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

[Jarkko Sakkinen]

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@xxxxxxxxx>
> > Reported-by: Craig Robson <craig@xxxxxxxxx>
> > Fixes: 7aee9c52d7ac ("tpm: tpm1: rewrite tpm1_get_random() using tpm_buf structure")
> > Cc: Laura Abbott <labbott@xxxxxxxxxx>
> > Cc: Tomas Winkler <tomas.winkler@xxxxxxxxx>
> > Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxxxx>

Applied to my master branch. Jason, Tomas, do you want me to add
reviewed-by's?

/Jarkko