netdev - [PATCH net v2] ppp: reject claimed-as-LCP but actually malformed packets

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Message-ID: <20240708115615.134770-1-dmantipov@yandex.ru>
Date: Mon,  8 Jul 2024 14:56:15 +0300
From: Dmitry Antipov <dmantipov@...dex.ru>
To: Simon Horman <horms@...nel.org>
Cc: "David S . Miller" <davem@...emloft.net>,
	"Ricardo B . Marliere" <ricardo@...liere.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	linux-ppp@...r.kernel.org,
	netdev@...r.kernel.org,
	lvc-project@...uxtesting.org,
	Dmitry Antipov <dmantipov@...dex.ru>,
	syzbot+ec0723ba9605678b14bf@...kaller.appspotmail.com
Subject: [PATCH net v2] 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@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ec0723ba9605678b14bf
Fixes: 44073187990d ("ppp: ensure minimum packet size in ppp_write()")
Signed-off-by: Dmitry Antipov <dmantipov@...dex.ru>
---
v2: style, comments, and metadata adjustments suggested by Simon Horman
---
 drivers/net/ppp/ppp_generic.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 0a65b6d690fe..eb9acfcaeb09 100644
--- 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:
-- 
2.45.2