linux-kernel - Re: [PATCH] firmware: arm_scmi: avoid double free in error flow

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Date:   Tue, 26 Nov 2019 10:48:50 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Wen Yang <wenyang@...ux.alibaba.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH] firmware: arm_scmi: avoid double free in error flow

On Tue, Nov 26, 2019 at 06:24:47PM +0800, Wen Yang wrote:
>
>
> On 2019/11/26 12:13 上午, Sudeep Holla wrote:
> > On Mon, Nov 25, 2019 at 11:54:09PM +0800, Wen Yang wrote:
> > > If device_register() fails, both put_device() and kfree()
> > > are called, ending with a double free of the scmi_dev.
> > >
> >
> > Correct.
> >
> > > Calling kfree() is needed only when a failure happens between the
> > > allocation of the scmi_dev and its registration, so move it to
> > > there and remove it from the error flow.
> > >
> >
> > kstrdup_const can fail and in that case device is not yet registered,
> > so we need to free. Since device_register() calls put_device() on failure
> > too, I would just drop it as it's unnecessary, not sure why I have added
> > it in the first place. Can you re-spin the patch dropping put_device
> > and renaming put_dev label to something like free_const.
> >

Please ignore the above completely. I have made some changes locally and
got completely confused when I looked at your patch and compared with
the modified context locally.

>
> Hi Sudeep,
> Thanks for your comments.
> Let's check the code like this:
>
> int device_register(struct device *dev)
> {
>         device_initialize(dev);   --> Initialize kobj-> kref to 1
>         return device_add(dev);
> }
>
> int device_add(struct device *dev)
> {
> ...
>         dev = get_device(dev);  --> kobj-> kref increases by 1
> ...
> done:
>         put_device(dev);  --> kobj-> kref decreases by 1 and is now 1
>         return error;
> ...
> }
>
> So we also need to call put_device (),
> and the original patch should be fine.
> Please kindly help to check again, thank you.
>

You are right, sorry for the confusion. I will apply your original patch.
Thanks again for the patch.

--
Regards,
Sudeep