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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
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Date: Thu, 3 Mar 2022 13:10:56 +0100
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Subject: [PATCH] grub-core/loader/i386/efi/linux.c: do not validate kernels
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twice
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On codebases that have shim-lock-verifier built into the grub core
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(like 2.06 upstream), shim-lock-verifier is in enforcing mode when
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booted with secureboot. It means that grub_cmd_linux() command
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attempts to perform shim validate upon opening linux kernel image,
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including kernel measurement. And the verifier correctly returns file
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open error when shim validate protocol is not present or shim fails to
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validate the kernel.
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This makes the call to grub_linuxefi_secure_validate() redundant, but
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also harmful. As validating the kernel image twice, extends the PCRs
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with the same measurement twice. Which breaks existing sealing
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policies when upgrading from grub2.04+rhboot+sb+linuxefi to
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grub2.06+rhboot+sb+linuxefi builds. It is also incorrect to measure
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the kernel twice.
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This patch must not be ported to older editions of grub code bases
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that do not have verifiers framework, or it is not builtin, or
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shim-lock-verifier is an optional module.
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This patch is tested to ensure that unsigned kernels are not possible
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to boot in secureboot mode when shim rejects kernel, or shim protocol
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is missing, and that the measurements become stable once again. The
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above also ensures that CVE-2020-15705 is not reintroduced.
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Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
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---
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grub-core/loader/i386/efi/linux.c | 13 -------------
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1 file changed, 13 deletions(-)
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diff --git a/grub-core/loader/i386/efi/linux.c b/grub-core/loader/i386/efi/linux.c
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index 3cf0f9b330..941df6400b 100644
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--- a/grub-core/loader/i386/efi/linux.c
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+++ b/grub-core/loader/i386/efi/linux.c
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@@ -30,7 +30,6 @@
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#include <grub/cpu/efi/memory.h>
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#include <grub/tpm.h>
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#include <grub/safemath.h>
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-#include <grub/efi/sb.h>
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GRUB_MOD_LICENSE ("GPLv3+");
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@@ -278,7 +277,6 @@ grub_cmd_linux (grub_command_t cmd __attribute__ ((unused)),
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grub_ssize_t start, filelen;
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void *kernel = NULL;
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int setup_header_end_offset;
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- int rc;
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grub_dl_ref (my_mod);
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@@ -308,17 +306,6 @@ grub_cmd_linux (grub_command_t cmd __attribute__ ((unused)),
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goto fail;
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}
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- if (grub_efi_get_secureboot () == GRUB_EFI_SECUREBOOT_MODE_ENABLED)
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- {
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- rc = grub_linuxefi_secure_validate (kernel, filelen);
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- if (rc <= 0)
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- {
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- grub_error (GRUB_ERR_INVALID_COMMAND,
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- N_("%s has invalid signature"), argv[0]);
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- goto fail;
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- }
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- }
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-
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lh = (struct linux_i386_kernel_header *)kernel;
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grub_dprintf ("linux", "original lh is at %p\n", kernel);
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