Re: [PATCH v2] tpm: Allow system suspend to continue when TPM suspend fails

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Jan 16, 2023 at 03:00:03PM +0100, Vlastimil Babka wrote:
> On 1/16/23 12:44, Jarkko Sakkinen wrote:
> > On Fri, Jan 06, 2023 at 04:01:56AM +0100, Jason A. Donenfeld wrote:
> >> TPM 1 is sometimes broken across system suspends, due to races or
> >> locking issues or something else that haven't been diagnosed or fixed
> >> yet, most likely having to do with concurrent reads from the TPM's
> >> hardware random number generator driver. These issues prevent the system
> >> from actually suspending, with errors like:
> >> 
> >>   tpm tpm0: A TPM error (28) occurred continue selftest
> >>   ...
> > 
> > <REMOVE>
> > 
> >>   tpm tpm0: A TPM error (28) occurred attempting get random
> >>   ...
> >>   tpm tpm0: Error (28) sending savestate before suspend
> >>   tpm_tis 00:08: PM: __pnp_bus_suspend(): tpm_pm_suspend+0x0/0x80 returns 28
> >>   tpm_tis 00:08: PM: dpm_run_callback(): pnp_bus_suspend+0x0/0x10 returns 28
> >>   tpm_tis 00:08: PM: failed to suspend: error 28
> >>   PM: Some devices failed to suspend, or early wake event detected
> > 
> > </REMOVE>
> > 
> > Unrelated to thix particular fix.
> 
> Not sure I understand.
> AFAIK this is not a proper fix, but a workaround for when laptop suspend no
> longer works because TPM fails to suspend. The error messages quoted above
> are very much related to the problem of suspend not working, and this patch
> did work as advertised at least for me. I see errors but they don't prevent
> suspend anymore:
> 
> https://lore.kernel.org/all/58d7a42c-9e6b-ab2a-617f-d5e373bf63cb@suse.cz/
> 
> >> This issue was partially fixed by 23393c646142 ("char: tpm: Protect
> >> tpm_pm_suspend with locks"), in a last minute 6.1 commit that Linus took
> >> directly because the TPM maintainers weren't available. However, it
> >> seems like this just addresses the most common cases of the bug, rather
> >> than addressing it entirely. So there are more things to fix still,
> >> apparently.
> >> 
> >> In lieu of actually fixing the underlying bug, just allow system suspend
> >> to continue, so that laptops still go to sleep fine. Later, this can be
> >> reverted when the real bug is fixed.
> >> 
> >> Link: https://lore.kernel.org/lkml/7cbe96cf-e0b5-ba63-d1b4-f63d2e826efa@suse.cz/
> >> Cc: stable@vger.kernel.org # 6.1+
> >> Reported-by: Vlastimil Babka <vbabka@suse.cz>
> >> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> >> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> >> ---
> >> This is basically untested and I haven't worked out if there are any
> >> awful implications of letting the system sleep when TPM suspend fails.
> >> Maybe some PCRs get cleared and that will make everything explode on
> >> resume? Maybe it doesn't matter? Somebody well versed in TPMology should
> >> probably [n]ack this approach.
> >> 
> >>  drivers/char/tpm/tpm-interface.c | 5 ++++-
> >>  1 file changed, 4 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> >> index d69905233aff..6df9067ef7f9 100644
> >> --- a/drivers/char/tpm/tpm-interface.c
> >> +++ b/drivers/char/tpm/tpm-interface.c
> >> @@ -412,7 +412,10 @@ int tpm_pm_suspend(struct device *dev)
> >>  	}
> >>  
> >>  suspended:
> >> -	return rc;
> >> +	if (rc)
> >> +		pr_err("Unable to suspend tpm-%d (error %d), but continuing system suspend\n",
> >> +		       chip->dev_num, rc);
> >> +	return 0;
> >>  }
> >>  EXPORT_SYMBOL_GPL(tpm_pm_suspend);
> >>  
> >> -- 
> >> 2.39.0
> >> 
> > 
> > This tpm_tis local issue, nothing to do with tpm_pm_suspend(). Executing
> > the selftest as part of wake up, is TPM 1.2 dTPM specific requirement, and
> > the call is located in tpm_tis_resume() [*].
> > 
> > [*] https://lore.kernel.org/lkml/Y8U1QxA4GYvPWDky@kernel.org/
> 
> Yes the changelog at the top does say "due to races or locking issues or
> something else that haven't been diagnosed or fixed yet"
> 
> I don't know what causes the TPM to start returning error 28 on resume and
> never recover from it. But it didn't happen before hwrng started using the
> TPM. Before that, it was probably just the selftest ever doing anything with
> the TPM, and on its own I don't recall it ever (before 6.1) failing and
> preventing further suspend/resume.

Would it be possible to test this theory by commenting out tpm_add_hwrng()
call from tpm_chip_register()?

Since they are called sequentially any sort of concurrency issue can be
probably ruled out.

One thing that I noticed is that probably it would be more safe-play to
move tpm_add_hwrng() call after creating the character device, as there's
no need to do it before anything else.

BR, Jarkko