[PATCH net 4/4] netlabel: validate CIPSO option against skb tail in netlbl_skbuff_getattr

[Paul Moore]

On Thu, May 14, 2026 at 12:52 PM Qi Tang <tpluszz77 at gmail.com> wrote:
>
> netlbl_skbuff_getattr() locates the CIPSO option in the IPv4 IP header
> via cipso_v4_optptr() and hands the bare pointer to cipso_v4_getattr().
> The consumer re-reads cipso[1] (option length), cipso[6] (tag type),
> and then cipso_v4_parsetag_*() re-reads further bytes from the skb.
>
> __ip_options_compile() validates these bytes only at parse time.  An
> nftables LOCAL_IN payload write reachable from an unprivileged user
> namespace can rewrite them after parse and before the SELinux/Smack
> peer-label consume path (selinux_sock_rcv_skb_compat ->
> selinux_netlbl_sock_rcv_skb -> netlbl_skbuff_getattr).  This is the
> IPv4 analogue of the CALIPSO IPv6 trust-after-modification fixed in
> the previous patch: the tag parsers walk the option using attacker-
> controlled length bytes, producing slab-out-of-bounds reads whose
> contents feed into the MLS access decision.
>
> Validate the option fits within skb_tail_pointer(skb) before invoking
> cipso_v4_getattr().
>
> Runtime confirmation (Smack peer-label policy + nft LOCAL_IN
> mutation of tag_len): UdpInDatagrams increments to 1 and recvfrom
> returns the payload, showing netlbl_skbuff_getattr ->
> cipso_v4_getattr -> cipso_v4_parsetag_rbm -> netlbl_bitmap_walk runs
> end-to-end past the option's true bound; with this patch the
> consume path short-circuits at the bounds check and the counter
> stays 0.
>
> Reported-by: Qi Tang <tpluszz77 at gmail.com>
> Reported-by: Tong Liu <lyutoon at gmail.com>
> Fixes: 04f81f0154e4 ("cipso: don't use IPCB() to locate the CIPSO IP option")
> Signed-off-by: Qi Tang <tpluszz77 at gmail.com>
> ---
>  net/netlabel/netlabel_kapi.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/net/netlabel/netlabel_kapi.c b/net/netlabel/netlabel_kapi.c
> index 4af8ab76964e0..ace561a2904a4 100644
> --- a/net/netlabel/netlabel_kapi.c
> +++ b/net/netlabel/netlabel_kapi.c
> @@ -1393,11 +1393,21 @@ int netlbl_skbuff_getattr(const struct sk_buff *skb,
>         unsigned char *ptr;
>
>         switch (family) {
> -       case AF_INET:
> +       case AF_INET: {
> +               const unsigned char *tail = skb_tail_pointer(skb);
> +               u8 opt_len, tag_len;
> +
>                 ptr = cipso_v4_optptr(skb);
> -               if (ptr && cipso_v4_getattr(ptr, secattr) == 0)
> +               if (!ptr || ptr + 8 > tail)
> +                       break;

Similar to my CALIPSO comments, I suspect we would want to return an
error here, yes?

Also, how did you arrive at the magic number of '8' above?

> +               opt_len = ptr[1];       /* total CIPSO option length */
> +               tag_len = ptr[7];       /* first tag length */
> +               if (ptr + opt_len > tail || ptr + 6 + tag_len > tail)
> +                       break;
> +               if (cipso_v4_getattr(ptr, secattr) == 0)
>                         return 0;
>                 break;
> +       }
>  #if IS_ENABLED(CONFIG_IPV6)
>         case AF_INET6: {
>                 const unsigned char *tail = skb_tail_pointer(skb);
> --
> 2.47.3

-- 
paul-moore.com