Subject: Re: [PATCH ipsec-next v7 07/16] xfrm: iptfs: add new iptfs xfrm mode impl

From: Florian Westphal <[email-masked]>
Date: Thu, 1 Aug 2024 14:13:10 +0200

Christian Hopps <[email-masked]> wrote:
> +static int __iptfs_init_state(struct xfrm_state *x,
> +			      struct xfrm_iptfs_data *xtfs)
> +{
> +	/* Modify type (esp) adjustment values */
> +
> +	if (x->props.family == AF_INET)
> +		x->props.header_len += sizeof(struct iphdr) + sizeof(struct ip_iptfs_hdr);
> +	else if (x->props.family == AF_INET6)
> +		x->props.header_len += sizeof(struct ipv6hdr) + sizeof(struct ip_iptfs_hdr);
> +	x->props.enc_hdr_len = sizeof(struct ip_iptfs_hdr);
> +
> +	/* Always have a module reference if x->mode_data is set */
> +	if (!try_module_get(x->mode_cbs->owner))
> +		return -EINVAL;

If the comment means that we already have a module owner ref taken
before this try_module_get, then this should use __module_get and
a mention where the first ref was taken.

If not, then this needs an explanation as to what prevents another cpu to
rmmod the owning module between the lookup in xfrm_init_state and the
module reference in __iptfs_init_state.

cpu0					cpu1
 xfrm_init_state
   -> xfrm_get_mode_cbs                 rmmod
     -> __iptfs_init_state		  xfrm_iptfs_fini
       <interrupt>                         xfrm_unregister_mode_cbs
                                            release memory
       <resume>
	try_module_get -> UaF