Re: [syzbot] KASAN: use-after-free Read in tipc_recvmsg

[Pavel Skripkin <paskripkin@gmail.com>]

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On Sun, 18 Jul 2021 10:15:19 -0700
syzbot <syzbot+e6741b97d5552f97c24d@syzkaller.appspotmail.com> wrote:

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    ab0441b4a920 Merge branch 'vmxnet3-version-6'
> git tree:       net-next
> console output:
> https://syzkaller.appspot.com/x/log.txt?x=1744ac6a300000 kernel
> config:  https://syzkaller.appspot.com/x/.config?x=da140227e4f25b17
> dashboard link:
> https://syzkaller.appspot.com/bug?extid=e6741b97d5552f97c24d syz
> repro:
> https://syzkaller.appspot.com/x/repro.syz?x=13973a74300000 C
> reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17ffc902300000
> 
> The issue was bisected to:
> 
> commit 67a3156453859ceb40dc4448b7a6a99ea0ad27c7
> Author: Rob Herring <robh@kernel.org>
> Date:   Thu May 27 19:45:47 2021 +0000
> 
>     of: Merge of_address_to_resource() and
> of_pci_address_to_resource() implementations
> 
> bisection log:
> https://syzkaller.appspot.com/x/bisect.txt?x=129b0438300000 final
> oops:     https://syzkaller.appspot.com/x/report.txt?x=119b0438300000
> console output:
> https://syzkaller.appspot.com/x/log.txt?x=169b0438300000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the
> commit: Reported-by:
> syzbot+e6741b97d5552f97c24d@syzkaller.appspotmail.com Fixes:
> 67a315645385 ("of: Merge of_address_to_resource() and
> of_pci_address_to_resource() implementations")
> 
> ==================================================================
> BUG: KASAN: use-after-free in tipc_recvmsg+0xf77/0xf90
> net/tipc/socket.c:1979 Read of size 4 at addr ffff8880328cf1c0 by
> task kworker/u4:0/8
> 

Since code accesing skb_cb after possible kfree_skb() call let's just
store bytes_read to variable and use it instead of acessing
skb_cb->bytes_read

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



With regards,
Pavel Skripkin

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From 9f81f8574bfc1183209022b405848e01c35b86e6 Mon Sep 17 00:00:00 2001
From: Pavel Skripkin <paskripkin@gmail.com>
Date: Fri, 23 Jul 2021 19:34:06 +0300
Subject: [PATCH] tipc: fix use-after-free in tipc_recvmsg

/* .. */

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---
 net/tipc/socket.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index 9b0b311c7ec1..0cf2468d209d 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -1886,6 +1886,7 @@ static int tipc_recvmsg(struct socket *sock, struct msghdr *m,
 	struct sk_buff *skb;
 	bool grp_evt;
 	long timeout;
+	unsigned int bytes_read;
 
 	/* Catch invalid receive requests */
 	if (unlikely(!buflen))
@@ -1973,10 +1974,13 @@ static int tipc_recvmsg(struct socket *sock, struct msghdr *m,
 		tipc_node_distr_xmit(sock_net(sk), &xmitq);
 	}
 
-	if (!skb_cb->bytes_read)
+	/* To avoid accesing skb_cb after tsk_advance_rx_queue */
+	bytes_read = skb_cb->bytes_read;
+
+	if (!bytes_read)
 		tsk_advance_rx_queue(sk);
 
-	if (likely(!connected) || skb_cb->bytes_read)
+	if (likely(!connected) || bytes_read)
 		goto exit;
 
 	/* Send connection flow control advertisement when applicable */
-- 
2.32.0