On 7/24/26 12:12 PM, Eelco Chaudron wrote:
>
>
> 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: [email protected]
>> Signed-off-by: Cen Zhang (Microsoft) <[email protected]>
>
> 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
>
>> ---
>> 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/[email protected]
>>
>> 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;
The 'eth' is also only used in the 'else' block, but defined at the top.
It doesn't matter where, but I think, we should keep them together.
>
>>
>> /* 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
>
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