Re: [PATCH 2/9] tpm: Allow PCR 23 to be restricted to kernel-only use
From: Matthew Garrett
Date: Sun Feb 28 2021 - 03:04:23 EST
On Wed, Feb 24, 2021 at 10:00:53AM -0800, James Bottomley wrote:
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On Sat, 2021年02月20日 at 01:32 +0000, Matthew Garrett wrote:
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> Under certain circumstances it might be desirable to enable the
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> creation of TPM-backed secrets that are only accessible to the
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> kernel. In an ideal world this could be achieved by using TPM
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> localities, but these don't appear to be available on consumer
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> systems.
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I don't understand this ... the localities seem to work fine on all the
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systems I have ... is this some embedded thing?
I haven't made it work on an HP Z440 or a Lenovo P520. So now I'm
wondering whether having chipsets with TXT support (even if it's turned
off) confuse this point. Sigh. I'd really prefer to use localities than
a PCR, so if it works on client platforms I'd be inclined to say we'll
do a self-test and go for that, and workstation vendors can just
recommend their customers use UPSes or something.
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> An alternative is to simply block userland from modifying one of the
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> resettable PCRs, leaving it available to the kernel. If the kernel
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> ensures that no userland can access the TPM while it is carrying out
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> work, it can reset PCR 23, extend it to an arbitrary value, create or
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> load a secret, and then reset the PCR again. Even if userland somehow
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> obtains the sealed material, it will be unable to unseal it since PCR
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> 23 will never be in the appropriate state.
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This seems a bit arbitrary: You're removing this PCR from user space
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accessibility, but PCR 23 is defined as "Application Support" how can
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we be sure no application will actually want to use it (and then fail)?
Absolutely no way of guaranteeing that, and enabling this option is
certainly an ABI break.
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Since PCRs are very scarce, why not use a NV index instead. They're
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still a bounded resource, but most TPMs have far more of them than they
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do PCRs, and the address space is much bigger so picking a nice
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arbitrary 24 bit value reduces the chance of collisions.
How many write cycles do we expect the NV to survive? But I'll find a
client system with a TPM and play with locality support there - maybe we
can just avoid this problem anyway.