l2tp: fix reading optional fields of L2TPv3 [Linux 5.0]

l2tp: fix reading optional fields of L2TPv3 [Linux 5.0]

This Linux kernel change "l2tp: fix reading optional fields of L2TPv3" is included in the Linux 5.0 release. This change is authored by Jacob Wen <jian.w.wen [at] oracle.com> on Wed Jan 30 14:55:14 2019 +0800. The commit for this change in Linux stable tree is 4522a70 (patch).

l2tp: fix reading optional fields of L2TPv3

Use pskb_may_pull() to make sure the optional fields are in skb linear
parts, so we can safely read them later.

It's easy to reproduce the issue with a net driver that supports paged
skb data. Just create a L2TPv3 over IP tunnel and then generates some
network traffic.
Once reproduced, rx err in /sys/kernel/debug/l2tp/tunnels will increase.

Changes in v4:
1. s/l2tp_v3_pull_opt/l2tp_v3_ensure_opt_in_linear/
2. s/tunnel->version != L2TP_HDR_VER_2/tunnel->version == L2TP_HDR_VER_3/
3. Add 'Fixes' in commit messages.

Changes in v3:
1. To keep consistency, move the code out of l2tp_recv_common.
2. Use "net" instead of "net-next", since this is a bug fix.

Changes in v2:
1. Only fix L2TPv3 to make code simple.
   To fix both L2TPv3 and L2TPv2, we'd better refactor l2tp_recv_common.
   It's complicated to do so.
2. Reloading pointers after pskb_may_pull

Fixes: f7faffa3ff8e ("l2tp: Add L2TPv3 protocol support")
Fixes: 0d76751fad77 ("l2tp: Add L2TPv3 IP encapsulation (no UDP) support")
Fixes: a32e0eec7042 ("l2tp: introduce L2TPv3 IP encapsulation support for IPv6")
Signed-off-by: Jacob Wen <[email protected]>
Acked-by: Guillaume Nault <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

There are 30 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 net/l2tp/l2tp_core.c |  4 ++++
 net/l2tp/l2tp_core.h | 20 ++++++++++++++++++++
 net/l2tp/l2tp_ip.c   |  3 +++
 net/l2tp/l2tp_ip6.c  |  3 +++
 4 files changed, 30 insertions(+)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 26f1d43..dd5ba0c 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -884,6 +884,10 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
        goto error;
    }

+   if (tunnel->version == L2TP_HDR_VER_3 &&
+       l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr))
+       goto error;
+
    l2tp_recv_common(session, skb, ptr, optr, hdrflags, length);
    l2tp_session_dec_refcount(session);

diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 9c9afe9..b2ce902 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -301,6 +301,26 @@ static inline bool l2tp_tunnel_uses_xfrm(const struct l2tp_tunnel *tunnel)
 }
 #endif

+static inline int l2tp_v3_ensure_opt_in_linear(struct l2tp_session *session, struct sk_buff *skb,
+                          unsigned char **ptr, unsigned char **optr)
+{
+   int opt_len = session->peer_cookie_len + l2tp_get_l2specific_len(session);
+
+   if (opt_len > 0) {
+       int off = *ptr - *optr;
+
+       if (!pskb_may_pull(skb, off + opt_len))
+           return -1;
+
+       if (skb->data != *optr) {
+           *optr = skb->data;
+           *ptr = skb->data + off;
+       }
+   }
+
+   return 0;
+}
+
 #define l2tp_printk(ptr, type, func, fmt, ...)             \
 do {                                   \
    if (((ptr)->debug) & (type))                    \
diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index 35f6f86..d4c6052 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -165,6 +165,9 @@ static int l2tp_ip_recv(struct sk_buff *skb)
        print_hex_dump_bytes("", DUMP_PREFIX_OFFSET, ptr, length);
    }

+   if (l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr))
+       goto discard_sess;
+
    l2tp_recv_common(session, skb, ptr, optr, 0, skb->len);
    l2tp_session_dec_refcount(session);

diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index 237f1a4..0ae6899 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -178,6 +178,9 @@ static int l2tp_ip6_recv(struct sk_buff *skb)
        print_hex_dump_bytes("", DUMP_PREFIX_OFFSET, ptr, length);
    }

+   if (l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr))
+       goto discard_sess;
+
    l2tp_recv_common(session, skb, ptr, optr, 0, skb->len);
    l2tp_session_dec_refcount(session);

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