Re: [syzbot] general protection fault in smc_tx_sendmsg

[Guvenc Gulce]

On 21/06/2021 16:56, Pavel Skripkin wrote:
> On Sun, 20 Jun 2021 16:22:16 -0700
> syzbot <syzbot+5dda108b672b54141857@xxxxxxxxxxxxxxxxxxxxxxxxx> 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 <fw@xxxxxxxxx>
> > 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

Thanks for analyzing the cause. Your approach would work but I would prefer that we
check the state of the socket before doing the statistics relevant if check. This will ensure
that smc_buf_create() was already called.
I am testing the fix at the moment which would look like the following:

diff --git a/net/smc/smc_tx.c b/net/smc/smc_tx.c
index 075c4f4b41cf..289025cd545a 100644
--- a/net/smc/smc_tx.c
+++ b/net/smc/smc_tx.c
@@ -154,6 +154,9 @@ int smc_tx_sendmsg(struct smc_sock *smc, struct msghdr *msg, size_t len)
                goto out_err;
        }

+       if (sk->sk_state == SMC_INIT)
+               return -ENOTCONN;
+
        if (len > conn->sndbuf_desc->len)
                SMC_STAT_RMB_TX_SIZE_SMALL(smc, !conn->lnk);