Secure AuthenticationのトピックRe: Concurrency issue with hmac init-update-finish

Concurrency issue with hmac init-update-finish

bug_squasher_77 — Fri, 18 Nov 2022 07:43:27 GMT

Following up from

While doing some stress testing, I witnessed wrong HMAC would return.

Further investigation revealed that if one process is killed while calling sss_mac_update()
Very next process would return wrong HMAC.

For example, see the code in the screenshot,

It will randomly kill the current process just after sss_mac_update, without cleanup. (Simulate systemd watchdog reset…)

The next hmac calculation gives wrong result without any error.

Digging into the source code, I see that CryptoObject is created inside sss_mac_init() and deleted inside sss_mac_context_free()

However, there is logic to reuse the context in sss_mac_init that is causing the issue.

There could be a way to disable this behavior. Instead of reusing, I want to be able to delete existing and create new CryptoObject everytime there is mac_init

Right now, I have implemented this workaround,

I am not sure if I should be using internal APIs or stick to first level sss_mac* APIs.

Summary.

- ~~Should I use~~ ~~SSSFTR_SE05X_CREATE_DELETE_CRYPTOOBJ ? The documentation says I need to initialize all CryptoObject at time of provisioning. I don't think it will achieve what I am looking for~~
- Should I use my workaround and use internal APIs?
- I want to control this behavior programmatically (or via compile time if not possible programmatically), so a feature request for se05x dev team.

Edit: I tested SSS_FTR_SE05x_CREATE_DELETE_CRYPTOOBJ=0, it simply refuses to calculate hmac with init-update-finish. Which is not what I am looking for.

Re: Concurrency issue with hmac init-update-finish

Kan_Li — Mon, 21 Nov 2022 06:12:41 GMT

Hi @bug_squasher_77 ,

Actually there is a macro of EX_SSS_BOOT_DO_ERASE which enables similar operation as your workaround in the MW.

Maybe you can reuse this mechanism as well? or you prefer to do it in your user application?

Have a great day,
Kan

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Re: Concurrency issue with hmac init-update-finish

bug_squasher_77 — Mon, 21 Nov 2022 06:21:01 GMT

Hey @Kan_Li ,

This also deletes all stored keys and performs factory reset - Not usable in production.

Re: Concurrency issue with hmac init-update-finish

Kan_Li — Mon, 21 Nov 2022 09:31:38 GMT

Hi @bug_squasher_77 ,

I know, but maybe you can adjust it according to your application.

Have a great day,
Kan