linux-kernel - [PATCH 1/2] l2tp: ensure sessions are freed after their PPPOL2TP socket

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Message-Id: <20200402173158.7798-2-will@kernel.org>
Date:   Thu,  2 Apr 2020 18:31:57 +0100
From:   Will Deacon <will@...nel.org>
To:     gregkh@...uxfoundation.org
Cc:     stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, g.nault@...halink.fr,
        "David S . Miller" <davem@...emloft.net>,
        Will Deacon <will@...nel.org>
Subject: [PATCH 1/2] l2tp: ensure sessions are freed after their PPPOL2TP socket

From: Guillaume Nault <g.nault@...halink.fr>

commit cdd10c9627496ad25c87ce6394e29752253c69d3 upstream.

If l2tp_tunnel_delete() or l2tp_tunnel_closeall() deletes a session
right after pppol2tp_release() orphaned its socket, then the 'sock'
variable of the pppol2tp_session_close() callback is NULL. Yet the
session is still used by pppol2tp_release().

Therefore we need to take an extra reference in any case, to prevent
l2tp_tunnel_delete() or l2tp_tunnel_closeall() from freeing the session.

Since the pppol2tp_session_close() callback is only set if the session
is associated to a PPPOL2TP socket and that both l2tp_tunnel_delete()
and l2tp_tunnel_closeall() hold the PPPOL2TP socket before calling
pppol2tp_session_close(), we're sure that pppol2tp_session_close() and
pppol2tp_session_destruct() are paired and called in the right order.
So the reference taken by the former will be released by the later.

Signed-off-by: Guillaume Nault <g.nault@...halink.fr>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Will Deacon <will@...nel.org>
---
 net/l2tp/l2tp_ppp.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 16b63e60396f..d919b3e6b548 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -437,11 +437,11 @@ static void pppol2tp_session_close(struct l2tp_session *session)

 	BUG_ON(session->magic != L2TP_SESSION_MAGIC);

-	if (sock) {
+	if (sock)
 		inet_shutdown(sock, SEND_SHUTDOWN);
-		/* Don't let the session go away before our socket does */
-		l2tp_session_inc_refcount(session);
-	}
+
+	/* Don't let the session go away before our socket does */
+	l2tp_session_inc_refcount(session);
 }

 /* Really kill the session socket. (Called from sock_put() if
-- 
2.26.0.rc2.310.g2932bb562d-goog