[PATCH v5 7/8] x86/virt/tdx: Require the module to assert it has the NO_RBP_MOD mitigation

[Kai Huang]

Old TDX modules can clobber RBP in the TDH.VP.ENTER SEAMCALL.  However
RBP is used as frame pointer in the x86_64 calling convention, and
clobbering RBP could result in bad things like being unable to unwind
the stack if any non-maskable exceptions (NMI, #MC etc) happens in that
gap.

A new "NO_RBP_MOD" feature was introduced to more recent TDX modules to
not clobber RBP.  This feature is reported in the TDX_FEATURES0 global
metadata field via bit 18.

Don't initialize the TDX module if this feature is not supported [1].

Link: https://lore.kernel.org/all/fc0e8ab7-86d4-4428-be31-82e1ece6dd21@xxxxxxxxx/ [1]
Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>
Reviewed-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---

v4 -> v5:
 - Rebase due to patch 3 change.

v3 -> v4:
 - Move reading TDX_FEATURES0 code to this patch.
 - Change patch title and use permalink - Dan.

 Hi Dan, Ardian, Nikolay,

 The code to read TDX_FEATURES0 was not included in this patch when you
 gave your tag.  I didn't remove them.  Please let me know if you want
 me to remove your tag.  Thanks!

v2 -> v3:
 - check_module_compatibility() -> check_features().
 - Improve error message.

 https://lore.kernel.org/kvm/cover.1721186590.git.kai.huang@xxxxxxxxx/T/#md9e2eeef927838cbf20d7b361cdbea518b8aec50

---
 arch/x86/virt/vmx/tdx/tdx.c | 36 ++++++++++++++++++++++++++++++++++++
 arch/x86/virt/vmx/tdx/tdx.h | 16 ++++++++++++++++
 2 files changed, 52 insertions(+)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 130ddac47f64..c877d02ca057 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -292,6 +292,21 @@ static int __read_sys_metadata_field(u64 field_id, void *val, int size)
 	__read_sys_metadata_field(_field_id, _val, sizeof(*(_val)));	\
 })
 
+static int get_tdx_sys_info_features(struct tdx_sys_info_features *sysinfo_features)
+{
+	int ret = 0;
+
+#define READ_SYS_INFO(_field_id, _member)				\
+	ret = ret ?: read_sys_metadata_field(MD_FIELD_ID_##_field_id,	\
+					&sysinfo_features->_member)
+
+	READ_SYS_INFO(TDX_FEATURES0, tdx_features0);
+
+#undef READ_SYS_INFO
+
+	return ret;
+}
+
 static int get_tdx_sys_info_version(struct tdx_sys_info_version *sysinfo_version)
 {
 	int ret = 0;
@@ -335,6 +350,10 @@ static int get_tdx_sys_info(struct tdx_sys_info *sysinfo)
 {
 	int ret;
 
+	ret = get_tdx_sys_info_features(&sysinfo->features);
+	if (ret)
+		return ret;
+
 	ret = get_tdx_sys_info_version(&sysinfo->version);
 	if (ret)
 		return ret;
@@ -364,6 +383,18 @@ static void print_basic_sys_info(struct tdx_sys_info *sysinfo)
 	print_sys_info_version(&sysinfo->version);
 }
 
+static int check_features(struct tdx_sys_info *sysinfo)
+{
+	u64 tdx_features0 = sysinfo->features.tdx_features0;
+
+	if (!(tdx_features0 & TDX_FEATURES0_NO_RBP_MOD)) {
+		pr_err("frame pointer (RBP) clobber bug present, upgrade TDX module\n");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 /* Calculate the actual TDMR size */
 static int tdmr_size_single(u16 max_reserved_per_tdmr)
 {
@@ -1145,6 +1176,11 @@ static int init_tdx_module(void)
 
 	print_basic_sys_info(&sysinfo);
 
+	/* Check whether the kernel can support this module */
+	ret = check_features(&sysinfo);
+	if (ret)
+		return ret;
+
 	/*
 	 * To keep things simple, assume that all TDX-protected memory
 	 * will come from the page allocator.  Make sure all pages in the
diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
index 0203528da024..18c54e1e3a4a 100644
--- a/arch/x86/virt/vmx/tdx/tdx.h
+++ b/arch/x86/virt/vmx/tdx/tdx.h
@@ -31,6 +31,7 @@
  *
  * See the "global_metadata.json" in the "TDX 1.5 ABI definitions".
  */
+#define MD_FIELD_ID_TDX_FEATURES0		0x0A00000300000008ULL
 #define MD_FIELD_ID_BUILD_DATE			0x8800000200000001ULL
 #define MD_FIELD_ID_BUILD_NUM			0x8800000100000002ULL
 #define MD_FIELD_ID_MINOR_VERSION		0x0800000100000003ULL
@@ -105,6 +106,20 @@ struct tdmr_info {
  * those used by the kernel are.
  */
 
+/*
+ * Class "TDX Module Info".
+ *
+ * This class also contains other fields like SYS_ATTRIBUTES and the
+ * NUM_TDX_FEATURES.  For now only TDX_FEATURES0 is needed, but still
+ * keep the structure to follow the spec (and for future extension).
+ */
+struct tdx_sys_info_features {
+	u64 tdx_features0;
+};
+
+/* Bit definitions of TDX_FEATURES0 metadata field */
+#define TDX_FEATURES0_NO_RBP_MOD	_BITULL(18)
+
 /* Class "TDX Module Version" */
 struct tdx_sys_info_version {
 	u16 major;
@@ -123,6 +138,7 @@ struct tdx_sys_info_tdmr {
 };
 
 struct tdx_sys_info {
+	struct tdx_sys_info_features	features;
 	struct tdx_sys_info_version	version;
 	struct tdx_sys_info_tdmr	tdmr;
 };
-- 
2.46.2