Re: [PATCH net v3] net/openvswitch: check Ethernet header length in key_extract()
From: Eelco Chaudron <echaudro@redhat.com>
Date: 2026-07-24 10:12:14
Also in:
lkml
On 24 Jul 2026, at 1:37, Cen Zhang (Microsoft) wrote:
When a packet arrives on an ARPHRD_NONE device (e.g. TUN),
ovs_flow_key_extract() trusts the user-provided skb->protocol field: if
it is ETH_P_TEB, the packet is classified as MAC_PROTO_ETHERNET and
key_extract() is called without ensuring the skb has ETH_HLEN (14) bytes
of linear data. key_extract() unconditionally pulls 2 * ETH_ALEN bytes
for MAC addresses and parse_ethertype() pulls 2 more, either of which
triggers a kernel BUG in __skb_pull() when the linear area is too small.
kernel BUG at include/linux/skbuff.h:2848!
RIP: 0010:key_extract+0xa7e/0xd90 net/openvswitch/flow.c:933
ovs_flow_key_extract+0x419/0xa70
ovs_vport_receive+0x222/0x390
netdev_frame_hook+0x3e0/0x630
tun_get_user+0x2d0c/0x38e0
Fixed by adding checks in key_extract() before pulling the Ethernet
header.
Fixes: 217ac77a3c25 ("openvswitch: allow L3 netdev ports")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Cen Zhang (Microsoft) <redacted>Hi Cen, Thanks for the new revision. The doc comment for key_extract() also needs updating. It currently states: * The caller must ensure that skb->len >= ETH_HLEN. The comment should be removed or replaced with a note that the function returns -EINVAL when the frame is too short to contain an Ethernet header. One more comment below. Cheers, Eelco
quoted hunk ↗ jump to hunk
--- v3: Use check_header() per review, fix format issue. v2: Moved the check into key_extract() per Ilya Maximets. Link: https://lore.kernel.org/all/20260721143602.64677-1-blbllhy@gmail.com (local) net/openvswitch/flow.c | 5 +++++ 1 file changed, 5 insertions(+)diff --git a/net/openvswitch/flow.c b/net/openvswitch/flow.c index 66366982f604..fc4002353e17 100644 --- a/net/openvswitch/flow.c +++ b/net/openvswitch/flow.c@@ -911,6 +911,7 @@ static int key_extract_l3l4(struct sk_buff *skb, struct sw_flow_key *key) static int key_extract(struct sk_buff *skb, struct sw_flow_key *key) { struct ethhdr *eth; + int err;
Since err is only used in the else block, it should be scoped there:
} else {
int err = check_header(skb, ETH_HLEN);
if (unlikely(err))
return err;
quoted hunk ↗ jump to hunk
/* Flags are always used as part of stats */ key->tp.flags = 0;@@ -926,6 +927,10 @@ static int key_extract(struct sk_buff *skb, struct sw_flow_key *key) skb_reset_network_header(skb); key->eth.type = skb->protocol; } else { + err = check_header(skb, ETH_HLEN); + if (unlikely(err)) + return err; + eth = eth_hdr(skb); ether_addr_copy(key->eth.src, eth->h_source); ether_addr_copy(key->eth.dst, eth->h_dest);-- 2.53.0