x86/virt/tdx: Require the module to assert it has the NO_RBP_MOD mitigation

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.  KVM will need to use the TDH.VP.ENTER SEAMCALL to run
TDX guests.  It won't be safe to run TDX guests w/o this feature.  To
prevent it, just don't initialize the TDX module if this feature is not
supported [1].

Note the bit definitions of TDX_FEATURES0 are not auto-generated in
tdx_global_metadata.h.  Manually define a macro for it in "tdx.h".

Signed-off-by: Kai Huang <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Nikolay Borisov <[email protected]>
Reviewed-by: Adrian Hunter <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Link: https://lore.kernel.org/[email protected]/ [1]
Link: https://lore.kernel.org/all/76ae5025502c84d799e3a56a6fc4f69a82da8f93.1734188033.git.kai.huang%40intel.com

Author: kaihuang

arch/x86/virt/vmx/tdx/tdx.c

@@ -272,6 +272,18 @@ static int read_sys_metadata_field(u64 field_id, u64 *data)
 
 #include "tdx_global_metadata.c"
 
+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)
 {
@@ -1055,6 +1067,11 @@ static int init_tdx_module(void)
 	if (ret)
 		return ret;
 
+	/* 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

arch/x86/virt/vmx/tdx/tdx.h

@@ -2,6 +2,7 @@
 #ifndef _X86_VIRT_TDX_H
 #define _X86_VIRT_TDX_H
 
+#include <linux/bits.h>
 #include "tdx_global_metadata.h"
 
 /*
@@ -51,6 +52,9 @@ struct tdmr_info {
 	DECLARE_FLEX_ARRAY(struct tdmr_reserved_area, reserved_areas);
 } __packed __aligned(TDMR_INFO_ALIGNMENT);
 
+/* Bit definitions of TDX_FEATURES0 metadata field */
+#define TDX_FEATURES0_NO_RBP_MOD	BIT(18)
+
 /*
  * Do not put any hardware-defined TDX structure representations below
  * this comment!