Re: [PATCH net v2] ipv6: sr: enforce exact attribute length for SEG6_ATTR_DST

[Hangbin Liu <hangbin.liu@linux.dev>]

On Mon, Sep 21, 2026 at 04:40:25AM +0000, Hui Peng wrote:
> In seg6_genl_policy, SEG6_ATTR_DST is defined with .type = NLA_BINARY and
> .len = sizeof(struct in6_addr). For NLA_BINARY, .len only enforces the
> maximum payload length and permits shorter payloads (e.g., 0 bytes).
> When seg6_genl_set_tunsrc() copies sizeof(struct in6_addr) bytes via
> kmemdup(val, sizeof(*val), GFP_KERNEL), a short SEG6_ATTR_DST attribute
> triggers a 16-byte out-of-bounds read past skb->tail into uninitialized
> skb->head memory, which is stored in sdata->tun_src and leaked back to
> userspace via SEG6_CMD_GET_TUNSRC.
> 
> Switch SEG6_ATTR_DST in seg6_genl_policy to
> NLA_POLICY_EXACT_LEN(sizeof(struct in6_addr)) so that generic netlink
> validation rejects any attribute whose length is not exactly
> sizeof(struct in6_addr) with -ERANGE.
> 
> Tested in QEMU against Linux 7.3.0-rc3 by sending a SEG6_CMD_SET_TUNSRC
> Generic Netlink message with a 0-byte SEG6_ATTR_DST attribute followed
> by SEG6_CMD_GET_TUNSRC. On the unfixed kernel, SEG6_CMD_SET_TUNSRC
> succeeds (err = 0) and SEG6_CMD_GET_TUNSRC leaks 16 bytes of
> uninitialized kernel heap memory (tun_src =
> 836a61ecc4d25a1042a8d60411cfb378); with this patch applied,
> SEG6_CMD_SET_TUNSRC is rejected by netlink policy validation with
> -ERANGE (-34) and tun_src remains zeroed.
> 
> Fixes: 915d7e5e5930 ("ipv6: sr: add code base for control plane support of SR-IPv6")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>
> ---
> Changes in v2:
> - Drop the redundant nla_len(info->attrs[SEG6_ATTR_DST]) !=
>   sizeof(struct in6_addr) check in seg6_genl_set_tunsrc() since
>   NLA_POLICY_EXACT_LEN() in seg6_genl_policy already enforces the exact
>   length, as pointed out by Hangbin Liu.
> 
>  net/ipv6/seg6.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv6/seg6.c b/net/ipv6/seg6.c
> index 62a7eb779202..8c2b156c227a 100644
> --- a/net/ipv6/seg6.c
> +++ b/net/ipv6/seg6.c
> @@ -138,8 +138,8 @@ void seg6_icmp_srh(struct sk_buff *skb, struct inet6_skb_parm *opt)
>  static struct genl_family seg6_genl_family;
>  
>  static const struct nla_policy seg6_genl_policy[SEG6_ATTR_MAX + 1] = {
> -	[SEG6_ATTR_DST]				= { .type = NLA_BINARY,
> -		.len = sizeof(struct in6_addr) },
> +	[SEG6_ATTR_DST]				=
> +		NLA_POLICY_EXACT_LEN(sizeof(struct in6_addr)),
>  	[SEG6_ATTR_DSTLEN]			= { .type = NLA_S32, },
>  	[SEG6_ATTR_HMACKEYID]		= { .type = NLA_U32, },
>  	[SEG6_ATTR_SECRET]			= { .type = NLA_BINARY, },
> -- 
> 2.49.0

Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>