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TPM provides zero practical security

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Re: TPM provides zero practical security

#11

Post by donmcronald »

> the signature of the BIOS is checked against a public key whose hash is stored in fuses

> Each of dozens (up to hundreds) of UEFI drivers written by various OEMs with varying levels of competence and care are loaded

Doesn't the BIOS signature encompass those drivers? Put another way, isn't the BIOS vendor attesting those drivers are non-malicious with their signature?

I think the TPM will turn out to be a net negative for consumers since it's going to get used to get used for attestations users can't control (ie: against the will of the user), but there are some benefits. Having a BitLocker key unlocked via a PIN where the TPM can protect against brute force attacks is useful for me. That alone covers most of my threat model which is having my data extracted from a lost or stolen PC.

Re: TPM provides zero practical security

#12

Post by Nextgrid »

Unfortunately this sounds like a typical pro-Linux rant with the usual scare words such as "Microsoft", "UEFI", "secure boot", etc. To be clear, I am attacking the piece itself, not the author.

The reason there is no explicit threat model defined in the TPM specs is because it defines a general-purpose hardware security module. It is up to integrator to define the threat model (TPM's security properties also depend on the rest of the system) and the application.

Even if a TPM is not perfect and depends on other pieces of the puzzle to also be secure, it at least opens the possibility of making it secure in the future once those vulnerabilities are discovered & fixed. Furthermore, even in this vulnerable state, it still increases the effort required for a successful attack.

Support for TPM-backed full disk encryption means you can now have FDE on by default for everyone with no usability impact at all. Even if it's not secure and a dedicated attack will still break it, it means a casual attacker can't just pull a drive or reboot the machine and run chntpw or steal sensitive data from discarded drives that haven't been properly wiped.

I like TPMs. I like the fact that a rogue datacenter employee or intruder can't just pull one of my servers' drives out and get sensitive data. I like not having to worry about having sensitive keys on the filesystem somewhere because every secret is in memory and is ultimately derived from the TPM doing remote attestation at boot and handing ephemeral keys. I like not having to worry about unattended reboots or entering LUKS passphrases remotely.

Re: TPM provides zero practical security

#13

Post by mgbmtl »

marcosdumay wrote: post #4

On the one usage scenario that benefits a PC user, the TPM makes for a really bad yubikey. You can't carry it between computers, you can't back it up, and you are certain to lose it at some point when the computer breaks of gets outdated. That means it either requires a second protocol for authentication, or that you will lose your accounts with all kinds of services all the time.

Depends on the implementation? For many services, I register my Yubikey, but also the Android fingerprint authentication (as well as TOTP as another fallback).

So for example, if I login to Gitlab on my phone, I can use my fingerprint (lockscreen auth). It's more convenient than using the TOTP app.

Similarly, I could register a TPM from my desktop that could be the same as using the fingerprint auth? It would only work from that desktop, but it's the same logic as my phone, and in a sense, that's a nice benefit.

Re: TPM provides zero practical security

#14

Post by [deleted] »

awesomeMilou wrote: post #6

At this point I don't understand why hardware vendors can't just do it like Apple. Put a small ARM SoC with some firmware in ROM onto the mainboard that starts before the main CPU and initializes it, ensuring that the system is in a known state before any components boot.

[deleted]

Re: TPM provides zero practical security

#15

Post by g_p »

mschuster91 wrote: post #8
awesomeMilou wrote: post #6

At this point I don't understand why hardware vendors can't just do it like Apple. Put a small ARM SoC with some firmware in ROM onto the mainboard that starts before the main CPU and initializes it, ensuring that the system is in a known state before any components boot.

That's actually how modern Intel and AMD CPUs work.

Indeed, and TPM + secure boot broadly define how these built-in firmware TPMs (fTPMs) implement verification and validation of system components.

TPM is the specification and standard for a predictable way this is implemented, and most modern CPUs do this as you say, with option ROM validation, UEFI firmware integrity checking, etc.

Re: TPM provides zero practical security

#17

Post by helloooooooo »

Ask Google exactly how they enforce their zero trust, VPN-less remote work environment. Hint: it has to do with the TPM. DRTM + Device Certificates + TLS Token Binding is a huge deal for proving that the endpoint is trusted, and that the principal actually logging in is using an approved device. DRTM prevents boot time tampering by assuring that the measured boot state is consistent with what the network expects.

Re: TPM provides zero practical security

#18

Post by candiddevmike »

Nextgrid wrote: post #12

Unfortunately this sounds like a typical pro-Linux rant with the usual scare words such as "Microsoft", "UEFI", "secure boot", etc. To be clear, I am attacking the piece itself, not the author. The reason there is no explicit threat model defined in the TPM specs is because it defines a general-purpose hardware security module. It is up to integrator to define the threat model (TPM's security properties also depend o…

What's the point of TPM-backed full disk encryption with no usability impact (meaning password/pin-less) for the average user who is more likely to get their device stolen vs some covert disk image shenanigan?

Re: TPM provides zero practical security

#19

Post by [deleted] »

ak217 wrote: post #3

> You can also use the TPM + PIN as a sort of Yubikey That's not zero. In my mind that's the main thing a TPM is really useful for. It's a secure enclave for a private key used for U2F/WebAuthn style attestation. I agree that the threat model not being explicitly discussed is a huge miss. But to that point, a TPM is still useful because it prevents someone who has hacked into my computer from commanding the TPM's aut…

[deleted]

Re: TPM provides zero practical security

#20

Post by Nextgrid »

candiddevmike wrote: post #18
Nextgrid wrote: post #12

Unfortunately this sounds like a typical pro-Linux rant with the usual scare words such as "Microsoft", "UEFI", "secure boot", etc. To be clear, I am attacking the piece itself, not the author. The reason there is no explicit threat model defined in the TPM specs is because it defines a general-purpose hardware security module. It is up to integrator to define the threat model (TPM's security properties also depend o…

What's the point of TPM-backed full disk encryption with no usability impact (meaning password/pin-less) for the average user who is more likely to get their device stolen vs some covert disk image shenanigan?

If the device is stolen it can still enforce OS-level authentication (including potentially phoning home, invalidating its access to remote resources, or erasing itself), except now you can't bypass it by rebooting and running chntpw.

Will this stop a dedicated attacker? Probably not, although a fTPM with an up to date OS would require the attacker to find an exploit for this machine's early boot firmware (UEFI, etc) or burn a Windows zero-day, both of which are very costly.

It does however prevent your casual thief from watching a YT video "how to reset windows password using linux live cd" and then getting access to your sensitive data (browser's saved passwords, etc), so it's a major improvement.

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