Re: [PATCH net-next v1] l2tp: fix reading optional fields

[Jacob Wen]

On 1/23/19 10:51 AM, Eric Dumazet wrote:

On 01/22/2019 06:30 PM, Jacob Wen wrote:

Use pskb_may_pull() to make sure the optional fields are in skb linear
parts.

Signed-off-by: Jacob Wen <jian.w....@oracle.com>
---
v1: fix warning: ISO C90 forbids mixed declarations and code
---
  net/l2tp/l2tp_core.c | 12 +++++++++++-
  1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 26f1d435696a..c0dfa2bcd218 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -627,6 +627,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct 
sk_buff *skb,

/* Parse and check optional cookie */

        if (session->peer_cookie_len > 0) {
+               if (!pskb_may_pull(skb, ptr - optr + session->peer_cookie_len))
+                       goto discard;

This is completely buggy.

After pskb_may_pull(), the ptr and optr pointers might point to freed data.

So the memcmp() might crash hard.

Fixed in v2. Thanks.

                if (memcmp(ptr, &session->peer_cookie[0], 
session->peer_cookie_len)) {
                        l2tp_info(tunnel, L2TP_MSG_DATA,
                                  "%s: cookie mismatch (%u/%u). Discarding.\n",
@@ -649,6 +651,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct 
sk_buff *skb,
        L2TP_SKB_CB(skb)->has_seq = 0;
        if (tunnel->version == L2TP_HDR_VER_2) {
                if (hdrflags & L2TP_HDRFLAG_S) {
+                       if (!pskb_may_pull(skb, ptr - optr + 4))
+                               goto discard;
                        ns = ntohs(*(__be16 *) ptr);
                        ptr += 2;
                        nr = ntohs(*(__be16 *) ptr);
@@ -663,8 +667,12 @@ void l2tp_recv_common(struct l2tp_session *session, struct 
sk_buff *skb,
                                 session->name, ns, nr, session->nr);
                }
        } else if (session->l2specific_type == L2TP_L2SPECTYPE_DEFAULT) {
-               u32 l2h = ntohl(*(__be32 *) ptr);
+               u32 l2h;
+
+               if (!pskb_may_pull(skb, ptr - optr + 4))
+                       goto discard;

+ l2h = ntohl(*(__be32 *) ptr);

                if (l2h & 0x40000000) {
                        ns = l2h & 0x00ffffff;

@@ -729,6 +737,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,

        if (tunnel->version == L2TP_HDR_VER_2) {
                /* If offset bit set, skip it. */
                if (hdrflags & L2TP_HDRFLAG_O) {
+                       if (!pskb_may_pull(skb, ptr - optr + 2))
+                               goto discard;
                        offset = ntohs(*(__be16 *)ptr);
                        ptr += 2 + offset;
                }

--
Jacob