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

JIRA: https://issues.redhat.com/browse/RHEL-51059
Upstream Status: linux.git
CVE: CVE-2024-41044

commit f2aeb73
Author: Dmitry Antipov <dmantipov@yandex.ru>
Date:   Mon Jul 8 14:56:15 2024 +0300

    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: 1da177e ("Linux-2.6.12-rc2")
    Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
    Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Signed-off-by: Guillaume Nault <gnault@redhat.com>

Author: Guillaume Nault

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
@@ -491,6 +492,15 @@ 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)
 {
@@ -513,6 +523,11 @@ 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: