grub2/SOURCES/0135-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     | 13 +++++++++----
 grub-core/loader/efi/chainloader.c |  1 +
 grub-core/loader/efi/linux.c       |  1 +
 grub-core/loader/i386/efi/linux.c  | 17 +++++++++++------
 4 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/grub-core/loader/arm64/linux.c b/grub-core/loader/arm64/linux.c
index 70a0075ec5..47f8cf0d84 100644
--- a/grub-core/loader/arm64/linux.c
+++ b/grub-core/loader/arm64/linux.c
@@ -34,6 +34,7 @@
 #include <grub/i18n.h>
 #include <grub/lib/cmdline.h>
 #include <grub/verify.h>
+#include <grub/efi/sb.h>
 
 GRUB_MOD_LICENSE ("GPLv3+");
 
@@ -363,11 +364,15 @@ 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_get_secureboot () == GRUB_EFI_SECUREBOOT_MODE_ENABLED)
     {
-      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 ac8dfd40c6..d41e8ea14a 100644
--- a/grub-core/loader/efi/chainloader.c
+++ b/grub-core/loader/efi/chainloader.c
@@ -1084,6 +1084,7 @@ 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 e8b9ecb17f..9260731c10 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 f992ceeef2..3cf0f9b330 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+");
 
@@ -101,7 +102,7 @@ kernel_alloc(grub_efi_uintn_t size, const char * const errmsg)
 
       pages = BYTES_TO_PAGES(size);
       grub_dprintf ("linux", "Trying to allocate %lu pages from %p\n",
-		    pages, (void *)max);
+		    (unsigned long)pages, (void *)(unsigned long)max);
 
       prev_max = max;
       addr = grub_efi_allocate_pages_real (max, pages,
@@ -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_get_secureboot () == GRUB_EFI_SECUREBOOT_MODE_ENABLED)
     {
-      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;
@@ -386,6 +390,7 @@ grub_cmd_linux (grub_command_t cmd __attribute__ ((unused)),
 
   setup_header_end_offset = *((grub_uint8_t *)kernel + 0x201);
   grub_dprintf ("linux", "copying %lu bytes from %p to %p\n",
+		(unsigned long)
 		MIN((grub_size_t)0x202+setup_header_end_offset,
 		    sizeof (*params)) - 0x1f1,
 		(grub_uint8_t *)kernel + 0x1f1,