[PATCH net v2] net: ip_tunnel: initialize `options_len` before referencing options

[Gris Ge <cnfourt@gmail.com>]

The following command triggers a kernel panic:

  ip link add d0 type dummy; ip link set d0 up
  ip route add 10.30.0.0/16 \
    encap ip id 300 geneve_opts 4660:66:11223344 dev d0

  memcpy: detected buffer overflow: 4 byte write of buffer size 0
  kernel BUG at lib/string_helpers.c:1044!
  ...
  ip_tun_parse_opts.part.0.cold+0x10/0x10
  ip_tun_build_state+0x116/0x2a0

On kernels built with GCC 15+ and `CONFIG_FORTIFY_SOURCE`, the fortified
`memcpy()` got 0 sized destination with request of 4 bytes length:

  static int ip_tun_parse_opts_geneve(...)
  {
      ...
      attr = tb[LWTUNNEL_IP_OPT_GENEVE_DATA];
      data_len = nla_len(attr); /* == 4 */

      struct geneve_opt *opt = ip_tunnel_info_opts(info) + opts_len;
      memcpy(opt->opt_data, nla_data(attr), data_len);
      /*     ^^^^^^^^^^^^^ 0 since options_len is assigned afterwards */

Fixed by initializing the counter before the options are referenced.
Matching what `tunnel_key_opts_set()` already does.

Fixes: bb5e62f2d547 ("net: Add options as a flexible array to struct ip_tunnel_info")
Cc: stable@vger.kernel.org
Signed-off-by: Gris Ge <cnfourt@gmail.com>
---
v2:
  - pass opt_len into ip_tun_set_opts() and initialize options_len there,
    matching tunnel_key_opts_set(), as suggested by Eric Dumazet.
v1: https://lore.kernel.org/netdev/20260912055304.1415016-1-cnfourt@gmail.com/

 net/ipv4/ip_tunnel_core.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index 5168d546ea2f..bab42b9e277f 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -680,8 +680,14 @@ static int ip_tun_get_optlen(struct nlattr *attr,
 }

 static int ip_tun_set_opts(struct nlattr *attr, struct ip_tunnel_info *info,
-			   struct netlink_ext_ack *extack)
+			   int opts_len, struct netlink_ext_ack *extack)
 {
+	/* `options_len` is the __counted_by() annotation of the `options`
+	 * flexible array, it must be initialized before parsing writes
+	 * into it.
+	 */
+	info->options_len = opts_len;
+
 	return ip_tun_parse_opts(attr, info, extack);
 }

@@ -712,7 +718,8 @@ static int ip_tun_build_state(struct net *net, struct nlattr *attr,

 	tun_info = lwt_tun_info(new_state);

-	err = ip_tun_set_opts(tb[LWTUNNEL_IP_OPTS], tun_info, extack);
+	err = ip_tun_set_opts(tb[LWTUNNEL_IP_OPTS], tun_info, opt_len,
+			      extack);
 	if (err < 0) {
 		lwtstate_free(new_state);
 		return err;
@@ -753,7 +760,6 @@ static int ip_tun_build_state(struct net *net, struct nlattr *attr,
 	}

 	tun_info->mode = IP_TUNNEL_INFO_TX;
-	tun_info->options_len = opt_len;

 	*ts = new_state;

@@ -1006,7 +1012,8 @@ static int ip6_tun_build_state(struct net *net, struct nlattr *attr,

 	tun_info = lwt_tun_info(new_state);

-	err = ip_tun_set_opts(tb[LWTUNNEL_IP6_OPTS], tun_info, extack);
+	err = ip_tun_set_opts(tb[LWTUNNEL_IP6_OPTS], tun_info, opt_len,
+			      extack);
 	if (err < 0) {
 		lwtstate_free(new_state);
 		return err;
@@ -1040,7 +1047,6 @@ static int ip6_tun_build_state(struct net *net, struct nlattr *attr,
 	}

 	tun_info->mode = IP_TUNNEL_INFO_TX | IP_TUNNEL_INFO_IPV6;
-	tun_info->options_len = opt_len;

 	*ts = new_state;

--
2.55.0