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Date: Wed, 7 Jun 2023 11:32:31 +0800
From: Hangyu Hua <>
Subject: Linux kernel: off-by-one in fl_set_geneve_opt

Hi guys,

I find a off-by-one bug in linux kernel's Flower
classifier(NET_CLS_FLOWER). It can cause denial-of-service and privilege 

# Details:

static int fl_set_geneve_opt(const struct nlattr *nla, struct 
fl_flow_key *key,
      int depth, int option_len,
      struct netlink_ext_ack *extack)
struct nlattr *tb[TCA_FLOWER_KEY_ENC_OPT_GENEVE_MAX + 1];
struct nlattr *class = NULL, *type = NULL, *data = NULL;
struct geneve_opt *opt;
int err, data_len = 0;

if (option_len > sizeof(struct geneve_opt))
data_len = option_len - sizeof(struct geneve_opt);

opt = (struct geneve_opt *)&key->[key->enc_opts.len]; <--- [1]
memset(opt, 0xff, option_len);
opt->length = data_len / 4;
opt->r1 = 0;
opt->r2 = 0;
opt->r3 = 0;

int new_len = key->enc_opts.len;

data_len = nla_len(data);
if (data_len < 4) {
NL_SET_ERR_MSG(extack, "Tunnel key geneve option data is less than 4
bytes long");
return -ERANGE;
if (data_len % 4) {
NL_SET_ERR_MSG(extack, "Tunnel key geneve option data is not a
multiple of 4 bytes long");
return -ERANGE;

new_len += sizeof(struct geneve_opt) + data_len;
if (new_len > FLOW_DIS_TUN_OPTS_MAX) { <--- [2]
NL_SET_ERR_MSG(extack, "Tunnel options exceeds max size");
return -ERANGE;
opt->length = data_len / 4;
memcpy(opt->opt_data, nla_data(data), data_len); <--- [3]

We can see that opt use key->enc_opts.len to get its pointer from
key->[] in [1]. Then length will be set to "data_len /
4". The bug is that if we send two TCA_FLOWER_KEY_ENC_OPTS_GENEVE
packets and their total size is 252 bytes(key->enc_opts.len = 252)
then key->enc_opts.len = opt->length = data_len / 4 when the third
TCA_FLOWER_KEY_ENC_OPTS_GENEVE packet enters fl_set_geneve_opt. This
can bypass the check in [2] and cause out of bound write in
[3](opt->opt_data = key->[257]).

# Patch

I already contacted the linux security team and made a patch:




In order to avoid confusion i will publish it after I get CVE.


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