grub2/0247-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  | 14 +++++++++-----
 4 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/grub-core/loader/arm64/linux.c b/grub-core/loader/arm64/linux.c
index 8791b35b6fe..a18c4874c4f 100644
--- a/grub-core/loader/arm64/linux.c
+++ b/grub-core/loader/arm64/linux.c
@@ -383,11 +383,15 @@ 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 3dfa41bb49d..26f4fa9dc6a 100644
--- a/grub-core/loader/efi/chainloader.c
+++ b/grub-core/loader/efi/chainloader.c
@@ -1083,6 +1083,7 @@ 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 e09f824862b..927d89a90d7 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 c0a98d152ff..bf7ceca10ff 100644
--- a/grub-core/loader/i386/efi/linux.c
+++ b/grub-core/loader/i386/efi/linux.c
@@ -30,6 +30,7 @@
 #include <grub/cpu/efi/memory.h>
 #include <grub/tpm.h>
 #include <grub/safemath.h>
+#include <grub/efi/sb.h>
 
 GRUB_MOD_LICENSE ("GPLv3+");
 
@@ -307,12 +308,15 @@ grub_cmd_linux (grub_command_t cmd __attribute__ ((unused)),
       goto fail;
     }
 
-  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;