[PATCH v5 10/11] OvmfPkg/AmdSev: add BlobVerifierLibSevHashes

["Dov Murik" <dovmurik@linux.ibm.com>]

Add an implementation for BlobVerifierLib that locates the SEV hashes
table and verifies that the calculated hashes of the kernel, initrd, and
cmdline blobs indeed match the expected hashes stated in the hashes
table.

If there's a missing hash or a hash mismatch then EFI_ACCESS_DENIED is
returned which will cause a failure to load a kernel image.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ashish Kalra <ashish.kalra@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3457
Co-developed-by: James Bottomley <jejb@linux.ibm.com>
Signed-off-by: James Bottomley <jejb@linux.ibm.com>
Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
---
 OvmfPkg/AmdSev/BlobVerifierLibSevHashes/BlobVerifierLibSevHashes.inf |  37 ++++
 OvmfPkg/AmdSev/BlobVerifierLibSevHashes/BlobVerifierSevHashes.c      | 202 ++++++++++++++++++++
 2 files changed, 239 insertions(+)

diff --git a/OvmfPkg/AmdSev/BlobVerifierLibSevHashes/BlobVerifierLibSevHashes.inf b/OvmfPkg/AmdSev/BlobVerifierLibSevHashes/BlobVerifierLibSevHashes.inf
new file mode 100644
index 000000000000..76ca0b8154ce
--- /dev/null
+++ b/OvmfPkg/AmdSev/BlobVerifierLibSevHashes/BlobVerifierLibSevHashes.inf
@@ -0,0 +1,37 @@
+## @file
+#
+#  Blob verifier library that uses SEV hashes table.  The hashes table holds the
+#  allowed hashes of the kernel, initrd, and cmdline blobs.
+#
+#  Copyright (C) 2021, IBM Corp
+#
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 1.29
+  BASE_NAME                      = BlobVerifierLibSevHashes
+  FILE_GUID                      = 59e713b5-eff3-46a7-8d8b-46f4c004ad7b
+  MODULE_TYPE                    = BASE
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = BlobVerifierLib
+  CONSTRUCTOR                    = BlobVerifierLibSevHashesConstructor
+
+[Sources]
+  BlobVerifierSevHashes.c
+
+[Packages]
+  CryptoPkg/CryptoPkg.dec
+  MdePkg/MdePkg.dec
+  OvmfPkg/OvmfPkg.dec
+
+[LibraryClasses]
+  BaseCryptLib
+  BaseMemoryLib
+  DebugLib
+  PcdLib
+
+[FixedPcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdQemuHashTableBase
+  gUefiOvmfPkgTokenSpaceGuid.PcdQemuHashTableSize
diff --git a/OvmfPkg/AmdSev/BlobVerifierLibSevHashes/BlobVerifierSevHashes.c b/OvmfPkg/AmdSev/BlobVerifierLibSevHashes/BlobVerifierSevHashes.c
new file mode 100644
index 000000000000..b9054879b07f
--- /dev/null
+++ b/OvmfPkg/AmdSev/BlobVerifierLibSevHashes/BlobVerifierSevHashes.c
@@ -0,0 +1,202 @@
+/** @file
+
+  Blob verifier library that uses SEV hashes table.  The hashes table holds the
+  allowed hashes of the kernel, initrd, and cmdline blobs.
+
+  Copyright (C) 2021, IBM Corporation
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#include <Library/BaseCryptLib.h>
+#include <Library/BaseLib.h>
+#include <Library/BaseMemoryLib.h>
+#include <Library/DebugLib.h>
+#include <Library/BlobVerifierLib.h>
+
+/**
+  The SEV Hashes table must be in encrypted memory and has the table
+  and its entries described by
+
+  <GUID>|UINT16 <len>|<data>
+
+  With the whole table GUID being 9438d606-4f22-4cc9-b479-a793d411fd21
+
+  The current possible table entries are for the kernel, the initrd
+  and the cmdline:
+
+  4de79437-abd2-427f-b835-d5b172d2045b  kernel
+  44baf731-3a2f-4bd7-9af1-41e29169781d  initrd
+  97d02dd8-bd20-4c94-aa78-e7714d36ab2a  cmdline
+
+  The size of the entry is used to identify the hash, but the
+  expectation is that it will be 32 bytes of SHA-256.
+**/
+
+#define SEV_HASH_TABLE_GUID \
+  (GUID) { 0x9438d606, 0x4f22, 0x4cc9, { 0xb4, 0x79, 0xa7, 0x93, 0xd4, 0x11, 0xfd, 0x21 } }
+#define SEV_KERNEL_HASH_GUID \
+  (GUID) { 0x4de79437, 0xabd2, 0x427f, { 0xb8, 0x35, 0xd5, 0xb1, 0x72, 0xd2, 0x04, 0x5b } }
+#define SEV_INITRD_HASH_GUID \
+  (GUID) { 0x44baf731, 0x3a2f, 0x4bd7, { 0x9a, 0xf1, 0x41, 0xe2, 0x91, 0x69, 0x78, 0x1d } }
+#define SEV_CMDLINE_HASH_GUID \
+  (GUID) { 0x97d02dd8, 0xbd20, 0x4c94, { 0xaa, 0x78, 0xe7, 0x71, 0x4d, 0x36, 0xab, 0x2a } }
+
+STATIC CONST EFI_GUID mSevKernelHashGuid = SEV_KERNEL_HASH_GUID;
+STATIC CONST EFI_GUID mSevInitrdHashGuid = SEV_INITRD_HASH_GUID;
+STATIC CONST EFI_GUID mSevCmdlineHashGuid = SEV_CMDLINE_HASH_GUID;
+
+#pragma pack (1)
+typedef struct {
+  GUID   Guid;
+  UINT16 Len;
+  UINT8  Data[];
+} HASH_TABLE;
+#pragma pack ()
+
+STATIC HASH_TABLE *mHashesTable;
+STATIC UINT16 mHashesTableSize;
+
+STATIC
+CONST GUID*
+FindBlobEntryGuid (
+  IN  CONST CHAR16    *BlobName
+  )
+{
+  if (StrCmp (BlobName, L"kernel") == 0) {
+    return &mSevKernelHashGuid;
+  } else if (StrCmp (BlobName, L"initrd") == 0) {
+    return &mSevInitrdHashGuid;
+  } else if (StrCmp (BlobName, L"cmdline") == 0) {
+    return &mSevCmdlineHashGuid;
+  } else {
+    return NULL;
+  }
+}
+
+/**
+  Verify blob from an external source.
+
+  @param[in] BlobName           The name of the blob
+  @param[in] Buf                The data of the blob
+  @param[in] BufSize            The size of the blob in bytes
+
+  @retval EFI_SUCCESS           The blob was verified successfully.
+  @retval EFI_ACCESS_DENIED     The blob could not be verified, and therefore
+                                should be considered non-secure.
+**/
+EFI_STATUS
+EFIAPI
+VerifyBlob (
+  IN  CONST CHAR16    *BlobName,
+  IN  CONST VOID      *Buf,
+  IN  UINT32          BufSize
+  )
+{
+  CONST GUID *Guid;
+  INT32 Remaining;
+  HASH_TABLE *Entry;
+
+  if (mHashesTable == NULL || mHashesTableSize == 0) {
+    DEBUG ((DEBUG_ERROR,
+      "%a: Verifier called but no hashes table discoverd in MEMFD\n",
+      __FUNCTION__));
+    return EFI_ACCESS_DENIED;
+  }
+
+  Guid = FindBlobEntryGuid (BlobName);
+  if (Guid == NULL) {
+    DEBUG ((DEBUG_ERROR, "%a: Unknown blob name \"%s\"\n", __FUNCTION__,
+      BlobName));
+    return EFI_ACCESS_DENIED;
+  }
+
+  //
+  // Remaining is INT32 to catch underflow in case Entry->Len has a
+  // very high UINT16 value
+  //
+  for (Entry = mHashesTable, Remaining = mHashesTableSize;
+       Remaining >= sizeof *Entry && Remaining >= Entry->Len;
+       Remaining -= Entry->Len,
+       Entry = (HASH_TABLE *)((UINT8 *)Entry + Entry->Len)) {
+    UINTN EntrySize;
+    EFI_STATUS Status;
+    UINT8 Hash[SHA256_DIGEST_SIZE];
+
+    if (!CompareGuid (&Entry->Guid, Guid)) {
+      continue;
+    }
+
+    DEBUG ((DEBUG_INFO, "%a: Found GUID %g in table\n", __FUNCTION__, Guid));
+
+    EntrySize = Entry->Len - sizeof Entry->Guid - sizeof Entry->Len;
+    if (EntrySize != SHA256_DIGEST_SIZE) {
+      DEBUG ((DEBUG_ERROR, "%a: Hash has the wrong size %d != %d\n",
+        __FUNCTION__, EntrySize, SHA256_DIGEST_SIZE));
+      return EFI_ACCESS_DENIED;
+    }
+
+    //
+    // Calculate the buffer's hash and verify that it is identical to the
+    // expected hash table entry
+    //
+    Sha256HashAll (Buf, BufSize, Hash);
+
+    if (CompareMem (Entry->Data, Hash, EntrySize) == 0) {
+      Status = EFI_SUCCESS;
+      DEBUG ((DEBUG_INFO, "%a: Hash comparison succeeded for \"%s\"\n",
+        __FUNCTION__, BlobName));
+    } else {
+      Status = EFI_ACCESS_DENIED;
+      DEBUG ((DEBUG_ERROR, "%a: Hash comparison failed for \"%s\"\n",
+        __FUNCTION__, BlobName));
+    }
+    return Status;
+  }
+
+  DEBUG ((DEBUG_ERROR, "%a: Hash GUID %g not found in table\n", __FUNCTION__,
+    Guid));
+  return EFI_ACCESS_DENIED;
+}
+
+/**
+  Locate the SEV hashes table.
+
+  This function always returns success, even if the table can't be found.  The
+  subsequent VerifyBlob calls will fail if no table was found.
+
+  @retval RETURN_SUCCESS   The hashes table is set up correctly, or there is no
+                           hashes table
+**/
+RETURN_STATUS
+EFIAPI
+BlobVerifierLibSevHashesConstructor (
+  VOID
+  )
+{
+  HASH_TABLE *Ptr;
+  UINT32 Size;
+
+  mHashesTable = NULL;
+  mHashesTableSize = 0;
+
+  Ptr = (void *)(UINTN)FixedPcdGet64 (PcdQemuHashTableBase);
+  Size = FixedPcdGet32 (PcdQemuHashTableSize);
+
+  if (Ptr == NULL || Size < sizeof *Ptr ||
+      !CompareGuid (&Ptr->Guid, &SEV_HASH_TABLE_GUID) ||
+      Ptr->Len < sizeof *Ptr || Ptr->Len > Size) {
+    return RETURN_SUCCESS;
+  }
+
+  DEBUG ((DEBUG_INFO, "%a: Found injected hashes table in secure location\n",
+    __FUNCTION__));
+
+  mHashesTable = (HASH_TABLE *)Ptr->Data;
+  mHashesTableSize = Ptr->Len - sizeof Ptr->Guid - sizeof Ptr->Len;
+
+  DEBUG ((DEBUG_VERBOSE, "%a: mHashesTable=0x%p, Size=%u\n", __FUNCTION__,
+    mHashesTable, mHashesTableSize));
+
+  return RETURN_SUCCESS;
+}
-- 
2.25.1