[FFe] Dual-signed shim

[Dimitri John Ledkov]

[FFe] Dual-signed shim

shim-signed package currently ships two files

/usr/lib/shim/shimx64.efi.signed
/usr/lib/shim/shimx64.efi.dualsigned

The two shims are the same, but have different signatures.

.signed is signed with MS UEFI CA 2011 only

.dualsigned is signed with Canonical CA & MS UEFI CA 2011.

$ sbverify --list /usr/lib/shim/shimx64.efi.signed
warning: data remaining[1177936 vs 1341560]: gaps between PE/COFF sections?
signature 1
image signature issuers:
 - /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Corporation UEFI CA 2011
image signature certificates:
 - subject: /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Windows UEFI Driver Publisher
   issuer: /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Corporation UEFI CA 2011
 - subject: /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Corporation UEFI CA 2011
   issuer: /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Corporation Third Party Marketplace Root

$ sbverify --list /usr/lib/shim/shimx64.efi.dualsigned
warning: data remaining[1179856 vs 1343480]: gaps between PE/COFF sections?
signature 1
image signature issuers:
 - /C=GB/ST=Isle of Man/L=Douglas/O=Canonical Ltd./CN=Canonical Ltd. Master Certificate Authority
image signature certificates:
 - subject: /C=GB/ST=Isle of Man/O=Canonical Ltd./OU=Secure Boot/CN=Canonical Ltd. Secure Boot Signing (2017)
   issuer: /C=GB/ST=Isle of Man/L=Douglas/O=Canonical Ltd./CN=Canonical Ltd. Master Certificate Authority
signature 2
image signature issuers:
 - /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Corporation UEFI CA 2011
image signature certificates:
 - subject: /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Windows UEFI Driver Publisher
   issuer: /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Corporation UEFI CA 2011
 - subject: /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Corporation UEFI CA 2011
   issuer: /C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/CN=Microsoft Corporation Third Party Marketplace Root

In light of the current Boothole vulnerabilities, it is desirable to support a more constrained boot chain. Specifically, UEFI 2011 signed Canonical shim can be booted universally on most hardware but also means other shims from other vendors can boot too. But if we provide a shim signed by Canonical CA, one can remove MS UEFI 2011 key from db, add Canonical CA, and thus only boot shims provided by canonical. In such scenario machine will only be able to boot Windows and Ubuntu, and no other Linux. Furthermore Windows production key can be removed from db as well, if one wishes to disable booting Windows too.

Certain hardware manufacturers ship Canonical CA key in db already. Thus out-of-the-box shipping dual-signed shim would improve security there, by reducing attack-vectors / having more constrained TPM measurements.

I am requesting FFe to ship dualsigned shim as /usr/lib/shim/shimx64.efi.signed and use it by default.

Regression potential is as follows:

 - very old / initial implementations of Secureboot using very old UEFI SB specs from 2008 do not support multiple signatures on .efi binary. Thus this change, may result in certain older firmware unable to boot. It is not clear with hardware doesn't support multiple signatures. And whether or not the order of signatures helps at all (i.e. if MS signature or Canonical one is first)

Mitigation strategy in case of regressions:

 - revert back to single-signed shim

[Steve Langasek (vorlon)]

Because there is concrete risk of regression that depends on hardware-specific testing, I do not think this is appropriate for an FFe. The dual-signed object is available and particular system configurations can manually opt into it but we should not, during feature freeze, push this out to all users by default.

[Julian Andres Klode (juliank)]

FTR: NSA says[1] that most UEFI implementations only support one hash, which might be the first one or a random one, and upstream confirms that to some extent - PE 8.3 not having a coherent spec for alignment or padding of signatures, causing incompatibilites - so that seems like a no-go anyway.

[1] 4.3.2 " Most UEFI implementations only read one/the first signature in a binary. Remove or overwrite existing signatures before signing." -https://media.defense.gov/2020/Sep/15/2002497594/-1/-1/0/CTR-UEFI-SECURE-BOOT-CUSTOMIZATION-20200915.PDF/CTR-UEFI-SECURE-BOOT-CUSTOMIZATION-20200915.PDF