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

On Mon, 2018-09-17 at 12:35 -0400, Paul Moore wrote:

On Mon, Sep 17, 2018 at 11:12 AM Stefan Nuernberger [off-list ref]
wrote:

quoted

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 <redacted>
Reviewed-by: David Woodhouse <redacted>
Reviewed-by: Simon Veith <redacted>
Cc: stable@vger.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;
    ....
  }

You're right, that looks much better. I sent around a new patch.

quoted

                optlen -= taglen;
                optptr += taglen;
        }

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