netdev - [PATCH 2/2] can: af_can: prevent potential access of uninitialized member in canfd

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Message-Id: <20201103213906.24219-3-anant.thazhemadam@gmail.com>
Date:   Wed,  4 Nov 2020 03:09:06 +0530
From:   Anant Thazhemadam <anant.thazhemadam@...il.com>
To:     socketcan@...tkopp.net, mkl@...gutronix.de, davem@...emloft.net,
        kuba@...nel.org
Cc:     linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Anant Thazhemadam <anant.thazhemadam@...il.com>,
        syzbot+9bcb0c9409066696d3aa@...kaller.appspotmail.com
Subject: [PATCH 2/2] can: af_can: prevent potential access of uninitialized member in canfd_rcv()

In canfd_rcv(), cfd->len is uninitialized when skb->len = 0, and this
uninitialized cfd->len is accessed nonetheless by pr_warn_once().

Fix this uninitialized variable access by checking cfd->len's validity
condition (cfd->len > CANFD_MAX_DLEN) separately after the skb->len's
condition is checked, and appropriately modify the log messages that
are generated as well.
In case either of the required conditions fail, the skb is freed and
NET_RX_DROP is returned, same as before.

Fixes: d4689846881d ("can: af_can: canfd_rcv(): replace WARN_ONCE by pr_warn_once")
Reported-by: syzbot+9bcb0c9409066696d3aa@...kaller.appspotmail.com
Tested-by: Anant Thazhemadam <anant.thazhemadam@...il.com>
Signed-off-by: Anant Thazhemadam <anant.thazhemadam@...il.com>
---
This patch was locally tested using the reproducer and .config file
generated by syzbot.

 net/can/af_can.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/net/can/af_can.c b/net/can/af_can.c
index 8ea01524f062..d759334f8843 100644
--- a/net/can/af_can.c
+++ b/net/can/af_can.c
@@ -703,16 +703,25 @@ static int canfd_rcv(struct sk_buff *skb, struct net_device *dev,
 {
 	struct canfd_frame *cfd = (struct canfd_frame *)skb->data;
 
-	if (unlikely(dev->type != ARPHRD_CAN || skb->len != CANFD_MTU ||
-		     cfd->len > CANFD_MAX_DLEN)) {
-		pr_warn_once("PF_CAN: dropped non conform CAN FD skbuf: dev type %d, len %d, datalen %d\n",
+	if (unlikely(dev->type != ARPHRD_CAN || skb->len != CANFD_MTU)) {
+		pr_warn_once("PF_CAN: dropped non conform CAN FD skbuff: dev type %d, len %d\n",
+			     dev->type, skb->len);
+		goto free_skb;
+	}
+
+	/* This check is made separately since cfd->len would be uninitialized if skb->len = 0. */
+	if (unlikely(cfd->len > CANFD_MAX_DLEN)) {
+		pr_warn_once("PF_CAN: dropped non conform CAN FD skbuff: dev type %d, len %d, datalen %d\n",
 			     dev->type, skb->len, cfd->len);
-		kfree_skb(skb);
-		return NET_RX_DROP;
+		goto free_skb;
 	}
 
 	can_receive(skb, dev);
 	return NET_RX_SUCCESS;
+
+free_skb:
+	kfree_skb(skb);
+	return NET_RX_DROP;
 }
 
 /* af_can protocol functions */
-- 
2.25.1