linux-kernel - Re: use-after-free in srpt_enable

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Date:   Tue, 5 Jul 2022 09:39:33 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Hillf Danton <hdanton@...a.com>,
        Mike Christie <michael.christie@...cle.com>,
        "lizhijian@...itsu.com" <lizhijian@...itsu.com>,
        Leon Romanovsky <leon@...nel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "target-devel@...r.kernel.org" <target-devel@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: use-after-free in srpt_enable_tpg()

On Mon, Jul 04, 2022 at 09:34:07PM -0700, Bart Van Assche wrote:
> On 7/3/22 17:11, Hillf Danton wrote:
> > On Sun, 3 Jul 2022 07:55:05 -0700 Bart Van Assche wrote:
> > > However, I'm not sure that would make a
> > > significant difference since there is a similar while-loop in one of the
> > > callers of srpt_remove_one() (disable_device() in the RDMA core).
> > 
> > Hehe... feel free to shed light on how the loop in RDMA core is currently
> > making the loop in srpt more prone to uaf?
> 
> In my email I was referring to the following code in disable_device():
> 
>        wait_for_completion(&device->unreg_completion);
> 
> I think that code shows that device removal by the RDMA core is synchronous
> in nature. Even if the ib_srpt source code would be modified such that the
> objects referred by that code live longer, the wait loop in disable_device()
> would wait for the ib_device reference counts to drop to zero.

That is not really the "ib_device" reference count it is the
"registration" reference count.

IB has a system where drivers/ulp can create critical regions where
the ib device must be registered using the ib_device_try_get()/put
calls. "Must be registered" is useful in a number of places but should
not be held for a long period.

This is distinct from the normal struct device refcount that simply
keeps the ib_device memory alive.

Jason