Re: [syzbot] [net?] KASAN: use-after-free Read in __skb_flow_dissect (3)

[Hillf Danton]

On Mon, 01 Jan 2024 09:18:16 -0800
> syzbot found the following issue on:
> 
> HEAD commit:    f5837722ffec Merge tag 'mm-hotfixes-stable-2023-12-27-15-0..
> git tree:       upstream
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=122dfc65e80000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git  master

--- x/net/core/filter.c
+++ y/net/core/filter.c
@@ -2144,6 +2144,7 @@ static int __bpf_redirect_no_mac(struct
 	unsigned int mlen = skb_network_offset(skb);
 
 	if (unlikely(skb->len <= mlen)) {
+out:
 		kfree_skb(skb);
 		return -ERANGE;
 	}
@@ -2159,6 +2160,8 @@ static int __bpf_redirect_no_mac(struct
 		if (!skb_at_tc_ingress(skb))
 			skb_postpull_rcsum(skb, skb_mac_header(skb), mlen);
 	}
+	if (skb->len <= sizeof(struct iphdr))
+		goto out;
 	skb_pop_mac_header(skb);
 	skb_reset_mac_len(skb);
 	return flags & BPF_F_INGRESS ?
@@ -2431,7 +2434,7 @@ enum {
 BPF_CALL_3(bpf_clone_redirect, struct sk_buff *, skb, u32, ifindex, u64, flags)
 {
 	struct net_device *dev;
-	struct sk_buff *clone;
+	struct sk_buff *buf;
 	int ret;
 
 	if (unlikely(flags & (~(BPF_F_INGRESS) | BPF_F_REDIRECT_INTERNAL)))
@@ -2441,22 +2444,11 @@ BPF_CALL_3(bpf_clone_redirect, struct sk
 	if (unlikely(!dev))
 		return -EINVAL;
 
-	clone = skb_clone(skb, GFP_ATOMIC);
-	if (unlikely(!clone))
+	buf = skb_copy(skb, GFP_ATOMIC);
+	if (unlikely(!buf))
 		return -ENOMEM;
 
-	/* For direct write, we need to keep the invariant that the skbs
-	 * we're dealing with need to be uncloned. Should uncloning fail
-	 * here, we need to free the just generated clone to unclone once
-	 * again.
-	 */
-	ret = bpf_try_make_head_writable(skb);
-	if (unlikely(ret)) {
-		kfree_skb(clone);
-		return -ENOMEM;
-	}
-
-	return __bpf_redirect(clone, dev, flags);
+	return __bpf_redirect(buf, dev, flags);
 }
 
 static const struct bpf_func_proto bpf_clone_redirect_proto = {
--