secureboot: add check to validate signature db certificates before updating - #1146
secureboot: add check to validate signature db certificates before updating#1146Rolv-Apneseth wants to merge 1 commit into
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📝 WalkthroughWalkthroughThe change adds EFI-only Secure Boot certificate validation. Update preparation checks for the Microsoft UEFI CA 2023 certificate before continuing. A new qemu kola test exercises the validation on an x86_64 Secure Boot system. ChangesSecure Boot certificate validation
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The update guard may treat an unreadable Secure Boot status as safe and allow an update without validating the firmware signature database, potentially leaving affected Secure Boot systems unable to boot. This fail-open behavior should be corrected before merge. Sequence Diagram(s)sequenceDiagram
participant bootupctl
participant bootupd
participant secureboot
participant EFI_variables
bootupctl->>bootupd: request update
bootupd->>secureboot: validate_secureboot_for_update()
secureboot->>EFI_variables: read SecureBoot and db
EFI_variables-->>secureboot: Secure Boot state and signatures
secureboot-->>bootupd: continue or return validation error
bootupd-->>bootupctl: update result
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Full details: Linked Issues checkExplanation The changes implement the core objective in issue [ Full details: Commit Message ConventionExplanation The PR contains one non-merge commit: ✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@src/secureboot.rs`:
- Around line 20-28: Update is_secureboot_enabled to return an error when
sysfs::varstore_read cannot read SECURE_BOOT or var.data() is empty, and return
success with disabled status only when the readable first byte is zero. Update
validate_secureboot_for_update to propagate these errors while skipping the
signature database check only for a confirmed zero value, and add regression
coverage for both unknown-state failures and the readable-disabled path.
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Cargo.tomlsrc/bootupd.rssrc/main.rssrc/secureboot.rstests/kola/test-secureboot
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Bot suggestion to bail when unable to read Secure Boot variables was incorrect as proven by the CI failure. UEFI can be enabled while the Secure Boot variables don't even exist. |
| Ok(false) => bail!( | ||
| "Secure Boot is enabled but the Microsoft UEFI CA 2023 certificate was not \ | ||
| found in the firmware's signature database. Updating the shim could render this system \ | ||
| unbootable. Please update your system firmware by using, for example, fwupd." |
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I think the error message is good, but could be improved. Instead of focus on fwupd it could be a little more generic and give the user other options to be more clear, something like:
""Please update your system firmware using your firmware setup menu, fwupd, or similar tools."
WDYT?
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The fwupd maintainer did tell me that 99% of updates are done through some kind of UI not fwupdmgr... I was tailoring this more towards FCOS itself but maybe that's reasonable if we want bootupd to be distro-agnostic.
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Thanks for working on this, overall looks pretty good. I just made a suggestion about improving the message error, but is not a blocker. Approved. :) |
| return Ok(()); | ||
| }; | ||
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| match db_contains_ms_2023_cert() { |
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And we're just assuming that by the time someone updates bootupd with this patch, they will also have a need to update the corresponding shim?
That could break some legitimate use cases where someone e.g. pulls a newer bootupd for an unrelated bug fix but is on an older OS stream and hasn't yet planned to update shim (though arguably they should).
One option is we could try to extract the signing state of the shim binary? Another is we could offer a configuration knob (embedded in the image) that says whether this feature should be on or not.
Something like bootupctl backend install --require-ms-2023-cert that drops a bit of state in EFI.json which is honored here.
Then we change how we generate the base image to enable that flag by default, but anyone who wanted to opt out could do so with a custom image build.
And perhaps simpler it'd be nice to have e.g. bootupctl backend --require-ms-2023-cert=false which would mutate the JSON to drop that requirement
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The flag to disable it is a good idea. I believe your last point is to have it be enabled by default and we just ask users who do have the more niche use cases to manually disable it after they've updated bootupd, right? That would be my preferred approach.
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We've gotten burned in the past by enabling things like this by default. In this case, it's probably fine.
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I understand, it's just hat the intent here is specifically to help prevent people from breaking their own setups unknowingly. Seems like disabling updates is the safer default in this scenario.
I'll add the flag and necessary state updates. If I add a test, the only thing I can think of doing is checking the flag updates state as expected. Any better ideas?
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Making it a CLI option is also viable and perhaps the simplest...anyone who hits this and wants to bypass it could just pass that.
Kind of in the general case we need to have config options embedded in the image and overridable via dynamic invocations.
3 participants
Closes #1099
The main concern this resolves is that, with the Microsoft UEFI CA 2011 Certificate having expired, any further updates to the shim will not be dual-signed, and will only be signed by the 2023 key. This means that any system with Secure Boot enabled will be broken if
bootupdupdates the shim and the system does not have the certificate available in their firmware's signature database. So we should make surebootupdis aware of this requirement so we don't break user's Secure Boot systems.As stated in the issue, we wanted a way to incorporate some of the
sbchooserlogic to achieve this goal. However, I found this crate which suits our use-case perfectly. I discussed briefly with the creator and he agreed it was a good fit.I've implemented this in such a way that any update will be blocked if Secure Boot is enabled and the 2023 cert is not in the system's signature DB. So updating is allowed as long as any one of the following is true:
Something that might be nice but would add some complexity is only blocking updates if the shim is actually being updated, as that's the only one that matters here. Any thoughts on this? I think it's gonna be a brief enough window where this is actually helpful that keeping it as simple as possible will benefit us more in the long run. Plus Secure Boot systems with outdated certificates should indeed have their firmware updated as soon as possible. I'd be happy to try implement the module comparison logic if we think it's important to only block on shim updates though.
Also for testing, I have a positive case test, and tested the negative case manually. I can't think of a good way to test the negative case with kola. Maybe including a feature in kola to enable using the db with an older certificate (binary blob?), but I'm not sure that's worth it.