linux-kernel - Re: [PATCH v7] tpm: Check size of response before accessing data

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Message-ID: <20170120201423.aky44svd3vejxxis@intel.com>
Date:   Fri, 20 Jan 2017 22:14:23 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Stefan Berger <stefanb@...ux.vnet.ibm.com>
Cc:     tpmdd-devel@...ts.sourceforge.net,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7] tpm: Check size of response before accessing data

On Fri, Jan 20, 2017 at 07:55:22PM +0200, Jarkko Sakkinen wrote:
> On Fri, Jan 20, 2017 at 06:21:58PM +0200, Jarkko Sakkinen wrote:
> > On Fri, Jan 20, 2017 at 03:36:30PM +0200, Jarkko Sakkinen wrote:
> > > On Thu, Jan 19, 2017 at 07:19:12AM -0500, Stefan Berger wrote:
> > > > Make sure that we have not received less bytes than what is indicated
> > > > in the header of the TPM response. Also, check the number of bytes in
> > > > the response before accessing its data.
> > > > 
> > > > Signed-off-by: Stefan Berger <stefanb@...ux.vnet.ibm.com>
> > > 
> > > Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> > 
> > Oops. I found some odd stuff after all so hold on for a moment.
> > I could do these updates myself probably...
> > 
> > > >  ssize_t tpm_transmit_cmd(struct tpm_chip *chip, const void *cmd,
> > > > -			 int len, unsigned int flags, const char *desc)
> > > > +			 size_t len, size_t min_rsp_body_length,
> > > > +			 unsigned int flags, const char *desc)
> > 
> > BTW, maybe the cmd_length would be actually a better idea because
> > it gets mixes witht local variable.
> > 
> > > >  {
> > > >  	const struct tpm_output_header *header;
> > > >  	int err;
> > > > +	ssize_t length;
> > 
> > Maybe it would make sense to name this as rsp_length.
> > 
> > > >  
> > > > -	len = tpm_transmit(chip, (const u8 *)cmd, len, flags);
> > > > -	if (len <  0)
> > > > -		return len;
> > > > -	else if (len < TPM_HEADER_SIZE)
> > > > +	length = tpm_transmit(chip, (const u8 *)cmd, len, flags);
> > > > +	if (length <  0)
> > > > +		return length;
> > > > +	else if (length < TPM_HEADER_SIZE)
> > > >  		return -EFAULT;
> > > >  
> > > >  	header = cmd;
> > > > +	if (length < be32_to_cpu(header->length))
> > > > +		return -EFAULT;
> > 
> > Why '<' and not '!='? In what legit case length would be larger?
> > 
> > > >  
> > > >  	err = be32_to_cpu(header->return_code);
> > > >  	if (err != 0 && desc)
> > > >  		dev_err(&chip->dev, "A TPM error (%d) occurred %s\n", err,
> > > >  			desc);
> > > > +	if (err)
> > > > +		return err;
> > > >  
> > > > -	return err;
> > > > +	if (be32_to_cpu(header->length) <
> > > > +	    min_rsp_body_length + TPM_HEADER_SIZE)
> > > > +		return -EFAULT;
> > 
> > Why couldn't you use 'length' here?
> > 
> > /Jarkko
> 
> Anyway,
> 
> Tested-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>

Stefan, I updated the patch by doing '!=' check and renaming parameters
to 'buf' and 'bufsiz' as they are in tpm_transmit(). The current namesd
did not make sense because you pass a buffer that will also will store
the response.

Can you check that after my updates it looks OK to you?

/Jarkko