[PATCH v3 3/3] efi: Add API for retrieving the EFI secret for cryptodisk

[James Bottomley <jejb@linux.ibm.com>]

This module is designed to provide an efisecret command which
interrogates the EFI configuration table to find the location of the
confidential computing secret and tries to register the secret with
the cryptodisk.

The secret is stored in a boot allocated area, usually a page in size.
The layout of the secret injection area is a header

|GRUB_EFI_SECRET_TABLE_HEADER_GUID|len|

with entries of the form

|guid|len|data|

the guid corresponding to the disk encryption passphrase is
GRUB_EFI_DISKPASSWD_GUID and data must be a zero terminated string.
To get a high entropy string that doesn't need large numbers of
iterations, use a base64 encoding of 33 bytes of random data.

Signed-off-by: James Bottomley <jejb@linux.ibm.com>

---

v2: use callback to print failure message and destroy secret
v3: change to generic naming to use for TDX and SEV and use new mechanism
---
 grub-core/Makefile.core.def    |   8 ++
 grub-core/disk/efi/efisecret.c | 129 +++++++++++++++++++++++++++++++++
 include/grub/efi/api.h         |  15 ++++
 3 files changed, 152 insertions(+)
 create mode 100644 grub-core/disk/efi/efisecret.c

diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
index 68b9e9f68..0b7e365cd 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -785,6 +785,14 @@ module = {
   enable = efi;
 };
 
+module = {
+  name = efisecret;
+
+  common = disk/efi/efisecret.c;
+
+  enable = efi;
+};
+
 module = {
   name = lsefimmap;
 
diff --git a/grub-core/disk/efi/efisecret.c b/grub-core/disk/efi/efisecret.c
new file mode 100644
index 000000000..318af0784
--- /dev/null
+++ b/grub-core/disk/efi/efisecret.c
@@ -0,0 +1,129 @@
+#include <grub/err.h>
+#include <grub/misc.h>
+#include <grub/cryptodisk.h>
+#include <grub/efi/efi.h>
+#include <grub/efi/api.h>
+#include <grub/dl.h>
+
+GRUB_MOD_LICENSE ("GPLv3+");
+
+static grub_efi_packed_guid_t secret_guid = GRUB_EFI_SECRET_TABLE_GUID;
+static grub_efi_packed_guid_t tableheader_guid = GRUB_EFI_SECRET_TABLE_HEADER_GUID;
+static grub_efi_packed_guid_t diskpasswd_guid = GRUB_EFI_DISKPASSWD_GUID;
+
+/*
+ * EFI places the secret in the lower 4GB, so it uses a UINT32
+ * for the pointer which we have to transform to the correct type
+ */
+struct efi_secret {
+  grub_uint64_t base;
+  grub_uint64_t size;
+};
+
+struct secret_header {
+  grub_efi_packed_guid_t guid;
+  grub_uint32_t len;
+};
+
+struct secret_entry {
+  grub_efi_packed_guid_t guid;
+  grub_uint32_t len;
+  char data[0];
+};
+
+static grub_err_t
+grub_efi_secret_put (const char *arg __attribute__((unused)), int have_it,
+		     char **ptr)
+{
+  struct secret_entry *e = (struct secret_entry *)(*ptr - (long)&((struct secret_entry *)0)->data);
+
+  /* destroy the secret */
+  grub_memset (e, 0, e->len);
+  *ptr = NULL;
+
+  if (have_it)
+    return GRUB_ERR_NONE;
+
+  return  grub_error (GRUB_ERR_ACCESS_DENIED, "EFI secret failed to unlock any volumes");
+}
+
+static grub_err_t
+grub_efi_secret_find (struct efi_secret *s, char **secret_ptr)
+{
+  int len;
+  struct secret_header *h;
+  struct secret_entry *e;
+  unsigned char *ptr = (unsigned char *)(unsigned long)s->base;
+
+  /* the area must be big enough for a guid and a u32 length */
+  if (s->size < sizeof (*h))
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area is too small");
+
+  h = (struct secret_header *)ptr;
+  if (grub_memcmp(&h->guid, &tableheader_guid, sizeof (h->guid)))
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area does not start with correct guid\n");
+  if (h->len < sizeof (*h))
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area is too small\n");
+
+  len = h->len - sizeof (*h);
+  ptr += sizeof (*h);
+
+  while (len >= (int)sizeof (*e)) {
+    e = (struct secret_entry *)ptr;
+    if (e->len < sizeof(*e) || e->len > (unsigned int)len)
+      return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area is corrupt\n");
+
+    if (! grub_memcmp (&e->guid, &diskpasswd_guid, sizeof (e->guid))) {
+      int end = e->len - sizeof(*e);
+
+      /*
+       * the passphrase must be a zero terminated string because the
+       * password routines call grub_strlen () to find its size
+       */
+      if (e->data[end - 1] != '\0')
+	return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret area disk encryption password is not zero terminated\n");
+
+      *secret_ptr = e->data;
+      return GRUB_ERR_NONE;
+    }
+    ptr += e->len;
+    len -= e->len;
+  }
+  return grub_error (GRUB_ERR_BAD_ARGUMENT, "EFI secret aread does not contain disk decryption password\n");
+}
+
+static grub_err_t
+grub_efi_secret_get (const char *arg __attribute__((unused)), char **ptr)
+{
+  unsigned int i;
+
+  for (i = 0; i < grub_efi_system_table->num_table_entries; i++)
+    {
+      grub_efi_packed_guid_t *guid =
+	&grub_efi_system_table->configuration_table[i].vendor_guid;
+
+      if (! grub_memcmp (guid, &secret_guid, sizeof (grub_efi_packed_guid_t))) {
+	struct efi_secret *s =
+	  grub_efi_system_table->configuration_table[i].vendor_table;
+
+	return grub_efi_secret_find(s, ptr);
+      }
+    }
+  return grub_error (GRUB_ERR_BAD_ARGUMENT, "No secret found in the EFI configuration table");
+}
+
+static struct grub_secret_entry secret = {
+  .name = "efisecret",
+  .get = grub_efi_secret_get,
+  .put = grub_efi_secret_put,
+};
+
+GRUB_MOD_INIT(efisecret)
+{
+  grub_cryptodisk_add_secret_provider (&secret);
+}
+
+GRUB_MOD_FINI(efisecret)
+{
+  grub_cryptodisk_remove_secret_provider (&secret);
+}
diff --git a/include/grub/efi/api.h b/include/grub/efi/api.h
index 39733585b..53a47f658 100644
--- a/include/grub/efi/api.h
+++ b/include/grub/efi/api.h
@@ -299,6 +299,21 @@
     { 0x9a, 0x16, 0x00, 0x90, 0x27, 0x3f, 0xc1, 0x4d } \
   }
 
+#define GRUB_EFI_SECRET_TABLE_GUID \
+  { 0xadf956ad, 0xe98c, 0x484c, \
+    { 0xae, 0x11, 0xb5, 0x1c, 0x7d, 0x33, 0x64, 0x47} \
+  }
+
+#define GRUB_EFI_SECRET_TABLE_HEADER_GUID \
+  { 0x1e74f542, 0x71dd, 0x4d66, \
+    { 0x96, 0x3e, 0xef, 0x42, 0x87, 0xff, 0x17, 0x3b } \
+  }
+
+#define GRUB_EFI_DISKPASSWD_GUID \
+  { 0x736869e5, 0x84f0, 0x4973, \
+    { 0x92, 0xec, 0x06, 0x87, 0x9c, 0xe3, 0xda, 0x0b } \
+  }
+
 #define GRUB_EFI_ACPI_TABLE_GUID	\
   { 0xeb9d2d30, 0x2d88, 0x11d3, \
     { 0x9a, 0x16, 0x0, 0x90, 0x27, 0x3f, 0xc1, 0x4d } \
-- 
2.26.2