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Date:   Thu, 23 Sep 2021 12:34:42 +0300
From:   Dan Carpenter <dan.carpenter@oracle.com>
To:     Xin Long <lucien.xin@gmail.com>
Cc:     syzbot <syzbot+581aff2ae6b860625116@syzkaller.appspotmail.com>,
        davem <davem@davemloft.net>, Jakub Kicinski <kuba@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "linux-sctp @ vger . kernel . org" <linux-sctp@vger.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>,
        network dev <netdev@vger.kernel.org>,
        Neil Horman <nhorman@tuxdriver.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Vlad Yasevich <vyasevich@gmail.com>
Subject: Re: [syzbot] general protection fault in sctp_rcv
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On Wed, Sep 22, 2021 at 05:18:29PM +0800, Xin Long wrote:
> On Tue, Sep 21, 2021 at 12:09 PM syzbot
> <syzbot+581aff2ae6b860625116@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    98dc68f8b0c2 selftests: nci: replace unsigned int with int
> > git tree:       net
> > console output: https://syzkaller.appspot.com/x/log.txt?x=11fd443d300000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=c31c0936547df9ea
> > dashboard link: https://syzkaller.appspot.com/bug?extid=581aff2ae6b860625116
> > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+581aff2ae6b860625116@syzkaller.appspotmail.com
> >
> > general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
> > KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> > CPU: 0 PID: 11205 Comm: kworker/0:12 Not tainted 5.14.0-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Workqueue: ipv6_addrconf addrconf_dad_work
> > RIP: 0010:sctp_rcv_ootb net/sctp/input.c:705 [inline]
> by anyway, checking if skb_header_pointer() return NULL is always needed:
> @@ -702,7 +702,7 @@ static int sctp_rcv_ootb(struct sk_buff *skb)
>                 ch = skb_header_pointer(skb, offset, sizeof(*ch), &_ch);
> 
>                 /* Break out if chunk length is less then minimal. */
> -               if (ntohs(ch->length) < sizeof(_ch))
> +               if (!ch || ntohs(ch->length) < sizeof(_ch))
>                         break;
> 

The skb_header_pointer() function is annotated as __must_check but that
only means you have to use the return value.  These things would be
better as a Coccinelle or Smatch check.

I will create a Smatch warning for this.

regards,
dan carpenter