grub2/0236-linuxefi-fail-kernel-validation-without-shim-protoco.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Dimitri John Ledkov <xnox@ubuntu.com>
Date: Wed, 22 Jul 2020 11:31:43 +0100
Subject: [PATCH] linuxefi: fail kernel validation without shim protocol.

If certificates that signed grub are installed into db, grub can be
booted directly. It will then boot any kernel without signature
validation. The booted kernel will think it was booted in secureboot
mode and will implement lockdown, yet it could have been tampered.

This version of the patch skips calling verification, when booted
without secureboot. And is indented with gnu ident.

CVE-2020-15705

Reported-by: Mathieu Trudel-Lapierre <cyphermox@ubuntu.com>
Signed-off-by: Dimitri John Ledkov <xnox@ubuntu.com>
---
 grub-core/loader/arm64/linux.c     | 12 ++++++++----
 grub-core/loader/efi/chainloader.c |  1 +
 grub-core/loader/efi/linux.c       |  1 +
 grub-core/loader/i386/efi/linux.c  | 13 ++++++++-----
 4 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/grub-core/loader/arm64/linux.c b/grub-core/loader/arm64/linux.c
index 628b320618c..e8ce560a2d2 100644
--- a/grub-core/loader/arm64/linux.c
+++ b/grub-core/loader/arm64/linux.c
@@ -381,11 +381,15 @@ grub_cmd_linux (grub_command_t cmd __attribute__ ((unused)),
 
   grub_dprintf ("linux", "kernel @ %p\n", kernel_addr);
 
-  rc = grub_linuxefi_secure_validate (kernel_addr, kernel_size);
-  if (rc < 0)
+  if (grub_efi_secure_boot ())
     {
-      grub_error (GRUB_ERR_INVALID_COMMAND, N_("%s has invalid signature"), argv[0]);
-      goto fail;
+      rc = grub_linuxefi_secure_validate (kernel_addr, kernel_size);
+      if (rc <= 0)
+	{
+	  grub_error (GRUB_ERR_INVALID_COMMAND,
+		      N_("%s has invalid signature"), argv[0]);
+	  goto fail;
+	}
     }
 
   pe = (void *)((unsigned long)kernel_addr + lh.hdr_offset);
diff --git a/grub-core/loader/efi/chainloader.c b/grub-core/loader/efi/chainloader.c
index 8b99cf23e9d..a93edc975cd 100644
--- a/grub-core/loader/efi/chainloader.c
+++ b/grub-core/loader/efi/chainloader.c
@@ -1079,6 +1079,7 @@ grub_cmd_chainloader (grub_command_t cmd __attribute__ ((unused)),
 
       return 0;
     }
+  // -1 fall-through to fail
 
 fail:
   if (dev)
diff --git a/grub-core/loader/efi/linux.c b/grub-core/loader/efi/linux.c
index b56ea0bc041..5f326eb4f53 100644
--- a/grub-core/loader/efi/linux.c
+++ b/grub-core/loader/efi/linux.c
@@ -33,6 +33,7 @@ struct grub_efi_shim_lock
 };
 typedef struct grub_efi_shim_lock grub_efi_shim_lock_t;
 
+// Returns 1 on success, -1 on error, 0 when not available
 int
 grub_linuxefi_secure_validate (void *data, grub_uint32_t size)
 {
diff --git a/grub-core/loader/i386/efi/linux.c b/grub-core/loader/i386/efi/linux.c
index 2bd6663625d..cc6a503adb1 100644
--- a/grub-core/loader/i386/efi/linux.c
+++ b/grub-core/loader/i386/efi/linux.c
@@ -313,12 +313,15 @@ grub_cmd_linux (grub_command_t cmd __attribute__ ((unused)),
   grub_tpm_measure (kernel, filelen, GRUB_BINARY_PCR, "grub_linuxefi", "Kernel");
   grub_print_error();
 
-  rc = grub_linuxefi_secure_validate (kernel, filelen);
-  if (rc < 0)
+  if (grub_efi_secure_boot ())
     {
-      grub_error (GRUB_ERR_INVALID_COMMAND, N_("%s has invalid signature"),
-		  argv[0]);
-      goto fail;
+      rc = grub_linuxefi_secure_validate (kernel, filelen);
+      if (rc <= 0)
+	{
+	  grub_error (GRUB_ERR_INVALID_COMMAND,
+		      N_("%s has invalid signature"), argv[0]);
+	  goto fail;
+	}
     }
 
   lh = (struct linux_i386_kernel_header *)kernel;