netdev - Re: [PATCH net 2/2] ip6_gre: Fix skb_under_panic in __gre6

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Date:   Fri, 15 Apr 2022 19:11:33 +0200
From:   Jakub Kicinski <kuba@...nel.org>
To:     Peilin Ye <yepeilin.cs@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Peilin Ye <peilin.ye@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        Feng Zhou <zhoufeng.zf@...edance.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 2/2] ip6_gre: Fix skb_under_panic in __gre6_xmit()

On Thu, 14 Apr 2022 13:08:54 -0700 Peilin Ye wrote:
> > We should also reject using SEQ with collect_md, but that's a separate
> > issue.  
> 
> Could you explain this a bit more?  It seems that commit 77a5196a804e
> ("gre: add sequence number for collect md mode.") added this
> intentionally.

Interesting. Maybe a better way of dealing with the problem would be
rejecting SEQ if it's not set on the device itself.

When the device is set up without the SEQ bit enabled it disables Tx
locking (look for LLTX). This means that multiple CPUs can try to do
the tunnel->o_seqno++ in parallel. Not catastrophic but racy for sure.