...alix.com>
Subject: [PATCH 25/29] l2tp: don't BUG_ON session magic checks in l2tp_ppp

checkpatch advises that WARN_ON and recovery code are preferred over
BUG_ON which crashes the kernel.

l2tp_ppp.c's BUG_ON checks of the l2tp session structure's "magic" field
occur in code paths where it's reasonably easy to recover:

 * In the case of pppol2tp_sock_to_session, we can return NULL and the
   caller will bail out appropriately.  There is no change required to
   any of the callsites of this function since they already handle
   pppol2tp_sock_to_session returning NULL.

 * In the case of pppol2tp_session_destruct we can just avoid
   decrementing the reference count on the suspect session structure.
   In the worst case scenario this results in a memory leak, which is
   preferable to a crash.

Convert these uses of BUG_ON to WARN_ON accordingly.

Signed-off-by: Tom Parkin <tparkin@...alix.com>
---
 net/l2tp/l2tp_ppp.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index e58fe7e3b884..6cd1a422c426 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -163,8 +163,12 @@ static inline struct l2tp_session *pppol2tp_sock_to_session(struct sock *sk)
 		sock_put(sk);
 		goto out;
 	}
-
-	BUG_ON(session->magic != L2TP_SESSION_MAGIC);
+	if (session->magic != L2TP_SESSION_MAGIC) {
+		WARN_ON(session->magic != L2TP_SESSION_MAGIC);
+		session = NULL;
+		sock_put(sk);
+		goto out;
+	}

 out:
 	return session;
@@ -419,8 +423,9 @@ static void pppol2tp_session_destruct(struct sock *sk)

 	if (session) {
 		sk->sk_user_data = NULL;
-		BUG_ON(session->magic != L2TP_SESSION_MAGIC);
-		l2tp_session_dec_refcount(session);
+		WARN_ON(session->magic != L2TP_SESSION_MAGIC);
+		if (session->magic == L2TP_SESSION_MAGIC)
+			l2tp_session_dec_refcount(session);
 	}
 }

-- 
2.17.1