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

[Kees Cook <keescook@xxxxxxxxxxxx>]

On Mon, Apr 1, 2019 at 11:39 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Mon, Apr 01, 2019 at 11:32:11AM -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 an 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>
> >  drivers/char/tpm/tpm1-cmd.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
> > index 85dcf2654d11..faeb78ecf960 100644
> > +++ 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)
> >  {
> > @@ -524,7 +524,7 @@ int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
> >
> >       rc = tpm_buf_init(&buf, TPM_TAG_RQU_COMMAND, TPM_ORD_GET_RANDOM);
> >       if (rc)
> > -             return rc;
> > +             goto fail;
>
> ?? Does tpm_buf_init() return positive?? Shouldn't..
>
> I think you actually want this:

I didn't check that, but it was tpm_transmit_cmd() that was failing
(and would be covered by the error path change I sent).

>
> diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
> index 85dcf2654d1102..a01e6fba1aacb6 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
>   */
>  int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
>  {
> @@ -531,8 +531,10 @@ 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) {
> +                       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 e74c5b7b64bfbd..9fa498b4cf8816 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -328,8 +328,10 @@ 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) {
> +                       rc = -EIO;
>                         goto out;
> +               }
>
>                 out = (struct tpm2_get_random_out *)
>                         &buf.data[TPM_HEADER_SIZE];

Eeks, I missed tpm2. In both cases, though a negative error would be
masked. I was trying to leave those as-is and only mask positive
errors.


-- 
Kees Cook