linux-kernel - Re: [PATCH] tpm: don't destroy chip device prematurely

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Message-ID: <20161005162741.GA18636@obsidianresearch.com>
Date:   Wed, 5 Oct 2016 10:27:41 -0600
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     "Winkler, Tomas" <tomas.winkler@...el.com>,
        "tpmdd-devel@...ts.sourceforge.net" 
        <tpmdd-devel@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: don't destroy chip device prematurely

On Wed, Oct 05, 2016 at 01:02:34PM +0300, Jarkko Sakkinen wrote:

> I'll repeat my question: what worse can happen than returning -EPIPE?  I
> though the whole rw lock scheme was introduced just for this purpose.

I thought I explained this, if device_del is moved after ops = null
then if sysfs looses the race it will oops the kernel. device_del hard
fences sysfs.

> Why there's even that branch in tpm-dev.c if it's so bad to let it
> happen?

Because cdev_del and device_del do not guarentee that the cdev is
fenced. They just prevent new calls into open(). So the branch in
tpm-dev.c is necessary to avoid a kernel oops if user space holds the
fd open across unregister.

It is the same sitatuion you identified in the securityfs discussion -
user space holding the fd open across a driver unregister.

Jason