A GRUB2 regression update has been released for Ubuntu Linux 20.04 LTS, 18.04 LTS,16.04 LTS, and 14.04 ESM.

USN-4432-2: GRUB2 regression

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Ubuntu Security Notice USN-4432-2
August 04, 2020

grub2, grub2-signed regression
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 20.04 LTS
- Ubuntu 18.04 LTS
- Ubuntu 16.04 LTS
- Ubuntu 14.04 ESM

Summary:

USN-4432-1 introduced a regression in the GRUB2 bootloader.

Software Description:
- grub2: GRand Unified Bootloader
- grub2-signed: GRand Unified Bootloader

Details:

USN-4432-1 fixed vulnerabilities in GRUB2 affecting Secure Boot
environments. Unfortunately, the update introduced regressions for
some BIOS systems (either pre-UEFI or UEFI configured in Legacy mode),
preventing them from successfully booting. This update addresses
the issue.

Users with BIOS systems that installed GRUB2 versions from USN-4432-1
should verify that their GRUB2 installation has a correct understanding
of their boot device location and installed the boot loader correctly.

We apologize for the inconvenience.

Original advisory details:

Jesse Michael and Mickey Shkatov discovered that the configuration parser
in GRUB2 did not properly exit when errors were discovered, resulting in
heap-based buffer overflows. A local attacker could use this to execute
arbitrary code and bypass UEFI Secure Boot restrictions. (CVE-2020-10713)

Chris Coulson discovered that the GRUB2 function handling code did not
properly handle a function being redefined, leading to a use-after-free
vulnerability. A local attacker could use this to execute arbitrary code
and bypass UEFI Secure Boot restrictions. (CVE-2020-15706)

Chris Coulson discovered that multiple integer overflows existed in GRUB2
when handling certain filesystems or font files, leading to heap-based
buffer overflows. A local attacker could use these to execute arbitrary
code and bypass UEFI Secure Boot restrictions. (CVE-2020-14309,
CVE-2020-14310, CVE-2020-14311)

It was discovered that the memory allocator for GRUB2 did not validate
allocation size, resulting in multiple integer overflows and heap-based
buffer overflows when handling certain filesystems, PNG images or disk
metadata. A local attacker could use this to execute arbitrary code and
bypass UEFI Secure Boot restrictions. (CVE-2020-14308)

Mathieu Trudel-Lapierre discovered that in certain situations, GRUB2
failed to validate kernel signatures. A local attacker could use this
to bypass Secure Boot restrictions. (CVE-2020-15705)

Colin Watson and Chris Coulson discovered that an integer overflow
existed in GRUB2 when handling the initrd command, leading to a heap-based
buffer overflow. A local attacker could use this to execute arbitrary code
and bypass UEFI Secure Boot restrictions. (CVE-2020-15707)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 20.04 LTS:
grub-efi-amd64-bin 2.04-1ubuntu26.2
grub-efi-amd64-signed 1.142.4+2.04-1ubuntu26.2
grub-efi-arm-bin 2.04-1ubuntu26.2
grub-efi-arm64-bin 2.04-1ubuntu26.2
grub-efi-arm64-signed 1.142.4+2.04-1ubuntu26.2
grub-efi-ia32-bin 2.04-1ubuntu26.2

Ubuntu 18.04 LTS:
grub-efi-amd64-bin 2.02-2ubuntu8.17
grub-efi-amd64-signed 1.93.19+2.02-2ubuntu8.17
grub-efi-arm-bin 2.02-2ubuntu8.17
grub-efi-arm64-bin 2.02-2ubuntu8.17
grub-efi-arm64-signed 1.93.19+2.02-2ubuntu8.17
grub-efi-ia32-bin 2.02-2ubuntu8.17
grub-efi-ia64-bin 2.02-2ubuntu8.17

Ubuntu 16.04 LTS:
grub-efi-amd64-bin 2.02~beta2-36ubuntu3.27
grub-efi-amd64-signed 1.66.27+2.02~beta2-36ubuntu3.27
grub-efi-arm-bin 2.02~beta2-36ubuntu3.27
grub-efi-arm64-bin 2.02~beta2-36ubuntu3.27
grub-efi-arm64-signed 1.66.27+2.02~beta2-36ubuntu3.27
grub-efi-ia32-bin 2.02~beta2-36ubuntu3.27
grub-efi-ia64-bin 2.02~beta2-36ubuntu3.27

Ubuntu 14.04 ESM:
grub-efi-amd64-bin 2.02~beta2-9ubuntu1.17
grub-efi-amd64-signed 1.34.20+2.02~beta2-9ubuntu1.17
grub-efi-arm-bin 2.02~beta2-9ubuntu1.17
grub-efi-arm64-bin 2.02~beta2-9ubuntu1.17
grub-efi-ia32-bin 2.02~beta2-9ubuntu1.17
grub-efi-ia64-bin 2.02~beta2-9ubuntu1.17

Fully mitigating these vulnerabilities requires both an updated
GRUB2 boot loader and the application of a UEFI Revocation
List (dbx) to system firmware. Ubuntu will provide a packaged
dbx update at a later time, though system adminstrators may
choose to apply a third party dbx update before then. For more
details on mitigation steps and the risks entailed (especially for
dual/multi-boot scenarios), please see the Knowledge Base article at
  https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/GRUB2SecureBootBypass

References:
  https://usn.ubuntu.com/4432-2
  https://usn.ubuntu.com/4432-1
  https://launchpad.net/bugs/1889556
  https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/GRUB2SecureBootBypass

Package Information:
  https://launchpad.net/ubuntu/+source/grub2/2.04-1ubuntu26.2
  https://launchpad.net/ubuntu/+source/grub2-signed/1.142.4
  https://launchpad.net/ubuntu/+source/grub2/2.02-2ubuntu8.17
  https://launchpad.net/ubuntu/+source/grub2-signed/1.93.19
  https://launchpad.net/ubuntu/+source/grub2/2.02~beta2-36ubuntu3.27
  https://launchpad.net/ubuntu/+source/grub2-signed/1.66.27