Don't make shim abort when TPM log event fails (RHBZ #2002265) #414
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frozencemetery
approved these changes
Sep 10, 2021
mok.c
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| return efi_status; | ||
| if (EFI_ERROR(tpm_status)) { | ||
| dprint(L"tpm_measure_variable(\"%s\",%lu,0x%llx)->%r (ignored)\n", | ||
| v->name, FullDataSize, FullData, tpm_status); |
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I know the codebase isn't great about this and the problem predates your change, but it would be nice to keep lines <80 characters when it's easy to do so.
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In general I agree, but I also really don't like breaking output strings in the middle, as that makes them harder to grep for. So in this case not wrapping is the right thing IMO.
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Agreed on the output string, but I'm actually talking about the next line (that supplies the other args to dprint()).
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Oh, I see. github's elision of most of the whitespace has fooled me.
vathpela
requested changes
Sep 10, 2021
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I don't really like this approach. Firstly, I think this firmware is really broken. As I said in the private RH bugzilla:
Just reading code and not having any debugging information, this appears that when
"On without Pre-Boot Measurements" is selected, the firmware is registering a protocol
using EFI_TCG_PROTOCOL_GUID that has the expected methods. That protocol provides
a ->StatusCheck() method which returns EFI_SUCCESS and sets the capabilities to
{.TPMPresentFlag = 1, .TPMDeactivatedFlag = 0}. But then, it also provides a
->HashLogExtendEvent() method that returns EFI_UNSUPPORTED?
Is that what's happening? If so, this seems wrong to me. Surely one of the following is
the correct way to implement this:
- set TPMPresentFlag = 0
- set TPMDeactivatedFlag = 1
- don't register the protocol if it's not going to implement anything
But also I'd like a more targeted workaround than ignoring all errors. In this case we're getting EFI_UNSUPPORTED back from calls that should be present, indicating that ->StatusCheck() is lying to us and providing a stubbed out, broken implementation. If that is what's going on, what we should do is add a "tpm_defective" status variable that defaults to false, make tpm_present() always return false if it is set, and make tpm 1.2 calls in tpm.c set it to false if the protocol method they're using gives us EFI_UNSUPPORTED.
On Dell hardware booted in UEFI with option TPM 1.2 "On without Pre-Boot Measurements", it appears that `tpm_log_event()` fails with Unsupported, which causes Shim to abort due to believing it couldn't set up the MokListRT, MokListXRT and SbatLevelRT variables. This patch ignore the error when trying to write to the TPM and sets the TPM as 'defective' to not try to write to it anymore. Signed-off-by: Renaud Métrich <rmetrich@redhat.com>
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@vathpela Thanks for the review. I updated the code according to your suggestion. |
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Log excerpt of the proposed code: |
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On Dell hardware booted in UEFI with option TPM 1.2 "On without Pre-Boot
Measurements", it appears that
tpm_log_event()fails with Unsupported,which causes Shim to abort due to believing it couldn't set up the
MokListRT, MokListXRT and SbatLevelRT variables.
This patch mimics what is done in Grub code: just ignore failures
related to the TPM.
Below is the verbose output of Shim.