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Thu, 09 Apr 2026 04:41:56 -0700 (PDT) Received: from [192.168.88.32] ([150.228.25.243]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43d1e4d2738sm68094680f8f.24.2026.04.09.04.41.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Apr 2026 04:41:56 -0700 (PDT) Message-ID: Date: Thu, 9 Apr 2026 13:41:55 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net] l2tp: take a session reference in pppol2tp_ioctl() To: Yiqi Sun , jchapman@katalix.com Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, horms@kernel.org, netdev@vger.kernel.org References: <20260404133245.2391409-1-sunyiqixm@gmail.com> Content-Language: en-US From: Paolo Abeni In-Reply-To: <20260404133245.2391409-1-sunyiqixm@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/4/26 3:32 PM, Yiqi Sun wrote: > pppol2tp_ioctl() reads sock->sk->sk_user_data and dereferences the > returned l2tp_session without taking a reference on it. > > Since the ppp socket/session lifetime rework, session teardown runs > asynchronously and can clear sk_user_data and drop the last session > reference in parallel with ioctl(). This leaves ioctl() with a stale > session pointer and can trigger a use-after-free. It's not immediately obvious to me which are the actual code paths involved, please include a more accurate description of the relevant race. > > Fix this by using pppol2tp_sock_to_session() in pppol2tp_ioctl() and > dropping the session reference before returning. This matches the > existing getsockopt/setsockopt paths. > > Fixes: c5cbaef992d64 ("l2tp: refactor ppp socket/session relationship") > Signed-off-by: Yiqi Sun > --- > net/l2tp/l2tp_ppp.c | 88 +++++++++++++++++++++++++++------------------ > 1 file changed, 54 insertions(+), 34 deletions(-) > > diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c > index ae4543d5597b..e6d7d3537180 100644 > --- a/net/l2tp/l2tp_ppp.c > +++ b/net/l2tp/l2tp_ppp.c > @@ -1042,66 +1042,79 @@ static int pppol2tp_tunnel_copy_stats(struct pppol2tp_ioc_stats *stats, > static int pppol2tp_ioctl(struct socket *sock, unsigned int cmd, > unsigned long arg) > { > + struct sock *sk = sock->sk; > struct pppol2tp_ioc_stats stats; > struct l2tp_session *session; > + int err; > + > + err = -ENOTCONN; > + if (!sk->sk_user_data) > + goto end; > + > + err = -EBADF; > + session = pppol2tp_sock_to_session(sk); > + if (!session) > + goto end; Consolidating the checks here brings an user visible change, as unsupported cmds previously delivered -ENOIOCTLCMD. > > switch (cmd) { > case PPPIOCGMRU: > case PPPIOCGFLAGS: > - session = sock->sk->sk_user_data; > - if (!session) > - return -ENOTCONN; > - > if (WARN_ON(session->magic != L2TP_SESSION_MAGIC)) This check is already present in pppol2tp_sock_to_session() Similar chunks below. /P