[Bug 1604873] Re: MokSBStateRT strictly inferior to /proc/sys/kernel/moksbstate_disabled

[Steve Langasek]

** Description changed:

+ [SRU Justification]
+ In some cases, incorrect locally-set EFI variables can prevent the shim-signed package from detecting that SecureBoot is active on the system.  As a result, the user will not be prompted to disable SecureBoot, and will be left with non-functional dkms modules after reboot to the new kernel.
+ 
+ [Test case]
+ 1. Install Ubuntu on a system (or VM) with SecureBoot enabled.
+ 2. Create the /sys/firmware/efi/efivars/MokSBStateRT-605dab50-e046-4300-abb6-3dd810dd8b23 variable with a value of 1 XXX: figure out how to do this
+ 3. Install shim-signed from -updates.
+ 4. Install the dahdi-dkms package.
+ 5. Confirm that you are not prompted to disable secureboot.
+ 6. Install shim-signed from -proposed.
+ 7. Confirm that you *are* prompted to disable secureboot.
+ 8. Delete the /sys/firmware/efi/efivars/MokSBStateRT-605dab50-e046-4300-abb6-3dd810dd8b23 variable.
+ 
+ 
  update-secureboot-policy tries to check whether MOK's override has disabled SecureBoot state.  However, since the real variable in nvram is not accessible after boot, it needs to use a proxy for this information.  There are two that it tries to use:
-  - We've specified how shim can mirror the MokSBState variable to MokSBStateRT at boot time, to expose this information to the OS (but this is not implemented in current shim).
-  - The recent kernels which honor MokSBState also include support for exposing this value as  /proc/sys/kernel/moksbstate_disabled.
+  - We've specified how shim can mirror the MokSBState variable to MokSBStateRT at boot time, to expose this information to the OS (but this is not implemented in current shim).
+  - The recent kernels which honor MokSBState also include support for exposing this value as  /proc/sys/kernel/moksbstate_disabled.
  
  Neither of these is guaranteed to be present on any given system.
  However, if present, the kernel variable should be *unconditionally*
  preferred over the efi "shadow" variable - because the kernel variable
  is immutable, whereas MokSBStateRT is just another nvram variable that
  things can overwrite (though they shouldn't).
  
  We have heard at least one report internally of a system where something
  other than our shim is setting the value of MokSBStateRT and confusing
  update-secureboot-policy, so this will be a priority to also fix in SRU.

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https://bugs.launchpad.net/bugs/1604873

Title:
  MokSBStateRT strictly inferior to /proc/sys/kernel/moksbstate_disabled

Status in shim-signed package in Ubuntu:
  Fix Released
Status in shim-signed source package in Precise:
  New
Status in shim-signed source package in Trusty:
  New
Status in shim-signed source package in Wily:
  New
Status in shim-signed source package in Xenial:
  New

Bug description:
  [SRU Justification]
  In some cases, incorrect locally-set EFI variables can prevent the shim-signed package from detecting that SecureBoot is active on the system.  As a result, the user will not be prompted to disable SecureBoot, and will be left with non-functional dkms modules after reboot to the new kernel.

  [Test case]
  1. Install Ubuntu on a system (or VM) with SecureBoot enabled.
  2. Create the /sys/firmware/efi/efivars/MokSBStateRT-605dab50-e046-4300-abb6-3dd810dd8b23 variable with a value of 1 XXX: figure out how to do this
  3. Install shim-signed from -updates.
  4. Install the dahdi-dkms package.
  5. Confirm that you are not prompted to disable secureboot.
  6. Install shim-signed from -proposed.
  7. Confirm that you *are* prompted to disable secureboot.
  8. Delete the /sys/firmware/efi/efivars/MokSBStateRT-605dab50-e046-4300-abb6-3dd810dd8b23 variable.

  
  update-secureboot-policy tries to check whether MOK's override has disabled SecureBoot state.  However, since the real variable in nvram is not accessible after boot, it needs to use a proxy for this information.  There are two that it tries to use:
   - We've specified how shim can mirror the MokSBState variable to MokSBStateRT at boot time, to expose this information to the OS (but this is not implemented in current shim).
   - The recent kernels which honor MokSBState also include support for exposing this value as  /proc/sys/kernel/moksbstate_disabled.

  Neither of these is guaranteed to be present on any given system.
  However, if present, the kernel variable should be *unconditionally*
  preferred over the efi "shadow" variable - because the kernel variable
  is immutable, whereas MokSBStateRT is just another nvram variable that
  things can overwrite (though they shouldn't).

  We have heard at least one report internally of a system where
  something other than our shim is setting the value of MokSBStateRT and
  confusing update-secureboot-policy, so this will be a priority to also
  fix in SRU.

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