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

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Message-ID: <83c010a7b8821309a023e41efbabb7e96f156eec.camel@kernel.org>
Date:   Wed, 31 May 2023 20:17:49 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Stefan Berger <stefanb@...ux.ibm.com>,
        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, Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2] tpm: tpm_vtpm_proxy: do not reference kernel
 memory as user memory

On Tue, 2023-05-30 at 13:45 -0400, Stefan Berger wrote:
> 
> 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().
> 
> 
> https://elixir.bootlin.com/linux/latest/source/arch/x86/mm/tlb.c#L1279   simple_read_from_buffer will pass the pointer to the user buffer along and it ('to') ends up in copy_to_user(to, ...);
> 
> 
> Same here: https://elixir.bootlin.com/linux/latest/source/security/integrity/ima/ima_fs.c#L41

It is good to mention here that IMA uses __user tagged pointers
correctly, and it does not really compare to the vtpm driver code
by any possible means. So let's not add illegit comparison points.

BR, Jarkko