Canonical-signed GRUB 2.14 bypasses Secure Boot lockdown via serial MMIO

Canonical-signed GNU GRUB 2.14 admits a lockdown bypass that lets a local attacker load unsigned modules while UEFI Secure Boot lockdown still reports as active. The flaw is in the serial command: under lockdown it still accepts a caller-supplied 64-bit MMIO base address and does not check that the target is a real UART rather than ordinary RAM.

An attacker who controls a GRUB configuration source accepted without configuration authentication can use the lockdown-available gdbinfo command to recover the image base, point the serial MMIO base at the head of GRUB's authoritative file-verifier list, and trigger input-only serial initialization. That initialization clears the list head. GRUB's empty-verifier-list behavior then fails open, admitting unsigned native modules even though lockdown continues to report enabled.

The vector is not remote and is not available to ordinary users. Prerequisites include control of or ability to select the boot path, continued shim/SBAT admission of the affected image, a structurally valid unsigned module, knowledge of loaded image placement, and a serial input poll. Administrator-controlled boot configuration or controlled removable media are representative routes; root or physical access alone does not satisfy every requirement.

The issue was confirmed on Ubuntu's grub-efi-amd64-signed package (gcdx64.efi.signed from the 1.215+2.14-2ubuntu1 build). Sibling signed images from the same package share the serial implementation and lockdown command set. The MMIO serial logic is upstream GRUB code present in builds that include ns8250 MMIO support, so other signed distributions may be exposed as well. Luppa reported the finding on the oss-security list.