Re: [PATCH v2] tpm-dev-common: Reject too short writes

[Jarkko Sakkinen]

On Wed, Sep 06, 2017 at 03:42:33PM +0300, Jarkko Sakkinen wrote:
> On Mon, Sep 04, 2017 at 07:36:42PM +0200, Alexander Steffen wrote:
> > tpm_transmit() does not offer an explicit interface to indicate the number
> > of valid bytes in the communication buffer. Instead, it relies on the
> > commandSize field in the TPM header that is encoded within the buffer.
> > Therefore, ensure that a) enough data has been written to the buffer, so
> > that the commandSize field is present and b) the commandSize field does not
> > announce more data than has been written to the buffer.
> > 
> > This should have been fixed with CVE-2011-1161 long ago, but apparently
> > a correct version of that patch never made it into the kernel.
> > 
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Alexander Steffen <Alexander.Steffen@xxxxxxxxxxxx>
> > ---
> > v2:
> > - Moved all changes to tpm_common_write in a single patch.
> > 
> >  drivers/char/tpm/tpm-dev-common.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
> > index 610638a..ac25574 100644
> > --- a/drivers/char/tpm/tpm-dev-common.c
> > +++ b/drivers/char/tpm/tpm-dev-common.c
> > @@ -99,7 +99,8 @@ ssize_t tpm_common_write(struct file *file, const char __user *buf,
> >  	if (atomic_read(&priv->data_pending) != 0)
> >  		return -EBUSY;
> >  
> > -	if (in_size > TPM_BUFSIZE)
> > +	if (in_size > sizeof(priv->data_buffer) || in_size < 6 ||
> > +	    in_size < be32_to_cpu(*((__be32 *) (buf + 2))))
> >  		return -E2BIG;
> >  
> >  	mutex_lock(&priv->buffer_mutex);
> > -- 
> > 2.7.4
> > 
> 
> How did you test this change after you implemented it?
> 
> Just thinking what to add to https://github.com/jsakkine-intel/tpm2-scripts
> 
> /Jarkko

Just when I started to implement this that the bug fix itself does not
have yet the right semantics.

It should be just add a new check:

if (in_size != be32_to_cpu(*((__be32 *) (buf + 2))))
	return -EINVAL;

The existing check is correct. This was missing. The reason for this is
that we process whatever is in the in_size bytes as a full command.

Sorry I didn't notice before I started to implement a test case.

/Jarkko