llc: verify mac len before reading mac header

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. The below referenced commit shows that used
to, on top of Token Ring.

At least one of the three eth_hdr uses goes back to before the start
of git history. But the one that syzbot exercises is introduced in
this commit. That commit is old enough (2008), that effectively all
stable kernels should receive this.

Fixes: f83f176 ("[LLC]: skb allocation size for responses")
Reported-by: [email protected]
Signed-off-by: Willem de Bruijn <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: wdebruij

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))

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 1;
+
 	llc_pdu_decode_sa(skb, mac_da);
 	llc_pdu_decode_da(skb, mac_sa);
 	llc_pdu_decode_ssap(skb, &dsap);

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);