Re: [PATCH net] llc: verify mac len before reading mac header
From: Eric Dumazet <edumazet@google.com>
Date: 2023-10-25 07:09:24
On Tue, Oct 24, 2023 at 9:50 PM Willem de Bruijn [off-list ref] wrote:
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.
Reported-by: syzbot+a8c7be6dee0de1b669cc@syzkaller.appspotmail.comNote that the syzbot report is still private in syzbot infra. Reviewed-by: Eric Dumazet <edumazet@google.com>
quoted hunk ↗ jump to hunk
Signed-off-by: Willem de Bruijn <willemb@google.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