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[34.83.123.78]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-cc72aefa9c0sm1224118a12.29.2026.09.19.14.28.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 19 Sep 2026 14:28:56 -0700 (PDT) From: Hui Peng To: andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] ppp: pptp: fix out-of-bounds read on short payload_len in pptp_rcv_core() Date: Sat, 19 Sep 2026 21:28:55 +0000 Message-ID: <20260919212856.3241262-1-benquike@gmail.com> X-Mailer: git-send-email 2.55.0.1082.g2b9226bbc0-goog Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit In `pptp_rcv_core()`, `pskb_may_pull(skb, headersize + payload_len)` only verifies that `skb->len >= headersize + payload_len` without checking that `payload_len` itself is large enough for the subsequent PPP header accesses. When a GRE packet arrives with `payload_len == 0` (or `1`) and `skb->len == headersize + payload_len`, `pskb_may_pull()` succeeds and `payload = skb->data + headersize` points to `skb->tail`. Immediately afterwards: 1. If `seq` is out-of-window, `pptp_rcv_core()` reads `payload[0]`, `payload[1]`, `PPP_PROTOCOL(payload)` (`payload[2..3]`), and `payload[4]` (up to 5 bytes past `skb->tail`) to check for an LCP Echo packet. 2. If `seq` is in-window, `pptp_rcv_core()` reads `payload[0]` and `payload[1]` (up to 2 bytes past `skb->tail`) to check for the PPP address and control bytes. Require `payload_len >= 2` before pulling the payload, and require `payload_len >= 5` before inspecting `payload[0..4]` for LCP Echo frames. Fixes: 00959ade36ac ("PPTP: PPP over IPv4 (Point-to-Point Tunneling Protocol)") Assisted-by: LLM Signed-off-by: Hui Peng --- drivers/net/ppp/pptp.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/drivers/net/ppp/pptp.c b/drivers/net/ppp/pptp.c index a797a0606f6b..2f979294490d 100644 --- a/drivers/net/ppp/pptp.c +++ b/drivers/net/ppp/pptp.c @@ -310,6 +310,8 @@ static int pptp_rcv_core(struct sock *sk, struct sk_buff *skb) payload_len = ntohs(header->payload_len); seq = ntohl(header->seq); + if (payload_len < 2) + goto drop; /* check for incomplete packet (length smaller than expected) */ if (!pskb_may_pull(skb, headersize + payload_len)) goto drop; @@ -317,9 +319,11 @@ static int pptp_rcv_core(struct sock *sk, struct sk_buff *skb) payload = skb->data + headersize; /* check for expected sequence number */ if (seq < opt->seq_recv + 1 || WRAPPED(opt->seq_recv, seq)) { - if ((payload[0] == PPP_ALLSTATIONS) && (payload[1] == PPP_UI) && - (PPP_PROTOCOL(payload) == PPP_LCP) && - ((payload[4] == PPP_LCP_ECHOREQ) || (payload[4] == PPP_LCP_ECHOREP))) + if (payload_len >= 5 && + payload[0] == PPP_ALLSTATIONS && payload[1] == PPP_UI && + PPP_PROTOCOL(payload) == PPP_LCP && + (payload[4] == PPP_LCP_ECHOREQ || + payload[4] == PPP_LCP_ECHOREP)) goto allow_packet; } else { opt->seq_recv = seq;