intel/txt: Add `txt_get_chipset_dpr` function

Due to platform-specific constraints, it is not possible to enable DPR
by programming the MCH's DPR register in ramstage. Instead, assume it
has been programmed earlier and check that its value is valid. If it is,
then simply configure DPR in TXT public base with the same parameters.
Note that some bits only exist on MCH DPR, and thus need to be cleared.

Implement this function on most client platforms. For Skylake and newer,
place it in common System Agent code. Also implement it for Haswell, for
which the rest of Intel TXT support will be added in subsequent commits.

Do not error out if DPR is larger than expected. On some platforms, such
as Haswell, MRC decides the size of DPR, and cannot be changed easily.
Reimplementing MRC is easier than working around its limitations anyway.

Change-Id: I391383fb03bd6636063964ff249c75028e0644cf
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46490
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

2 files changed, 46 insertions, 7 deletions

diff --git a/src/security/intel/txt/ramstage.c b/src/security/intel/txt/ramstage.c
index 263bc9daa6ff..f532a2fbd827 100644
--- a/src/security/intel/txt/ramstage.c
+++ b/src/security/intel/txt/ramstage.c
@@ -13,6 +13,7 @@
 #include <types.h>
 
 #include "txt.h"
+#include "txt_platform.h"
 #include "txt_register.h"
 #include "txt_getsec.h"
 
@@ -233,16 +234,42 @@ static void lockdown_intel_txt(void *unused)
 	printk(BIOS_INFO, "TEE-TXT: DPR capable %x\n", dpr_capable);
 
 	if (dpr_capable) {
-		/* Protect 3 MiB below TSEG and lock register */
-		union dpr_register dpr = {
-			.lock = 1,
-			.size = 3,
-			.top  = tseg_base / MiB,
-		};
-		write64((void *)TXT_DPR, dpr.raw);
+		/* Verify the DPR settings on the MCH and mirror them to TXT public space */
+		union dpr_register dpr = txt_get_chipset_dpr();
+
+		printk(BIOS_DEBUG, "TEE-TXT: MCH DPR 0x%08x\n", dpr.raw);
+
+		printk(BIOS_DEBUG, "TEE-TXT: MCH DPR base @ 0x%08x size %u MiB\n",
+			(dpr.top - dpr.size) * MiB, dpr.size);
 
 		// DPR TODO: implement SA_ENABLE_DPR in the intelblocks
 
+		if (!dpr.lock) {
+			printk(BIOS_ERR, "TEE-TXT: MCH DPR not locked.\n");
+			return;
+		}
+
+		if (!dpr.epm || !dpr.prs) {
+			printk(BIOS_ERR, "TEE-TXT: MCH DPR protection not active.\n");
+			return;
+		}
+
+		if (dpr.size < 3) {
+			printk(BIOS_ERR, "TEE-TXT: MCH DPR configured size is too small.\n");
+			return;
+		}
+
+		if (dpr.top * MiB != tseg_base) {
+			printk(BIOS_ERR, "TEE-TXT: MCH DPR top does not equal TSEG base.\n");
+			return;
+		}
+
+		/* Clear reserved bits */
+		dpr.prs = 0;
+		dpr.epm = 0;
+
+		write64((void *)TXT_DPR, dpr.raw);
+
 		printk(BIOS_INFO, "TEE-TXT: TXT.DPR 0x%08x\n",
 		       read32((void *)TXT_DPR));
 	}
diff --git a/src/security/intel/txt/txt_platform.h b/src/security/intel/txt/txt_platform.h
new file mode 100644
index 000000000000..8881cab331d6
--- /dev/null
+++ b/src/security/intel/txt/txt_platform.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef __SECURITY_INTEL_TXT_PLATFORM_H__
+#define __SECURITY_INTEL_TXT_PLATFORM_H__
+
+#include <types.h>
+#include "txt_register.h"
+
+/* Prototypes to be defined in chipset code */
+union dpr_register txt_get_chipset_dpr(void);
+
+#endif /* __SECURITY_INTEL_TXT_PLATFORM_H__ */