[Bug 1895817] Re: [FFe] Dual-signed shim

[Julian Andres Klode]

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

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Title:
  [FFe] Dual-signed shim

Status in shim-signed package in Ubuntu:
  Won't Fix

Bug description:
  [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

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