netdev - Re: [PATCH net] net/ipv4: defensive cipso option parsing

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Message-ID: <CAHC9VhQ+VKPFmx3R7Ty60KAJhiZwnc2-ZKRYG9w2NSAH7vgnoQ@mail.gmail.com>
Date:   Mon, 17 Sep 2018 12:35:26 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     snu@...zon.com
Cc:     netdev@...r.kernel.org, aams@...zon.de, yujuan.qi@...iatek.com,
        stable@...r.kernel.org
Subject: Re: [PATCH net] net/ipv4: defensive cipso option parsing

On Mon, Sep 17, 2018 at 11:12 AM Stefan Nuernberger <snu@...zon.com> wrote:
> commit 40413955ee26 ("Cipso: cipso_v4_optptr enter infinite loop") fixed
> a possible infinite loop in the IP option parsing of CIPSO. The fix
> assumes that ip_options_compile filtered out all zero length options and
> that no other one-byte options beside IPOPT_END and IPOPT_NOOP exist.
> While this assumption currently holds true, add explicit checks for zero
> length and invalid length options to be safe for the future. Even though
> ip_options_compile should have validated the options, the introduction of
> new one-byte options can still confuse this code without the additional
> checks.
>
> Signed-off-by: Stefan Nuernberger <snu@...zon.com>
> Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>
> Reviewed-by: Simon Veith <sveith@...zon.de>
> Cc: stable@...r.kernel.org
> ---
>  net/ipv4/cipso_ipv4.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/cipso_ipv4.c b/net/ipv4/cipso_ipv4.c
> index 82178cc69c96..f291b57b8474 100644
> --- a/net/ipv4/cipso_ipv4.c
> +++ b/net/ipv4/cipso_ipv4.c
> @@ -1512,7 +1512,7 @@ static int cipso_v4_parsetag_loc(const struct cipso_v4_doi *doi_def,
>   *
>   * Description:
>   * Parse the packet's IP header looking for a CIPSO option.  Returns a pointer
> - * to the start of the CIPSO option on success, NULL if one if not found.
> + * to the start of the CIPSO option on success, NULL if one is not found.
>   *
>   */
>  unsigned char *cipso_v4_optptr(const struct sk_buff *skb)
> @@ -1522,9 +1522,11 @@ unsigned char *cipso_v4_optptr(const struct sk_buff *skb)
>         int optlen;
>         int taglen;
>
> -       for (optlen = iph->ihl*4 - sizeof(struct iphdr); optlen > 0; ) {
> +       for (optlen = iph->ihl*4 - sizeof(struct iphdr); optlen > 1; ) {
>                 switch (optptr[0]) {
>                 case IPOPT_CIPSO:
> +                       if (!optptr[1] || optptr[1] > optlen)
> +                               return NULL;
>                         return optptr;
>                 case IPOPT_END:
>                         return NULL;
> @@ -1534,6 +1536,10 @@ unsigned char *cipso_v4_optptr(const struct sk_buff *skb)
>                 default:
>                         taglen = optptr[1];
>                 }
> +
> +               if (!taglen || taglen > optlen)
> +                       break;

I tend to think that you reach a point where you simply need to trust
that the stack is doing the right thing and that by the time you hit a
certain point you can safely assume that the packet is well formed,
but I'm not going to fight about that here.

Regardless of the above, I don't like how you're doing the option
length check twice in this code, that looks ugly to me, I think we can
do better.  How about something like this:

  for (...) {
    switch(optptr[0]) {
    case IPOPT_END:
      return NULL;
    case IPOPT_NOOP:
      taglen = 1;
    default:
      taglen = optptr[1];
    }
    if (taglen == 0 || taglen > optlen)
      return NULL;
    if (optptr[0] == IPOPT_CIPSO)
      return optptr;
    ....
  }

>                 optlen -= taglen;
>                 optptr += taglen;
>         }

-- 
paul moore
www.paul-moore.com