[PATCH net v2 4/4] netlabel: validate CIPSO option against skb tail in netlbl_skbuff_getattr

[Paul Moore]

On Sun, May 24, 2026 at 12:15 AM 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().  The pre-tag-walk guard "ptr + 8 > tail" covers
> the CIPSO option header (type + length + DOI = 6 bytes) plus the
> first tag header (type + length = 2 bytes), which are the bytes
> cipso_v4_getattr() reads to dispatch on the tag.  When the bounds
> check fails the packet has been mutated after parse, so return
> -EINVAL rather than fall through to the unlabeled path.
>
> 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 returns -EINVAL at the bounds check and the counter
> stays 0.
>
> Cc: stable at vger.kernel.org
> 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 | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/net/netlabel/netlabel_kapi.c b/net/netlabel/netlabel_kapi.c
> index d0d6220b8d59d..c2d3ea751f4e1 100644
> --- a/net/netlabel/netlabel_kapi.c
> +++ b/net/netlabel/netlabel_kapi.c
> @@ -1393,11 +1393,24 @@ 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)
> +                       break;
> +               /* CIPSO header (type+len+DOI = 6) + first tag header (type+len = 2) */
> +               if (ptr + 8 > tail)
> +                       return -EINVAL;
> +               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)
> +                       return -EINVAL;
> +               if (cipso_v4_getattr(ptr, secattr) == 0)
>                         return 0;

I'd strongly prefer if you moved the tag length check into
cipso_v4_getattr().  As you've already validated the CIPSO option
length field it should be a fairly easy check, no need to test it
against the skb's tail pointer, just ensure the tag length doesn't go
past the end of the CIPSO option.

>                 break;
> +       }
>  #if IS_ENABLED(CONFIG_IPV6)
>         case AF_INET6: {
>                 const unsigned char *tail = skb_tail_pointer(skb);
> --
> 2.47.3

-- 
paul-moore.com