Re: [PATCH v3] tpm: Actually fail on TPM errors during "get random"

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

On Tue, Apr 02, 2019 at 07:13:52PM +0000, Winkler, Tomas wrote:
> 
> 
> > On Tue, Apr 02, 2019 at 02:46:25AM +0300, Jarkko Sakkinen wrote:
> > > On Mon, Apr 01, 2019 at 12:06:07PM -0700, Kees Cook wrote:
> > > > A "get random" may fail with a TPM error, but those codes were
> > > > returned as-is to the caller, which assumed the result was the
> > > > number of bytes that had been written to the target buffer, which
> > > > could lead to a kernel heap memory exposure and over-read.
> > > >
> > > > This fixes tpm1_get_random() to mask positive TPM errors into -EIO,
> > > > as before.
> > > >
> > > > [   18.092103] tpm tpm0: A TPM error (379) occurred attempting get
> > random
> > > > [   18.092106] usercopy: Kernel memory exposure attempt detected from
> > SLUB object 'kmalloc-64' (offset 0, size 379)!
> > > >
> > > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1650989
> > > > Reported-by: Phil Baker <baker1tex@gmail.com>
> > > > Reported-by: Craig Robson <craig@zhatt.com>
> > > > Fixes: 7aee9c52d7ac ("tpm: tpm1: rewrite tpm1_get_random() using
> > > > tpm_buf structure")
> > > > Cc: Laura Abbott <labbott@redhat.com>
> > > > Cc: Tomas Winkler <tomas.winkler@intel.com>
> > > > Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > > > Cc: stable@vger.kernel.org
> > > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > > ---
> > > > v3: fix never-succeed, limit checks to tpm cmd return (James, Jason)
> > > > v2: also fix tpm2 implementation (Jason Gunthorpe)
> > > > ---
> > > >  drivers/char/tpm/tpm1-cmd.c | 7 +++++--
> > > > drivers/char/tpm/tpm2-cmd.c | 7 +++++--
> > > >  2 files changed, 10 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/char/tpm/tpm1-cmd.c
> > > > b/drivers/char/tpm/tpm1-cmd.c index 85dcf2654d11..faacbe1ffa1a
> > > > 100644
> > > > --- a/drivers/char/tpm/tpm1-cmd.c
> > > > +++ b/drivers/char/tpm/tpm1-cmd.c
> > > > @@ -510,7 +510,7 @@ struct tpm1_get_random_out {
> > > >   *
> > > >   * Return:
> > > >   * *  number of bytes read
> > > > - * * -errno or a TPM return code otherwise
> > > > + * * -errno (positive TPM return codes are masked to -EIO)
> > > >   */
> > > >  int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)  {
> > > > @@ -531,8 +531,11 @@ int tpm1_get_random(struct tpm_chip *chip, u8
> > > > *dest, size_t max)
> > > >
> > > >  		rc = tpm_transmit_cmd(chip, &buf, sizeof(out->rng_data_len),
> > > >  				      "attempting get random");
> > > > -		if (rc)
> > > > +		if (rc) {
> > > > +			if (rc > 0)
> > > > +				rc = -EIO;
> > > >  			goto out;
> > > > +		}
> > > >
> > > >  		out = (struct tpm1_get_random_out
> > *)&buf.data[TPM_HEADER_SIZE];
> > > >
> > > > diff --git a/drivers/char/tpm/tpm2-cmd.c
> > > > b/drivers/char/tpm/tpm2-cmd.c index e74c5b7b64bf..8ffa6af61580
> > > > 100644
> > > > --- a/drivers/char/tpm/tpm2-cmd.c
> > > > +++ b/drivers/char/tpm/tpm2-cmd.c
> > > > @@ -301,7 +301,7 @@ struct tpm2_get_random_out {
> > > >   *
> > > >   * Return:
> > > >   *   size of the buffer on success,
> > > > - *   -errno otherwise
> > > > + *   -errno otherwise ((positive TPM return codes are masked to -EIO)
> > > >   */
> > > >  int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)  {
> > > > @@ -328,8 +328,11 @@ int tpm2_get_random(struct tpm_chip *chip, u8
> > *dest, size_t max)
> > > >  				       offsetof(struct tpm2_get_random_out,
> > > >  						buffer),
> > > >  				       "attempting get random");
> > > > -		if (err)
> > > > +		if (err) {
> > > > +			if (err > 0)
> > > > +				err = -EIO;
> > > >  			goto out;
> > > > +		}
> > > >
> > > >  		out = (struct tpm2_get_random_out *)
> > > >  			&buf.data[TPM_HEADER_SIZE];
> > > > --
> > > > 2.17.1
> > > >
> > > >
> > > > --
> > > > Kees Cook
> > >
> > > Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > 
> > Applied to my master branch. Jason, Tomas, do you want me to add reviewed-
> > by's?
> Sure, it fixes my patch.

Great, I'll add it. Thank you. Just want to be explicit with these
things as I consider them as if I was asking a signature from someone
:-)

/Jarkko