linux-kernel - Re: [EXTERNAL] Re: [PATCH V4 net] net: mana: Fix MANA VF unload when host is unresponsive

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Message-ID: <ZKxIFK2nQqV9AvIA@ziepe.ca>
Date:   Mon, 10 Jul 2023 15:04:04 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Haiyang Zhang <haiyangz@...rosoft.com>
Cc:     Alexander Lobakin <aleksander.lobakin@...el.com>,
        Souradeep Chakrabarti <schakrabarti@...rosoft.com>,
        souradeep chakrabarti <schakrabarti@...ux.microsoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        Long Li <longli@...rosoft.com>,
        Ajay Sharma <sharmaajay@...rosoft.com>,
        "leon@...nel.org" <leon@...nel.org>,
        "cai.huoqing@...ux.dev" <cai.huoqing@...ux.dev>,
        "ssengar@...ux.microsoft.com" <ssengar@...ux.microsoft.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [EXTERNAL] Re: [PATCH V4 net] net: mana: Fix MANA VF unload when
 host is unresponsive

On Thu, Jul 06, 2023 at 01:54:35PM +0000, Haiyang Zhang wrote:

> > This waiting loop is needed to let the pending Tx packets be sent. If
> > they weren't sent in 1 second, it most likely makes no sense already
> > whether they will be sent at all or not -- the destination host won't
> > wait for them for so long.
> > You say that it may happen only in case of HW issue. If so, I assume you
> > need to fix it some way, e.g. do a HW reset or so? If so, why bother
> > waiting for Tx completions if Tx is hung? You free all skbs later either
> > way, so there are no leaks.
> 
> At that point, we don't actually care if the pending packets are sent or not. 
> But if we free the queues too soon, and the HW is slow for unexpected 
> reasons, a delayed completion notice will DMA into the freed memory and 
> cause corruption. That's why we have a longer waiting time.

Aieiiie that is a horrible HW design to not have a strong fence of DMA.

"just wait and hope the HW doesn't UAF the kernel with DMA" is really
awful.

Jason