Re: [PATCH v3] tpm-dev-common: Reject too short writes

[Jarkko Sakkinen]

On Sat, Sep 09, 2017 at 12:37:39AM +0300, Jarkko Sakkinen wrote:
> On Fri, Sep 08, 2017 at 05:21:32PM +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.
> > v3:
> > - Access data copied from user space (priv->data_buffer) instead of user
> >   space data directly (buf).
> > - Changed return code to EINVAL.
> > 
> >  drivers/char/tpm/tpm-dev-common.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
> > index 610638a..461bf0b 100644
> > --- a/drivers/char/tpm/tpm-dev-common.c
> > +++ b/drivers/char/tpm/tpm-dev-common.c
> > @@ -110,6 +110,12 @@ ssize_t tpm_common_write(struct file *file, const char __user *buf,
> >  		return -EFAULT;
> >  	}
> >  
> > +	if (in_size < 6 ||
> > +	    in_size < be32_to_cpu(*((__be32 *) (priv->data_buffer + 2)))) {
> > +		mutex_unlock(&priv->buffer_mutex);
> > +		return -EINVAL;
> > +	}
> > +
> >  	/* atomic tpm command send and result receive. We only hold the ops
> >  	 * lock during this period so that the tpm can be unregistered even if
> >  	 * the char dev is held open.
> > -- 
> > 2.7.4
> > 
> 
> I'm not gonna fight about that "in_size < 6" check. I think it is not
> needed, I understand your point but still disagree but it is something
> where I can live with having it.
> 
> I kind of disagree also with allowing messages longer than the command
> size but it does not have to be in the scope of this commit and actually
> should be a separate discussion if we ever going to do something about
> it.
> 
> Thanks for the patience!
> 
> Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> 
> /Jarkko

Without your fix:

$ python -m unittest -v tpm2_smoke.SmokeTest.test_too_short_cmd
test_too_short_cmd (tpm2_smoke.SmokeTest) ... FAIL

======================================================================
FAIL: test_too_short_cmd (tpm2_smoke.SmokeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tpm2_smoke.py", line 157, in test_too_short_cmd
    self.assertEqual(rejected, True)
AssertionError: False != True

----------------------------------------------------------------------
Ran 1 test in 2.108s

FAILED (failures=1)

The test case expects to get a posix error, which it doesn't get.

With your fix:

$ python -m unittest -v tpm2_smoke.SmokeTest.test_too_short_cmd
test_too_short_cmd (tpm2_smoke.SmokeTest) ... ok

----------------------------------------------------------------------
Ran 1 test in 2.099s

OK

So looks good to me.

Tested-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>

Can you test the master branch with SPI TPM? I had to tinker your
commits a bit because of merge conflicts with Arnd's commit. I'll
put everything back to next as soon as I hear from you. Thanks

/Jarkko