Re: [PATCH] tpm: set TPM_CHIP_FLAG_SUSPENDED early

[Jerry Snitselaar]

On Thu, Oct 31, 2024 at 01:36:46AM +0200, Jarkko Sakkinen wrote:
> On Wed Oct 30, 2024 at 10:09 PM EET, Jerry Snitselaar wrote:
> > On Wed, Oct 30, 2024 at 12:36:47AM +0200, Jarkko Sakkinen wrote:
> > > Setting TPM_CHIP_FLAG_SUSPENDED in the end of tpm_pm_suspend() can be racy
> > > according to the bug report, as this leaves window for tpm_hwrng_read() to
> > > be called while the operation is in progress. Move setting of the flag
> > > into the beginning.
> > > 
> > > 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>
> > > ---
> > >  drivers/char/tpm/tpm-interface.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> > > index 8134f002b121..3f96bc8b95df 100644
> > > --- a/drivers/char/tpm/tpm-interface.c
> > > +++ b/drivers/char/tpm/tpm-interface.c
> > > @@ -370,6 +370,8 @@ int tpm_pm_suspend(struct device *dev)
> > >  	if (!chip)
> > >  		return -ENODEV;
> > >  
> > > +	chip->flags |= TPM_CHIP_FLAG_SUSPENDED;
> > > +
> > >  	if (chip->flags & TPM_CHIP_FLAG_ALWAYS_POWERED)
> > >  		goto suspended;
> > >  
> > > @@ -390,8 +392,6 @@ int tpm_pm_suspend(struct device *dev)
> > >  	}
> > >  
> > >  suspended:
> > > -	chip->flags |= TPM_CHIP_FLAG_SUSPENDED;
> > > -
> > >  	if (rc)
> > >  		dev_err(dev, "Ignoring error %d while suspending\n", rc);
> > >  	return 0;
> > > -- 
> > > 2.47.0
> > > 
> >
> > Reviewed-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>
> 
> Thanks but I actually started to look at the function:
> 
> https://elixir.bootlin.com/linux/v6.11.5/source/drivers/char/tpm/tpm-interface.c#L365
> 
> The absolutely safe-play way considering concurrency would be
> to do tpm_try_get_ops() before checking any flags. That way
> tpm_hwrng_read() is guaranteed not conflict.
> 
> So the way I would fix this instead would be to (untested
> wrote inline here):
> 
> int tpm_pm_suspend(struct device *dev)
> {
> 	struct tpm_chip *chip = dev_get_drvdata(dev);
> 	int rc = 0;
> 
> 	if (!chip)
> 		return -ENODEV;
> 
> 	rc = tpm_try_get_ops(chip);
> 	if (rc) {
> 		chip->flags = |= TPM_CHIP_FLAG_SUSPENDED;
> 		return rc;
> 	}
> 
> 	/* ... */
> 
> suspended:
> 	chip->flags |= TPM_CHIP_FLAG_SUSPENDED;
> 	tpm_put_ops(chip);
> 
> It does not really affect performance but guarantees that
> tpm_hwrng_read() is guaranteed either fully finish or
> never happens given that both sides take chip->lock.
> 
> So I'll put one more round of this and then this should be
> stable and fully fixed.
> 
> BR, Jarkko

Ah, yeah better to set it while it has the mutex. That should still be
'if (!rc)' after the tpm_try_get_ops() right? (I'm assuming that is just
a transcription error).

Regards,
Jerry