Re: [PATCH v2] tpm: Rework open/close/shutdown to avoid races

Jarkko Sakkinen
2022-03-07T16:45:05Z

On Mon, Mar 07, 2022 at 11:22:27AM -0500, James Bottomley wrote:
> On Mon, 2022-03-07 at 17:41 +0200, Jarkko Sakkinen wrote:
> [...]
> > James, would it possible for you to construct a proper patch from
> > this and send it so we could include it to the next PR (use my tree
> > as baseline)?
>
> Sure, There's not really any identifiable patch for a fixes tag, since
> the race really always existed.
>
> How about the below.
>
> James
>
> -------------------8>8>8><8<8<8------------------
>
> From 3e0e640e47e5728b68693c5d45bf3cdd0bff48ca Mon Sep 17 00:00:00 2001
> From: James Bottomley <James.Bottomley@HansenPartnership.com>
> Date: Mon, 7 Mar 2022 11:16:55 -0500
> Subject: [PATCH] tpm: use try_get_ops() in tpm-space.c
>
> As part of the series conversion to remove nested TPM operations:
>
> https://lore.kernel.org/all/20190205224723.19671-1-jarkko.sakkinen@linux.intel.com/
>
> exposure of the chip->tpm_mutex was removed from much of the upper
> level code. In this conversion, tpm2_del_space() was missed. This
> didn't matter much because it's usually called closely after a
> converted operation, so there's only a very tiny race window where the
> chip can be removed before the space flushing is done which causes a
> NULL deref on the mutex. However, there are reports of this window
> being hit in practice, so fix this by converting tpm2_del_space() to
> use tpm_try_get_ops(), which performs all the teardown checks before
> acquring the mutex.
>
> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> ---
> drivers/char/tpm/tpm2-space.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
> index 265ec72b1d81..ffb35f0154c1 100644
> --- a/drivers/char/tpm/tpm2-space.c
> +++ b/drivers/char/tpm/tpm2-space.c
> @@ -58,12 +58,12 @@ int tpm2_init_space(struct tpm_space *space, unsigned int buf_size)
>
> void tpm2_del_space(struct tpm_chip *chip, struct tpm_space *space)
> {
> - mutex_lock(&chip->tpm_mutex);
> - if (!tpm_chip_start(chip)) {
> +
> + if (tpm_try_get_ops(chip) == 0) {
> tpm2_flush_sessions(chip, space);
> - tpm_chip_stop(chip);
> + tpm_put_ops(chip);
> }
> - mutex_unlock(&chip->tpm_mutex);
> +
> kfree(space->context_buf);
> kfree(space->session_buf);
> }
> --
> 2.34.1
>
>

Thank you! LGTM.

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko