Re: [PATCH] tpm/tpm_crb: Avoid unaligned reads in crb_recv():

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Fri, Feb 01, 2019 at 07:20:42PM +0000, Winkler, Tomas wrote:
> > 
> > The current approach to read first 6 bytes from the response and then tail of
> > the response, can cause the 2nd memcpy_fromio() to do an unaligned read
> > (e.g. read 32-bit word from address aligned to a 16-bits), depending on how
> > memcpy_fromio() is implemented. If this happens, the read will fail and the
> > memory controller will fill the read with 1's.
> > 
> > This was triggered by 170d13ca3a2f, which should be probably refined to check
> > and react to the address alignment. Before that commit, on x86
> > memcpy_fromio() turned out to be memcpy(). By a luck GCC has done the right
> > thing (from tpm_crb's perspective) for us so far, but we should not rely on that.
> > Thus, it makes sense to fix this also in tpm_crb, not least because the fix can be
> > then backported to stable kernels and make them more robust when compiled
> > in differing environments.
> > 
> > Cc: stable@vger.kernel.org
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: James Morris <jmorris@namei.org>
> > Cc: Tomas Winkler <tomas.winkler@intel.com>
> > Cc: Jerry Snitselaar <jsnitsel@redhat.com>
> > Fixes: 30fc8d138e91 ("tpm: TPM 2.0 CRB Interface")
> > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > ---
> >  drivers/char/tpm/tpm_crb.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/char/tpm/tpm_crb.c b/drivers/char/tpm/tpm_crb.c index
> > 36952ef98f90..7f47e43aa9f1 100644
> > --- a/drivers/char/tpm/tpm_crb.c
> > +++ b/drivers/char/tpm/tpm_crb.c
> > @@ -288,18 +288,18 @@ static int crb_recv(struct tpm_chip *chip, u8 *buf,
> > size_t count)
> >  	unsigned int expected;
> > 
> >  	/* sanity check */
> > -	if (count < 6)
> > +	if (count < 8)
> >  		return -EIO;
> Why don't you already enforce reading at least the whole TPM header 10bytes, we are reading the whole buffer at one call anyway.
> Who every is asking for 8 bytes from the protocol level, is doing something wrong.
> 
> >  	if (ioread32(&priv->regs_t->ctrl_sts) & CRB_CTRL_STS_ERROR)
> >  		return -EIO;
> > 
> > -	memcpy_fromio(buf, priv->rsp, 6);
> > +	memcpy_fromio(buf, priv->rsp, 8);
> Maybe a short comment will spare someone looking into git history 
> >  	expected = be32_to_cpup((__be32 *) &buf[2]);
> > -	if (expected > count || expected < 6)
> > +	if (expected > count || expected < 8)
> Expected should be at least tpm header, right?
> >  		return -EIO;
> > 
> > -	memcpy_fromio(&buf[6], &priv->rsp[6], expected - 6);
> > +	memcpy_fromio(&buf[8], &priv->rsp[8], expected - 8);
> > 
> >  	return expected;
> >  }
> Otherwise ready the first 8 bytes looks good. 
> Thanks 
> Tomas

Your proposals look sane, thank you.

/Jarkko