[PATCH v2 3/3] efi: Add API for retrieving the AMD SEV injected secret for cryptodisk

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

This module is designed to provide a sevsecret command which
interrogates the EFI configuration table to find the location of the
sev secret injection and tries to register the secret with the
cryptodisk.

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

|GRUB_EFI_SEVSECRET_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
---
 grub-core/Makefile.core.def    |   8 ++
 grub-core/disk/efi/sevsecret.c | 132 +++++++++++++++++++++++++++++++++
 include/grub/efi/api.h         |  15 ++++
 3 files changed, 155 insertions(+)
 create mode 100644 grub-core/disk/efi/sevsecret.c

diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
index b5f47fc41..76ae5d8fc 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -784,6 +784,14 @@ module = {
   enable = efi;
 };
 
+module = {
+  name = sevsecret;
+
+  common = disk/efi/sevsecret.c;
+
+  enable = efi;
+};
+
 module = {
   name = lsefimmap;
 
diff --git a/grub-core/disk/efi/sevsecret.c b/grub-core/disk/efi/sevsecret.c
new file mode 100644
index 000000000..da59dd1ad
--- /dev/null
+++ b/grub-core/disk/efi/sevsecret.c
@@ -0,0 +1,132 @@
+#include <grub/err.h>
+#include <grub/command.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 sevsecret_guid = GRUB_EFI_SEVSECRET_TABLE_GUID;
+static grub_efi_packed_guid_t tableheader_guid = GRUB_EFI_SEVSECRET_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 sev_secret {
+  grub_uint32_t base;
+  grub_uint32_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_sevsecret_cb (int have_it, void *arg)
+{
+  struct secret_entry *e = arg;
+
+  /* destroy the secret */
+  grub_memset (e, 0, e->len);
+
+  if (have_it)
+    return GRUB_ERR_NONE;
+
+  return  grub_error (GRUB_ERR_ACCESS_DENIED, "SEV password failed to unlock any volumes");
+}
+
+static grub_err_t
+grub_efi_sevsecret_find (struct sev_secret *s)
+{
+  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, "SEV 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, "SEV secret area does not start with correct guid\n");
+  if (h->len < sizeof (*h))
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, "SEV 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, "SEV 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, "SEV secret area disk encryption password is not zero terminated\n");
+
+      return grub_cryptodisk_set_secret (e->data, grub_efi_sevsecret_cb, e);
+    }
+    ptr += e->len;
+    len -= e->len;
+  }
+  return grub_error (GRUB_ERR_BAD_ARGUMENT, "SEV secret aread does not contain disk decryption password\n");
+}
+
+static grub_err_t
+grub_efi_sevsecret_init (void)
+{
+  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, &sevsecret_guid, sizeof (grub_efi_packed_guid_t))) {
+	struct sev_secret *s =
+	  grub_efi_system_table->configuration_table[i].vendor_table;
+
+	return grub_efi_sevsecret_find(s);
+      }
+    }
+  return grub_error (GRUB_ERR_BAD_ARGUMENT, "No SEV secret found in the EFI configuration table");
+}
+
+static grub_err_t
+grub_cmd_sevsecret (grub_command_t cmd __attribute__ ((unused)),
+		    int argc __attribute__ ((unused)),
+		    char **args __attribute__ ((unused)))
+{
+  return grub_efi_sevsecret_init();
+}
+
+static grub_command_t cmd;
+
+GRUB_MOD_INIT(sevsecret)
+{
+  cmd = grub_register_command ("sevsecret", grub_cmd_sevsecret, 0,
+			       N_("Register a SEV secret with cryptomount if one exists"));
+}
+
+GRUB_MOD_FINI(sevsecret)
+{
+  grub_unregister_command (cmd);
+}
diff --git a/include/grub/efi/api.h b/include/grub/efi/api.h
index 1dcaa12f5..80eb880f8 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_SEVSECRET_TABLE_GUID \
+  { 0xadf956ad, 0xe98c, 0x484c, \
+    { 0xae, 0x11, 0xb5, 0x1c, 0x7d, 0x33, 0x64, 0x47} \
+  }
+
+#define GRUB_EFI_SEVSECRET_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