linux-kernel - Re: [PATCH v3] tpm: Lock TPM chip in tpm_pm

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Message-ID: <7pc2uu52wamyvhzfc27qnws546yxt33utfibtsjd7uv2djfxdt@jlyn3n55qkfx>
Date: Fri, 1 Nov 2024 14:09:55 -0700
From: Jerry Snitselaar <jsnitsel@...hat.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>, 
	stable@...r.kernel.org, Mike Seo <mikeseohyungjin@...il.com>, 
	"open list:TPM DEVICE DRIVER" <linux-integrity@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] tpm: Lock TPM chip in tpm_pm_suspend() first

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@...r.kernel.org # v6.4+
> > > Fixes: 99d464506255 ("tpm: Prevent hwrng from activating during resume")
> > > Reported-by: Mike Seo <mikeseohyungjin@...il.com>
> > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219383
> > > Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> > > ---
> > > 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@kernel.org/
> > > ---
> > >  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@...hat.com>