[SECURITY PATCH 23/73] disk/cryptodisk: Require authentication after TPM unlock for CLI access

[Daniel Kiper]

From: Michael Chang <mchang@suse.com>

The GRUB may use TPM to verify the integrity of boot components and the
result can determine whether a previously sealed key can be released. If
everything checks out, showing nothing has been tampered with, the key
is released and GRUB unlocks the encrypted root partition for the next
stage of booting.

However, the liberal Command Line Interface (CLI) can be misused by
anyone in this case to access files in the encrypted partition one way
or another. Despite efforts to keep the CLI secure by preventing utility
command output from leaking file content, many techniques in the wild
could still be used to exploit the CLI, enabling attacks or learning
methods to attack. It's nearly impossible to account for all scenarios
where a hack could be applied.

Therefore, to mitigate potential misuse of the CLI after the root device
has been successfully unlocked via TPM, the user should be required to
authenticate using the LUKS password. This added layer of security
ensures that only authorized users can access the CLI reducing the risk
of exploitation or unauthorized access to the encrypted partition.

Fixes: CVE-2024-49504

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 docs/grub.texi                | 30 +++++++++++++++
 grub-core/disk/cryptodisk.c   | 86 +++++++++++++++++++++++++++++++++++++++++++
 grub-core/kern/main.c         | 12 ++++++
 grub-core/normal/auth.c       | 30 +++++++++++++++
 grub-core/normal/main.c       |  4 ++
 grub-core/normal/menu_entry.c |  4 ++
 include/grub/auth.h           |  1 +
 include/grub/cryptodisk.h     |  3 ++
 include/grub/misc.h           |  2 +
 9 files changed, 172 insertions(+)

diff --git a/docs/grub.texi b/docs/grub.texi
index 200e747af..e914e022b 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -9119,6 +9119,36 @@ command through the swtpm control channel.
 # @kbd{swtpm_ioctl -s --unix swtpm-state/ctrl}
 @end example
 
+@subsection Command line and menuentry editor protection
+
+The TPM key protector provides full disk encryption support on servers or
+virtual machine images, meanwhile keeping the boot process unattended. This
+prevents service disruptions by eliminating the need for manual password input
+during startup, improving system uptime and continuity. It is achieved by TPM,
+which verifies the integrity of boot components by checking cryptographic
+hashes against securely stored values, to confirm the disks are unlocked in a
+trusted state.
+
+However, for users to access the system interactively, some form of
+authentication is still required, as the disks are not unlocked by an
+authorized user. This raised concerns about using an unprotected
+@samp{command-line interface} (@pxref{Command-line interface}), as anyone could
+execute commands to access decrypted data. To address this issue, the LUKS
+password is used to ensure that only authorized users are granted access to the
+interface. Additionally, the @samp{menu entry editor} (@pxref{Menu entry
+editor}) is also safeguarded by the LUKS password, as modifying a boot entry is
+effectively the same as altering the @file{grub.cfg} file read from encrypted
+files.
+
+It is worth mentioning that the built-in password support, as described in
+@samp{Authentication and Authorization in GRUB} (@pxref{Authentication and
+authorisation}), can also be used to protect the command-line interface from
+unauthorized access. However, it is not recommended to rely on this approach as
+it is an optional step. Setting it up requires additional manual intervention,
+which increases the risk of password leakage during the process. Moreover, the
+superuser list must be well maintained, and the password used cannot be
+synchronized with LUKS key rotation.
+
 @node Platform limitations
 @chapter Platform limitations
 
diff --git a/grub-core/disk/cryptodisk.c b/grub-core/disk/cryptodisk.c
index 5fc41979e..45adffdd9 100644
--- a/grub-core/disk/cryptodisk.c
+++ b/grub-core/disk/cryptodisk.c
@@ -1186,6 +1186,9 @@ grub_cryptodisk_scan_device_real (const char *name,
              ret = grub_cryptodisk_insert (dev, name, source);
              if (ret != GRUB_ERR_NONE)
                goto error;
+#ifndef GRUB_UTIL
+             grub_cli_set_auth_needed ();
+#endif
              goto cleanup;
            }
        }
@@ -1754,6 +1757,89 @@ luks_script_get (grub_size_t *sz)
   return ret;
 }
 
+#ifdef GRUB_MACHINE_EFI
+grub_err_t
+grub_cryptodisk_challenge_password (void)
+{
+  grub_cryptodisk_t cr_dev;
+
+  for (cr_dev = cryptodisk_list; cr_dev != NULL; cr_dev = cr_dev->next)
+    {
+      grub_cryptodisk_dev_t cr;
+      grub_disk_t source = NULL;
+      grub_err_t ret = GRUB_ERR_NONE;
+      grub_cryptodisk_t dev = NULL;
+      char *part = NULL;
+      struct grub_cryptomount_args cargs = {0};
+
+      cargs.check_boot = 0;
+      cargs.search_uuid = cr_dev->uuid;
+
+      source = grub_disk_open (cr_dev->source);
+
+      if (source == NULL)
+       {
+         ret = grub_errno;
+         goto error_out;
+       }
+
+      FOR_CRYPTODISK_DEVS (cr)
+      {
+       dev = cr->scan (source, &cargs);
+       if (grub_errno)
+         {
+           ret = grub_errno;
+           goto error_out;
+         }
+       if (dev == NULL)
+         continue;
+       break;
+      }
+
+      if (dev == NULL)
+       {
+         ret = grub_error (GRUB_ERR_BAD_MODULE, "no cryptodisk module can 
handle this device");
+         goto error_out;
+       }
+
+      part = grub_partition_get_name (source->partition);
+      grub_printf_ (N_("Enter passphrase for %s%s%s (%s): "), source->name,
+                   source->partition != NULL ? "," : "",
+                   part != NULL ? part : N_("UNKNOWN"), cr_dev->uuid);
+      grub_free (part);
+
+      cargs.key_data = grub_malloc (GRUB_CRYPTODISK_MAX_PASSPHRASE);
+      if (cargs.key_data == NULL)
+       {
+         ret = grub_errno;
+         goto error_out;
+       }
+
+      if (!grub_password_get ((char *) cargs.key_data, 
GRUB_CRYPTODISK_MAX_PASSPHRASE))
+       {
+         ret = grub_error (GRUB_ERR_BAD_ARGUMENT, "passphrase not supplied");
+         goto error_out;
+       }
+      cargs.key_len = grub_strlen ((char *) cargs.key_data);
+      ret = cr->recover_key (source, dev, &cargs);
+
+ error_out:
+      grub_disk_close (source);
+      if (dev != NULL)
+       cryptodisk_close (dev);
+      if (cargs.key_data)
+       {
+         grub_memset (cargs.key_data, 0, cargs.key_len);
+         grub_free (cargs.key_data);
+       }
+
+      return ret;
+    }
+
+  return GRUB_ERR_NONE;
+}
+#endif /* GRUB_MACHINE_EFI */
+
 struct grub_procfs_entry luks_script =
 {
   .name = "luks_script",
diff --git a/grub-core/kern/main.c b/grub-core/kern/main.c
index d29494d54..143a232b8 100644
--- a/grub-core/kern/main.c
+++ b/grub-core/kern/main.c
@@ -38,6 +38,7 @@
 #endif
 
 static bool cli_disabled = false;
+static bool cli_need_auth = false;
 
 grub_addr_t
 grub_modules_get_end (void)
@@ -247,6 +248,17 @@ grub_is_cli_disabled (void)
   return cli_disabled;
 }
 
+bool
+grub_is_cli_need_auth (void)
+{
+  return cli_need_auth;
+}
+
+void grub_cli_set_auth_needed (void)
+{
+  cli_need_auth = true;
+}
+
 static void
 check_is_cli_disabled (void)
 {
diff --git a/grub-core/normal/auth.c b/grub-core/normal/auth.c
index d94020186..71b361bc0 100644
--- a/grub-core/normal/auth.c
+++ b/grub-core/normal/auth.c
@@ -25,6 +25,10 @@
 #include <grub/time.h>
 #include <grub/i18n.h>
 
+#ifdef GRUB_MACHINE_EFI
+#include <grub/cryptodisk.h>
+#endif
+
 struct grub_auth_user
 {
   struct grub_auth_user *next;
@@ -201,6 +205,32 @@ grub_username_get (char buf[], unsigned buf_size)
 }
 
 grub_err_t
+grub_auth_check_cli_access (void)
+{
+  if (grub_is_cli_need_auth () == true)
+    {
+#ifdef GRUB_MACHINE_EFI
+      static bool authenticated = false;
+
+      if (authenticated == false)
+       {
+         grub_err_t ret;
+
+         ret = grub_cryptodisk_challenge_password ();
+         if (ret == GRUB_ERR_NONE)
+           authenticated = true;
+         return ret;
+       }
+      return GRUB_ERR_NONE;
+#else
+      return GRUB_ACCESS_DENIED;
+#endif
+    }
+
+  return GRUB_ERR_NONE;
+}
+
+grub_err_t
 grub_auth_check_authentication (const char *userlist)
 {
   char login[1024];
diff --git a/grub-core/normal/main.c b/grub-core/normal/main.c
index bd4431000..90879dc21 100644
--- a/grub-core/normal/main.c
+++ b/grub-core/normal/main.c
@@ -453,9 +453,13 @@ grub_cmdline_run (int nested, int force_auth)
     }
   while (err && force_auth);
 
+  if (err == GRUB_ERR_NONE)
+    err = grub_auth_check_cli_access ();
+
   if (err)
     {
       grub_print_error ();
+      grub_wait_after_message ();
       grub_errno = GRUB_ERR_NONE;
       return;
     }
diff --git a/grub-core/normal/menu_entry.c b/grub-core/normal/menu_entry.c
index ade56be2b..8b0d17e3f 100644
--- a/grub-core/normal/menu_entry.c
+++ b/grub-core/normal/menu_entry.c
@@ -1255,9 +1255,13 @@ grub_menu_entry_run (grub_menu_entry_t entry)
 
   err = grub_auth_check_authentication (NULL);
 
+  if (err == GRUB_ERR_NONE)
+    err = grub_auth_check_cli_access ();
+
   if (err)
     {
       grub_print_error ();
+      grub_wait_after_message ();
       grub_errno = GRUB_ERR_NONE;
       return;
     }
diff --git a/include/grub/auth.h b/include/grub/auth.h
index 747334451..21d5190f0 100644
--- a/include/grub/auth.h
+++ b/include/grub/auth.h
@@ -33,5 +33,6 @@ grub_err_t grub_auth_unregister_authentication (const char 
*user);
 grub_err_t grub_auth_authenticate (const char *user);
 grub_err_t grub_auth_deauthenticate (const char *user);
 grub_err_t grub_auth_check_authentication (const char *userlist);
+grub_err_t grub_auth_check_cli_access (void);
 
 #endif /* ! GRUB_AUTH_HEADER */
diff --git a/include/grub/cryptodisk.h b/include/grub/cryptodisk.h
index 59b461e7a..5bb15751d 100644
--- a/include/grub/cryptodisk.h
+++ b/include/grub/cryptodisk.h
@@ -203,4 +203,7 @@ grub_util_get_geli_uuid (const char *dev);
 grub_cryptodisk_t grub_cryptodisk_get_by_uuid (const char *uuid);
 grub_cryptodisk_t grub_cryptodisk_get_by_source_disk (grub_disk_t disk);
 
+#ifdef GRUB_MACHINE_EFI
+grub_err_t grub_cryptodisk_challenge_password (void);
+#endif
 #endif
diff --git a/include/grub/misc.h b/include/grub/misc.h
index 14d8f37ac..e087e7b3e 100644
--- a/include/grub/misc.h
+++ b/include/grub/misc.h
@@ -431,6 +431,8 @@ grub_uint64_t EXPORT_FUNC(grub_divmod64) (grub_uint64_t n,
                                          grub_uint64_t *r);
 
 extern bool EXPORT_FUNC(grub_is_cli_disabled) (void);
+extern bool EXPORT_FUNC(grub_is_cli_need_auth) (void);
+extern void EXPORT_FUNC(grub_cli_set_auth_needed) (void);
 
 /* Must match softdiv group in gentpl.py.  */
 #if !defined(GRUB_MACHINE_EMU) && (defined(__arm__) || defined(__ia64__) || \
-- 
2.11.0