Subject: Re: [syzbot] general protection fault in smc_tx_sendmsg

From: Pavel Skripkin <[email-masked]>
Date: Mon, 21 Jun 2021 17:56:03 +0300

On Sun, 20 Jun 2021 16:22:16 -0700
syzbot <[email-masked]> wrote:

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    0c337952 Merge tag 'wireless-drivers-next-2021-06-16'
> of g.. git tree:       net-next
> console output:
> https://syzkaller.appspot.com/x/log.txt?x=1621de10300000 kernel
> config:  https://syzkaller.appspot.com/x/.config?x=a6380da8984033f1
> dashboard link:
> https://syzkaller.appspot.com/bug?extid=5dda108b672b54141857 syz
> repro:
> https://syzkaller.appspot.com/x/repro.syz?x=121d2d20300000 C
> reproducer:   https://syzkaller.appspot.com/x/repro.c?x=100bd768300000
> 
> The issue was bisected to:
> 
> commit f9006acc8dfe59e25aa75729728ac57a8d84fc32
> Author: Florian Westphal <[email-masked]>
> Date:   Wed Apr 21 07:51:08 2021 +0000
> 
>     netfilter: arp_tables: pass table pointer via nf_hook_ops
> 

I think, bisection is wrong this time :)

It should be e0e4b8fa533858532f1b9ea9c6a4660d09beb37a ("net/smc: Add SMC
statistics support")

Some debug results:

syzkaller repro just opens the socket and calls sendmsg. Ftrace log:

 0)               |  smc_create() {
 0)               |    smc_sock_alloc() {
 0) + 88.493 us   |      smc_hash_sk();
 0) ! 131.487 us  |    }
 0) ! 189.912 us  |  }
 0)               |  smc_sendmsg() {
 0)   2.808 us    |    smc_tx_sendmsg();
 0) ! 148.484 us  |  }

That means, that smc_buf_create() wasn't called at all, so we need to
check sndbuf_desc before dereferencing

Something like this should work

diff --git a/net/smc/smc_tx.c b/net/smc/smc_tx.c
index 075c4f4b4..e24071b12 100644
--- a/net/smc/smc_tx.c
+++ b/net/smc/smc_tx.c
@@ -154,7 +154,7 @@ int smc_tx_sendmsg(struct smc_sock *smc, struct msghdr *msg, size_t len)
 		goto out_err;
 	}
 
-	if (len > conn->sndbuf_desc->len)
+	if (conn->sndbuf_desc && len > conn->sndbuf_desc->len)
 		SMC_STAT_RMB_TX_SIZE_SMALL(smc, !conn->lnk);
 
 	if (len > conn->peer_rmbe_size)

Thoughts?

+CC Guvenc Gulce 

With regards,
Pavel Skripkin