Re: [PATCH v2 10/11] OvmfPkg: add SevHashesBlobVerifierLib


Dov Murik
 

On 19/07/2021 20:28, Tom Lendacky wrote:
On 7/6/21 3:55 AM, Dov Murik wrote:
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: Laszlo Ersek <lersek@redhat.com>
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/Library/BlobVerifierLib/SevHashesBlobVerifierLib.inf | 36 ++++
OvmfPkg/Library/BlobVerifierLib/SevHashesBlobVerifier.c | 199 ++++++++++++++++++++
2 files changed, 235 insertions(+)

diff --git a/OvmfPkg/Library/BlobVerifierLib/SevHashesBlobVerifierLib.inf b/OvmfPkg/Library/BlobVerifierLib/SevHashesBlobVerifierLib.inf
new file mode 100644
index 000000000000..b060d6a1b545
--- /dev/null
+++ b/OvmfPkg/Library/BlobVerifierLib/SevHashesBlobVerifierLib.inf
@@ -0,0 +1,36 @@
+## @file
+#
+# Blob verifier library that uses SEV hashes table.
+#
+# Copyright (C) 2021, IBM Corp
+#
+# SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
Same comments here as were made on the Null library instance.
Indeed.


+[Defines]
+ INF_VERSION = 0x00010005
+ BASE_NAME = SevHashesBlobVerifierLib
But is this BASE_NAME okay?

Or should it be BlobVerifierLibSevHashes ?


+ FILE_GUID = 59e713b5-eff3-46a7-8d8b-46f4c004ad7b
+ MODULE_TYPE = BASE
+ VERSION_STRING = 1.0
+ LIBRARY_CLASS = BlobVerifierLib
+ CONSTRUCTOR = SevHashesBlobVerifierLibConstructor
+
+[Sources]
+ SevHashesBlobVerifier.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/Library/BlobVerifierLib/SevHashesBlobVerifier.c b/OvmfPkg/Library/BlobVerifierLib/SevHashesBlobVerifier.c
new file mode 100644
index 000000000000..961ee29f5df3
--- /dev/null
+++ b/OvmfPkg/Library/BlobVerifierLib/SevHashesBlobVerifier.c
@@ -0,0 +1,199 @@
+/** @file
+
+ Blob verifier library that uses SEV hashes table.
+
+ 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 BlobName The name of the blob
+ @param Buf The data of the blob
+ @param 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,
+ UINT32 BufSize
+ )
+{
+ CONST GUID *Guid;
+ INT32 Len;
Any reason for this not to be a UINT16 like the struct or mHashesTableSize?
Detect overflows in the `for` loop below?

If a (bad) Entry->Len is 0xffff, then adding it to Len will overflow the
UINT16 and the Len < mHashesTableSize condition is still true.


+ HASH_TABLE *Entry;
+ UINT8 Hash[SHA256_DIGEST_SIZE];
+
+ 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;
+ }
+
+ Sha256HashAll (Buf, BufSize, Hash);
Maybe search for and find the Guid (done in the for loop below) before
calling Sha256HashAll?
Yep; I'll move it just before CompareMem below.

Thanks,
-Dov


Thanks,
Tom

+
+ for (Entry = mHashesTable, Len = 0;
+ Len < (INT32)mHashesTableSize;
+ Len += Entry->Len,
+ Entry = (HASH_TABLE *)((UINT8 *)Entry + Entry->Len)) {
+ UINTN EntrySize;
+ EFI_STATUS Status;
+
+ if (!CompareGuid (&Entry->Guid, Guid)) {
+ continue;
+ }
+
+ DEBUG ((DEBUG_INFO, "%a: Found GUID %g in table\n", __FUNCTION__, Guid));
+
+ //
+ // Verify that the buffer's calculated hash is identical to the expected
+ // hash table entry
+ //
+ 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;
+ }
+
+ 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 verifier tables were set up correctly
+**/
+RETURN_STATUS
+EFIAPI
+SevHashesBlobVerifierLibConstructor (
+ VOID
+ )
+{
+ HASH_TABLE *Ptr = (void *)(UINTN)FixedPcdGet64 (PcdQemuHashTableBase);
+ UINT32 Size = FixedPcdGet32 (PcdQemuHashTableSize);
+
+ mHashesTable = NULL;
+ mHashesTableSize = 0;
+
+ if (Ptr == NULL || Size == 0) {
+ return RETURN_SUCCESS;
+ }
+
+ if (!CompareGuid (&Ptr->Guid, &SEV_HASH_TABLE_GUID)) {
+ return RETURN_SUCCESS;
+ }
+
+ if (Ptr->Len < (sizeof Ptr->Guid + sizeof Ptr->Len)) {
+ 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;
+}

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