ppp: reject claimed-as-LCP but actually malformed packets

Since 'ppp_async_encode()' assumes valid LCP packets (with code
from 1 to 7 inclusive), add 'ppp_check_packet()' to ensure that
LCP packet has an actual body beyond PPP_LCP header bytes, and
reject claimed-as-LCP but actually malformed data otherwise.

Reported-by: syzbot+ec0723ba9605678b14bf@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ec0723ba9605678b14bf
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 0a65b6d690feb9fb5d4f1a2046d6195ac0bd39f9..eb9acfcaeb097496b5e28c87af13f5b4091a9bed 100644 (file)
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -70,6 +70,7 @@
 #define MPHDRLEN_SSN   4       /* ditto with short sequence numbers */
 
 #define PPP_PROTO_LEN  2
+#define PPP_LCP_HDRLEN 4
 
 /*
  * An instance of /dev/ppp can be associated with either a ppp
@@ -493,6 +494,15 @@ static ssize_t ppp_read(struct file *file, char __user *buf,
        return ret;
 }
 
+static bool ppp_check_packet(struct sk_buff *skb, size_t count)
+{
+       /* LCP packets must include LCP header which 4 bytes long:
+        * 1-byte code, 1-byte identifier, and 2-byte length.
+        */
+       return get_unaligned_be16(skb->data) != PPP_LCP ||
+               count >= PPP_PROTO_LEN + PPP_LCP_HDRLEN;
+}
+
 static ssize_t ppp_write(struct file *file, const char __user *buf,
                         size_t count, loff_t *ppos)
 {
@@ -515,6 +525,11 @@ static ssize_t ppp_write(struct file *file, const char __user *buf,
                kfree_skb(skb);
                goto out;
        }
+       ret = -EINVAL;
+       if (unlikely(!ppp_check_packet(skb, count))) {
+               kfree_skb(skb);
+               goto out;
+       }
 
        switch (pf->kind) {
        case INTERFACE: