netdev - Re: net/dst_cache.c: preemption bug in net/dst

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Date:   Wed, 18 Sep 2019 15:49:04 +0530
From:   Bharath Vedartham <linux.bhar@...il.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     davem <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        allison@...utok.net, tglx@...utronix.de,
        network dev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jon Maloy <jon.maloy@...csson.com>
Subject: Re: net/dst_cache.c: preemption bug in net/dst_cache.c

On Mon, Sep 09, 2019 at 05:48:25PM +0800, Xin Long wrote:
> On Fri, Aug 23, 2019 at 3:58 PM Bharath Vedartham <linux.bhar@...il.com> wrote:
> >
> > Hi all,
> >
> > I just want to bring attention to the syzbot bug [1]
> >
> > Even though syzbot claims the bug to be in net/tipc, I feel it is in
> > net/dst_cache.c. Please correct me if I am wrong.
> >
> > This bug is being triggered a lot of times by syzbot since the day it
> > was reported. Also given that this is core networking code, I felt it
> > was important to bring this to attention.
> >
> > It looks like preemption needs to be disabled before using this_cpu_ptr
> > or maybe we would be better of using a get_cpu_var and put_cpu_var combo
> > here.
> b->media->send_msg (tipc_udp_send_msg)
> -> tipc_udp_xmit() -> dst_cache_get()
> 
> send_msg() is always called under the protection of rcu_read_lock(), which
> already disabled preemption. If not, there must be some unbalanced calls of
> disable/enable preemption elsewhere.
> 
> Agree that this could be a serious issue, do you have any reproducer for this?
> 
> Thanks.
Hi Xin,

Sorry for the delayed response. I do not have a reproducer for this. You
can submit a patch to syzbot which can run the patch on the same system
on which it found the bug.

Thank you
Bharath
> >
> > [1] https://syzkaller.appspot.com/bug?id=dc6352b92862eb79373fe03fdf9af5928753e057
> >
> > Thank you
> > Bharath