linux-kernel - Re: [PATCH v3 4/4] tpm: Allow for exclusive TPM access when using /dev/tpm<n>

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Message-ID: <aP_Tfl9B-CW49ukN@kernel.org>
Date: Mon, 27 Oct 2025 22:18:06 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Roberto Sassu <roberto.sassu@...weicloud.com>,
	Jonathan McDowell <noodles@...th.li>,
	Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
	zohar@...ux.ibm.com
Subject: Re: [PATCH v3 4/4] tpm: Allow for exclusive TPM access when using
 /dev/tpm<n>

On Mon, Oct 27, 2025 at 04:09:35PM -0400, James Bottomley wrote:
> On Mon, 2025-10-27 at 21:38 +0200, Jarkko Sakkinen wrote:
> > On Mon, Oct 20, 2025 at 01:53:30PM +0200, Roberto Sassu wrote:
> [...]
> > > Hi Jonathan
> > > 
> > > do I understand it correctly, that a process might open the TPM
> > > with O_EXCL, and this will prevent IMA from extending a PCR until
> > > that process closes the file descriptor?
> > > 
> > > If yes, this might be a concern, and I think an additional API to
> > > prevent such behavior would be needed (for example when IMA is
> > > active, i.e. there is a measurement policy loaded).
> > 
> > Also this would be a problem with hwrng.
> > 
> > This probably needs to be refined somehow. I don't have a solution at
> > hand but "invariant" is that in-kernel caller should override user
> > space exclusion, even when O_EXCL is used.
> 
> Also, are we sure we need O_EXCL in the first place?  A well
> functioning TPM is supposed to be able to cope with field upgrade while
> it receives other commands.  When it's in this state, it's supposed to
> return TPM_RC_UPGRADE to inappropriate commands, so if we made sure we
> can correctly handle that in the kernel, that might be enough to get
> all this to work correctly without needing an exclusive open.
> 
> Of course, Field Upgrade is likely to be the least well tested of any
> TPM capability, so there's a good chance at least one TPM out there
> isn't going to behave as the standard says it should.
> 
> Regards,
> 
> James

I get that depending on configuration someone really would want to
have guaranteed exclusive access to the device. Since it is opt-in
via O_EXCL, I don't have anything in principle againts adding it.

The patch set needs rework but feature itself is totally fine.

BR, Jarkko