Re: [PATCH] netfilter: ipset: harden payload calculation in call_ad()

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Fri, Mar 13, 2026 at 01:01:32PM -0500, David Baum wrote:
> call_ad() computes the netlink error payload size with
> min(SIZE_MAX, sizeof(*errmsg) + nlmsg_len(nlh)), but min(SIZE_MAX, x)
> is always x, so the guard is a no-op.
> 
> Replace it with an explicit negative-length check and
> check_add_overflow() so the addition is validated before being passed
> to nlmsg_new().
> 
> Signed-off-by: David Baum <davidbaum461@gmail.com>
> ---
>  net/netfilter/ipset/ip_set_core.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/ipset/ip_set_core.c b/net/netfilter/ipset/ip_set_core.c
> index a2fe711cb5e3..11d3854d9b11 100644
> --- a/net/netfilter/ipset/ip_set_core.c
> +++ b/net/netfilter/ipset/ip_set_core.c
> @@ -10,6 +10,7 @@
>  #include <linux/module.h>
>  #include <linux/moduleparam.h>
>  #include <linux/ip.h>
> +#include <linux/overflow.h>
>  #include <linux/skbuff.h>
>  #include <linux/spinlock.h>
>  #include <linux/rculist.h>
> @@ -1763,13 +1764,18 @@ call_ad(struct net *net, struct sock *ctnl, struct sk_buff *skb,
>  		struct nlmsghdr *rep, *nlh = nlmsg_hdr(skb);
>  		struct sk_buff *skb2;
>  		struct nlmsgerr *errmsg;
> -		size_t payload = min(SIZE_MAX,
> -				     sizeof(*errmsg) + nlmsg_len(nlh));
> +		int nlmsg_payload_len = nlmsg_len(nlh);
> +		size_t payload;
>  		int min_len = nlmsg_total_size(sizeof(struct nfgenmsg));
>  		struct nlattr *cda[IPSET_ATTR_CMD_MAX + 1];
>  		struct nlattr *cmdattr;
>  		u32 *errline;
>  
> +		if (nlmsg_payload_len < 0 ||

Hm, nlh was already sanitized by nfnetlink?

> +		    check_add_overflow(sizeof(*errmsg),
> +				       (size_t)nlmsg_payload_len, &payload))

Maybe cap this to int:

                int payload = sizeof(struct nlmsgerr);
                ...
        
                if (check_add_overflow(payload, nlmsg_payload_len, &payload))

> +			return -ENOMEM;
> +
>  		skb2 = nlmsg_new(payload, GFP_KERNEL);
>  		if (!skb2)
>  			return -ENOMEM;