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

From: Pavel Skripkin <[email-masked]>
Date: Fri, 23 Jul 2021 19:49:32 +0300

On Fri, 23 Jul 2021 19:36:11 +0300
Pavel Skripkin <[email-masked]> wrote:

> On Sun, 18 Jul 2021 10:15:19 -0700
> syzbot <[email-masked]> 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 <[email-masked]>
> > 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:
> > [email-masked] 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
> 
> 

Oops... The buggy code is in -next tree.

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

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

/* .. */

Signed-off-by: Pavel Skripkin <[email-masked]>
---
 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