[PATCH v1 8/8] x86/virt/tdx: Verify the C member size against the metadata field ID

[Chao Gao]

Each TDX global metadata field ID encodes the size of that field.
read_sys_metadata_table() stores each value at the width recorded in
the table, which TD_SYSINFO_MAP() derives from the destination C member.
Nothing checks that the two agree.

A table entry naming the wrong field ID, or a struct member declared at
the wrong width, would silently truncate the value read from the TDX
module. That is a kernel-side bug rather than a TDX module problem.

Add macros to extract the encoded size from a field ID, and use them in
TD_SYSINFO_MAP() to assert that it matches the member size.  Both are
compile-time constants, so the check costs nothing at runtime.

Note that BUILD_BUG_ON() cannot be used in a structure initializer; use
BUILD_BUG_ON_ZERO() instead, which yields 0 and so can be folded into the
.size initializer without changing its value.

No functional change intended.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
---
 arch/x86/virt/vmx/tdx/tdx.c |  9 ++++++++-
 arch/x86/virt/vmx/tdx/tdx.h | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 4bf21848df62..59099cc15f7a 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -357,11 +357,18 @@ struct tdx_sys_field {
 	u8  size;
 };
 
+/*
+ * The size encoded in the field ID and the size of the destination C
+ * member must agree; BUILD_BUG_ON_ZERO() enforces this at compile time.
+ */
 #define TD_SYSINFO_MAP(_field_id, _struct, _member)				\
 	{									\
 		.field_id = MD_FIELD_ID_##_field_id,				\
 		.offset   = offsetof(struct _struct, _member),			\
-		.size     = sizeof_field(struct _struct, _member),		\
+		.size     = sizeof_field(struct _struct, _member) +		\
+			    BUILD_BUG_ON_ZERO(					\
+				sizeof_field(struct _struct, _member) !=	\
+				MD_FIELD_ID_ELE_SIZE(MD_FIELD_ID_##_field_id)),	\
 	}
 
 /*
diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
index 5f567cb6c07a..c612b1cf7c14 100644
--- a/arch/x86/virt/vmx/tdx/tdx.h
+++ b/arch/x86/virt/vmx/tdx/tdx.h
@@ -109,6 +109,21 @@
 #define MD_FIELD_ID_CPUID_CONFIG_LEAVES		0x9900000300000400ULL
 #define MD_FIELD_ID_CPUID_CONFIG_VALUES		0x9900000300000500ULL
 
+/*
+ * Sub-field definitions of MD_FIELD_ID.
+ *
+ * See "MD_FIELD_ID (Metadata Field Identifier / Sequence Header)
+ * Definition" in the Intel TDX Module ABI spec.
+ *
+ *  - Bit 33:32: ELEMENT_SIZE_CODE -- log2 of a single metadata
+ *                                    element's size in bytes
+ */
+#define MD_FIELD_ID_ELE_SIZE_CODE(field_id)	\
+	(((field_id) & GENMASK_ULL(33, 32)) >> 32)
+
+#define MD_FIELD_ID_ELE_SIZE(field_id)		\
+	(1 << MD_FIELD_ID_ELE_SIZE_CODE(field_id))
+
 /* TDX page types */
 #define	PT_NDA		0x0
 #define	PT_RSVD		0x1
-- 
2.52.0