Subject: Re: [PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

From: Simon Horman <[email-masked]>
Date: Wed, 22 Jan 2025 15:51:31 +0000

On Wed, Jan 22, 2025 at 04:49:17PM +0300, Dan Carpenter wrote:
> The "payload" variable is type size_t, however the nlmsg_total_size()
> function will a few bytes to it and then truncate the result to type
> int.  That means that if "payload" is more than UINT_MAX the alloc_skb()
> function might allocate a buffer which is smaller than intended.
> 
> Cc: [email-masked]
> Fixes: bfa83a9e03cf ("[NETLINK]: Type-safe netlink messages/attributes interface")
> Signed-off-by: Dan Carpenter <[email-masked]>
> ---
>  include/net/netlink.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/net/netlink.h b/include/net/netlink.h
> index e015ffbed819..ca7a8152e6d4 100644
> --- a/include/net/netlink.h
> +++ b/include/net/netlink.h
> @@ -1015,6 +1015,8 @@ static inline struct nlmsghdr *nlmsg_put_answer(struct sk_buff *skb,
>   */
>  static inline struct sk_buff *nlmsg_new(size_t payload, gfp_t flags)
>  {
> +	if (payload > INT_MAX)
> +		return NULL;
>  	return alloc_skb(nlmsg_total_size(payload), flags);

Hi Dan,

I wonder if this is sufficient.

If payload is INT_MAX then won't the call to nlmsg_msg_size() inside
nlmsg_total_size() overflow. And likewise, it feels that NLMSG_ALIGN
could overflow somehow.