linux-kernel - Re: [PATCH] tpm: Disable hwrng for TPM 1 if PM

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Message-ID: <Y7dPV5BK6jk1KvX+@zx2c4.com>
Date:   Thu, 5 Jan 2023 23:29:43 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thorsten Leemhuis <regressions@...mhuis.info>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Peter Huewe <peterhuewe@....de>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Jan Dabros <jsd@...ihalf.com>,
        regressions@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>,
        linux-integrity@...r.kernel.org,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Johannes Altmanninger <aclopte@...il.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] tpm: Disable hwrng for TPM 1 if PM_SLEEP is enabled

On Thu, Jan 05, 2023 at 01:58:48PM -0800, Linus Torvalds wrote:
> On Thu, Jan 5, 2023 at 6:48 AM Jason A. Donenfeld <Jason@...c4.com> wrote:
> >
> > TPM 1's support for its hardware RNG is broken across system suspends,
> > due to races or locking issues or something else that haven't been
> > diagnosed or fixed yet. These issues prevent the system from actually
> > suspending. So disable the driver in this case. Later, when this is
> > fixed properly, we can remove this.
> 
> How about just keeping it enabled, but not making it a fatal error if
> the TPM saving doesn't work? IOW, just print the warning, and then
> "return 0" from the suspend function.

You're right that returning 0 from the pm notifier would make the
problem that users actually care about -- laptop doesn't sleep when you
close the lid -- go away.

>From a random.c perspective, the RNG is already initialized when the
driver loads, which will be before suspend bricks the driver. So even if
the behavior afterwards is a buggy driver handing all zeros to random.c,
it won't really matter much; random.c can deal with that
cryptographically. I have no idea if this is actually the case with the
driver's error condition. But if it is, it's good that it doesn't
matter.

So okay, I'll roll a patch to do that when I get home. I'm writing on my
phone now, but from memory it's just changing a 'return rc;' into
'return 0;'.

Then the TPM folks can fix the underlying issue at their leisure
whenever.

Jason