From: Willem de Bruijn willemb@google.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. 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: f83f1768f833 ("[LLC]: skb allocation size for responses") Reported-by: syzbot+a8c7be6dee0de1b669cc@syzkaller.appspotmail.com Signed-off-by: Willem de Bruijn willemb@google.com

---

Changes v1->v2 - fix return value in llc_sap_action_send_test_r - add Fixes tag - cc: stable@vger.kernel.org --- 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..06fb8e6944b06 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 1; + 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);