netdev - [PATCH net] llc: verify mac len before reading mac header

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Date: Tue, 24 Oct 2023 15:49:36 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net,
	kuba@...nel.org,
	edumazet@...gle.com,
	pabeni@...hat.com,
	Willem de Bruijn <willemb@...gle.com>,
	syzbot+a8c7be6dee0de1b669cc@...kaller.appspotmail.com
Subject: [PATCH net] llc: verify mac len before reading mac header

From: Willem de Bruijn <willemb@...gle.com>

LLC reads the mac header with eth_hdr without verifying that the skb
has an Ethernet header.

Syzbot was able to enter llc_rcv on a tun device. Tun can insert
packets without mac len and with user configurable skb->protocol
(passing a tun_pi header when not configuring IFF_NO_PI).

    BUG: KMSAN: uninit-value in llc_station_ac_send_test_r net/llc/llc_station.c:81 [inline]
    BUG: KMSAN: uninit-value in llc_station_rcv+0x6fb/0x1290 net/llc/llc_station.c:111
    llc_station_ac_send_test_r net/llc/llc_station.c:81 [inline]
    llc_station_rcv+0x6fb/0x1290 net/llc/llc_station.c:111
    llc_rcv+0xc5d/0x14a0 net/llc/llc_input.c:218
    __netif_receive_skb_one_core net/core/dev.c:5523 [inline]
    __netif_receive_skb+0x1a6/0x5a0 net/core/dev.c:5637
    netif_receive_skb_internal net/core/dev.c:5723 [inline]
    netif_receive_skb+0x58/0x660 net/core/dev.c:5782
    tun_rx_batched+0x3ee/0x980 drivers/net/tun.c:1555
    tun_get_user+0x54c5/0x69c0 drivers/net/tun.c:2002

Add a mac_len test before all three eth_hdr(skb) calls under net/llc.

There are further uses in include/net/llc_pdu.h. All these are
protected by a test skb->protocol == ETH_P_802_2. Which does not
protect against this tun scenario.

But the mac_len test added in this patch in llc_fixup_skb will
indirectly protect those too. That is called from llc_rcv before any
other LLC code.

It is tempting to just add a blanket mac_len check in llc_rcv, but
not sure whether that could break valid LLC paths that do not assume
an Ethernet header. 802.2 LLC may be used on top of non-802.3
protocols in principle.

Reported-by: syzbot+a8c7be6dee0de1b669cc@...kaller.appspotmail.com
Signed-off-by: Willem de Bruijn <willemb@...gle.com>
---
 net/llc/llc_input.c   | 10 ++++++++--
 net/llc/llc_s_ac.c    |  3 +++
 net/llc/llc_station.c |  3 +++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/llc/llc_input.c b/net/llc/llc_input.c
index 7cac441862e21..51bccfb00a9cd 100644
--- a/net/llc/llc_input.c
+++ b/net/llc/llc_input.c
@@ -127,8 +127,14 @@ static inline int llc_fixup_skb(struct sk_buff *skb)
 	skb->transport_header += llc_len;
 	skb_pull(skb, llc_len);
 	if (skb->protocol == htons(ETH_P_802_2)) {
-		__be16 pdulen = eth_hdr(skb)->h_proto;
-		s32 data_size = ntohs(pdulen) - llc_len;
+		__be16 pdulen;
+		s32 data_size;
+
+		if (skb->mac_len < ETH_HLEN)
+			return 0;
+
+		pdulen = eth_hdr(skb)->h_proto;
+		data_size = ntohs(pdulen) - llc_len;
 
 		if (data_size < 0 ||
 		    !pskb_may_pull(skb, data_size))
diff --git a/net/llc/llc_s_ac.c b/net/llc/llc_s_ac.c
index 79d1cef8f15a9..7923c064773cc 100644
--- a/net/llc/llc_s_ac.c
+++ b/net/llc/llc_s_ac.c
@@ -153,6 +153,9 @@ int llc_sap_action_send_test_r(struct llc_sap *sap, struct sk_buff *skb)
 	int rc = 1;
 	u32 data_size;
 
+	if (skb->mac_len < ETH_HLEN)
+		return 0;
+
 	llc_pdu_decode_sa(skb, mac_da);
 	llc_pdu_decode_da(skb, mac_sa);
 	llc_pdu_decode_ssap(skb, &dsap);
diff --git a/net/llc/llc_station.c b/net/llc/llc_station.c
index 05c6ae0920534..f506542925109 100644
--- a/net/llc/llc_station.c
+++ b/net/llc/llc_station.c
@@ -76,6 +76,9 @@ static int llc_station_ac_send_test_r(struct sk_buff *skb)
 	u32 data_size;
 	struct sk_buff *nskb;
 
+	if (skb->mac_len < ETH_HLEN)
+		goto out;
+
 	/* The test request command is type U (llc_len = 3) */
 	data_size = ntohs(eth_hdr(skb)->h_proto) - 3;
 	nskb = llc_alloc_frame(NULL, skb->dev, LLC_PDU_TYPE_U, data_size);
-- 
2.42.0.758.gaed0368e0e-goog