Re: [PATCH v3] tpm: Lock TPM chip in tpm_pm_suspend() first

[Jarkko Sakkinen]

On Fri Nov 1, 2024 at 11:09 PM EET, Jerry Snitselaar wrote:
> On Fri, Nov 01, 2024 at 11:07:15PM +0200, Jarkko Sakkinen wrote:
> > On Fri Nov 1, 2024 at 10:23 PM EET, Jerry Snitselaar wrote:
> > > On Fri, Nov 01, 2024 at 02:21:56AM +0200, Jarkko Sakkinen wrote:
> > > > Setting TPM_CHIP_FLAG_SUSPENDED in the end of tpm_pm_suspend() can be racy
> > > > according, as this leaves window for tpm_hwrng_read() to be called while
> > > > the operation is in progress. The recent bug report gives also evidence of
> > > > this behaviour.
> > > > 
> > > > Aadress this by locking the TPM chip before checking any chip->flags both
> > > > in tpm_pm_suspend() and tpm_hwrng_read(). Move TPM_CHIP_FLAG_SUSPENDED
> > > > check inside tpm_get_random() so that it will be always checked only when
> > > > the lock is reserved.
> > > > 
> > > > Cc: stable@xxxxxxxxxxxxxxx # v6.4+
> > > > Fixes: 99d464506255 ("tpm: Prevent hwrng from activating during resume")
> > > > Reported-by: Mike Seo <mikeseohyungjin@xxxxxxxxx>
> > > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219383
> > > > Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > > > ---
> > > > v3:
> > > > - Check TPM_CHIP_FLAG_SUSPENDED inside tpm_get_random() so that it is
> > > >   also done under the lock (suggested by Jerry Snitselaar).
> > > > v2:
> > > > - Addressed my own remark:
> > > >   https://lore.kernel.org/linux-integrity/D59JAI6RR2CD.G5E5T4ZCZ49W@xxxxxxxxxx/
> > > > ---
> > > >  drivers/char/tpm/tpm-chip.c      |  4 ----
> > > >  drivers/char/tpm/tpm-interface.c | 32 ++++++++++++++++++++++----------
> > > >  2 files changed, 22 insertions(+), 14 deletions(-)
> > > > 
> > > > diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> > > > index 1ff99a7091bb..7df7abaf3e52 100644
> > > > --- a/drivers/char/tpm/tpm-chip.c
> > > > +++ b/drivers/char/tpm/tpm-chip.c
> > > > @@ -525,10 +525,6 @@ static int tpm_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait)
> > > >  {
> > > >  	struct tpm_chip *chip = container_of(rng, struct tpm_chip, hwrng);
> > > >  
> > > > -	/* Give back zero bytes, as TPM chip has not yet fully resumed: */
> > > > -	if (chip->flags & TPM_CHIP_FLAG_SUSPENDED)
> > > > -		return 0;
> > > > -
> > > >  	return tpm_get_random(chip, data, max);
> > > >  }
> > > >  
> > > > diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> > > > index 8134f002b121..b1daa0d7b341 100644
> > > > --- a/drivers/char/tpm/tpm-interface.c
> > > > +++ b/drivers/char/tpm/tpm-interface.c
> > > > @@ -370,6 +370,13 @@ int tpm_pm_suspend(struct device *dev)
> > > >  	if (!chip)
> > > >  		return -ENODEV;
> > > >  
> > > > +	rc = tpm_try_get_ops(chip);
> > > > +	if (rc) {
> > > > +		/* Can be safely set out of locks, as no action cannot race: */
> > > > +		chip->flags |= TPM_CHIP_FLAG_SUSPENDED;
> > > > +		goto out;
> > > > +	}
> > > > +
> > > >  	if (chip->flags & TPM_CHIP_FLAG_ALWAYS_POWERED)
> > > >  		goto suspended;
> > > >  
> > > > @@ -377,21 +384,19 @@ int tpm_pm_suspend(struct device *dev)
> > > >  	    !pm_suspend_via_firmware())
> > > >  		goto suspended;
> > > >  
> > > > -	rc = tpm_try_get_ops(chip);
> > > > -	if (!rc) {
> > > > -		if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> > > > -			tpm2_end_auth_session(chip);
> > > > -			tpm2_shutdown(chip, TPM2_SU_STATE);
> > > > -		} else {
> > > > -			rc = tpm1_pm_suspend(chip, tpm_suspend_pcr);
> > > > -		}
> > > > -
> > > > -		tpm_put_ops(chip);
> > > > +	if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> > > > +		tpm2_end_auth_session(chip);
> > > > +		tpm2_shutdown(chip, TPM2_SU_STATE);
> > > > +		goto suspended;
> > > >  	}
> > > >  
> > > > +	rc = tpm1_pm_suspend(chip, tpm_suspend_pcr);
> > > > +
> > >
> > >
> > > I imagine the above still be wrapped in an else with the if (chip->flags & TPM_CHIP_FLAG_TPM2)
> > > otherwise it will call tpm1_pm_suspend for both tpm1 and tpm2 devices, yes?
> > >
> > > So:
> > >
> > > 	if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> > > 		tpm2_end_auth_session(chip);
> > > 		tpm2_shutdown(chip, TPM2_SU_STATE);
> > > 		goto suspended;
> > > 	} else {
> > > 		rc = tpm1_pm_suspend(chip, tpm_suspend_pcr);
> > > 	}
> > >
> > >
> > > Other than that I think it looks good.
> > 
> > It should be fine because after tpm2_shutdown() is called there is "goto
> > suspended;". This is IMHO more readable as it matches the structure of
> > previous exits before it. In future if this needs to be improved it will
> > easier to move the logic to a helper function (e.g. __tpm_pm_suspend())
> > where gotos are substituted with return-statements.
> > 
> > BR, Jarkko
> > 
>
> Heh, yep.
>
> Reviewed-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>

Thanks!

BR, Jarkko