linux-kernel - RE: [PATCH RFC v2] tpm: tpm_vtpm_proxy: do not reference kernel memory as user memory

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Date:   Wed, 31 May 2023 07:47:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Stefan Berger' <stefanb@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>
CC:     Jason Gunthorpe <jgg@...dia.com>,
        Alejandro Cabrera <alejandro.cabreraaldaya@...i.fi>,
        Jarkko Sakkinen <jarkko.sakkinen@...i.fi>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH RFC v2] tpm: tpm_vtpm_proxy: do not reference kernel
 memory as user memory

From: Stefan Berger
> Sent: 30 May 2023 18:46
> 
> On 5/29/23 22:01, Jarkko Sakkinen wrote:
> > From: Jarkko Sakkinen <jarkko.sakkinen@...i.fi>
> >
> 
> > -	rc = copy_to_user(buf, proxy_dev->buffer, len);
> > +	if (buf)
> > +		rc = copy_to_user(buf, proxy_dev->buffer, len);
> > +
> 
> Looking through other drivers it seems buf is always expected to be a valid non-NULL pointer on
> file_operations.read().

If the user passes NULL the copy_to/from_user() fails and
-EFAULT is returned.

Adding the NULL check makes the request silently succeed.
I doubt that is anywhere near right when you ignore copy_from_user().

I'm not sure what the rational/subject is about either.
copy_to/from_user() calls access_ok() and will fail on
a kernel address.

	David

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