linux-kernel - Re: [syzbot] [net?] KASAN: use-after-free Read in __skb_flow

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Message-Id: <20240118221554.1749-1-hdanton@sina.com>
Date: Fri, 19 Jan 2024 06:15:54 +0800
From: Hillf Danton <hdanton@...a.com>
To: syzbot <syzbot+bfde3bef047a81b8fde6@...kaller.appspotmail.com>
Cc: linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [net?] KASAN: use-after-free Read in __skb_flow_dissect (3)

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
@@ -2148,17 +2148,6 @@ static int __bpf_redirect_no_mac(struct
 		return -ERANGE;
 	}
 
-	if (mlen) {
-		__skb_pull(skb, mlen);
-
-		/* At ingress, the mac header has already been pulled once.
-		 * At egress, skb_pospull_rcsum has to be done in case that
-		 * the skb is originated from ingress (i.e. a forwarded skb)
-		 * to ensure that rcsum starts at net header.
-		 */
-		if (!skb_at_tc_ingress(skb))
-			skb_postpull_rcsum(skb, skb_mac_header(skb), mlen);
-	}
 	skb_pop_mac_header(skb);
 	skb_reset_mac_len(skb);
 	return flags & BPF_F_INGRESS ?
--