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

[<Alexander.Steffen@infineon.com>]

> On Wed, Sep 06, 2017 at 02:19:28PM +0000,
> Alexander.Steffen@infineon.com wrote:
> > > 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@vger.kernel.org
> > > > > Signed-off-by: Alexander Steffen
> <Alexander.Steffen@infineon.com>
> > > > > ---
> > > > > 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
> >
> > I already had test cases that failed with some of my TPMs under some
> circumstances. I'll try to come up with a concise description of what those
> tests do or send you a patch directly for your tests. GitHub pull requests are
> okay for that repository? (I already have one waiting there.)
> >
> > > >
> > > > /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.
> >
> > There are two problems with this solution:
> >
> > 1. You need to check for in_size < 6, otherwise you read data that has
> > not been written there during that request. I haven't tested this, but
> > I'd expect it to fail for example if you try to send the two commands
> > "00 00 00 00 00 02" and "00 00". The first will be rejected with
> > EINVAL, because 6 (in_size) != 2 (commandSize). But the second will
> > pass that check, because now in_size happens to match the commandSize
> > that has only been written to the buffer for the first command.
> 
> AFAIK tpm_transmit checks that the command has at least the header.

This was only a simple example, it will fail with other values as well. Just add 8 to both size fields and append 8 null bytes, and you will pass the length check in tpm_transmit but still have incorrect data. Also, it is probably no good style to omit checks for obvious errors, just because an unrelated check in a completely different location also happens to catch the problem under some circumstances. tpm_common_write discards the relevant information (in_size), so all other parts of the code need to be able to rely on it to have validated it properly.

> > 2. You may not reject commands where in_size > commandSize, because
> > TIS/PTP require the TPM to throw away extra bytes (and process the
> > command as usual), not fail the command. You can see that in the State
> > Transition Table (Table 18 in TIS 1.3), line 20, with the TPM in
> > Reception state and Expect=0, writing more data does not change the
> > state ("Write is not expected. Drop write. TPM ignores this state
> > transition."). Of course, since we do not pass on in_size, but only
> > commandSize the TPM will never see those extra bytes, but the external
> > behavior (for user space applications) is identical.
> 
> OK, this is more relevant. What is the legit case to send extra bytes?

For me: testing that my TPM implementation behaves correctly :) I can run the same test cases against the TPM using the Linux driver and several other, unrelated means. I'd like to avoid having special cases for communication paths in there, just because in one case I have a more direct channel to the TPM whereas in the other the Linux driver interferes with the communication and rejects the data before the TPM sees it. For "normal" usage from Linux applications this is not relevant, but it does not break anything either.

I've just noticed that the v2 patch is broken, because the code incorrectly tries to access data from user space. Interestingly, the tests worked fine on x86_64 and aarch64, only armv7l was broken (and that result somehow got lost, so I assumed that the patch was fine). I'll send a fixed patch soon.

Alexander