cpu/x86/smm: Add helper functions to verify SMM access

* Add a function to check if a region overlaps with SMM.
* Add a function to check if a pointer points to SMM.
* Document functions in Documentation/security/smm

To be used to verify data accesses in SMM.

Change-Id: Ia525d2bc685377f50ecf3bdcf337a4c885488213
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Christian Walter <christian.walter@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41084
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/security/index.md

@@ -13,3 +13,7 @@ This section describes documentation about the security architecture of coreboot
 - [Intel TXT in general](intel/txt.md)
 - [Intel TXT Initial Boot Block](intel/txt_ibb.md)
 - [Intel Authenticated Code Modules](intel/acm.md)
+
+## SMM
+
+- [System Management Mode](smm.md)

Documentation/security/smm.md

@@ -0,0 +1,29 @@
+# x86 System Managment Mode
+
+## Introduction
+
+The code running in System Management Mode (SMM) provides runtime services
+to applications running in [ring0]. It has a higher privilege level than
+[ring0] and resides in the SMRAM region which cannot be accessed from [ring0].
+
+SMM can be entered by issuing System Managment Interrupts (SMIs).
+
+## Secure data exchange
+
+In order to not leak SMM internals or accidentally overwrite parts of SMM,
+[ring0] provided data (pointers, offsets, sizes, ...) must be checked before
+using them in SMM.
+
+There exist two methods to verify data:
+
+```C
+/* Returns true if the region overlaps with the SMM */
+bool smm_region_overlaps_handler(struct region *r);
+```
+
+```C
+/* Returns true if the memory pointed to overlaps with SMM reserved memory. */
+static inline bool smm_points_to_smram(const void *ptr, const size_t len);
+```
+
+[ring0]: https://en.wikipedia.org/wiki/Protection_ring

src/cpu/x86/smm/smihandler.c

@@ -2,6 +2,7 @@
 
 #include <arch/io.h>
 #include <console/console.h>
+#include <commonlib/region.h>
 #include <cpu/x86/smm.h>
 #include <cpu/x86/smi_deprecated.h>
 #include <cpu/amd/amd64_save_state.h>
@@ -119,6 +120,13 @@ static inline void *smm_save_state(uintptr_t base, int arch_offset, int node)
 	return (void *)base;
 }
 
+bool smm_region_overlaps_handler(const struct region *r)
+{
+	const struct region r_smm = {SMM_BASE, SMM_DEFAULT_SIZE};
+
+	return region_overlap(&r_smm, r);
+}
+
 /**
  * @brief Interrupt handler for SMI#
  *
@@ -129,7 +137,7 @@ void smi_handler(u32 smm_revision)
 {
 	unsigned int node;
 	smm_state_save_area_t state_save;
-	u32 smm_base = 0xa0000; /* ASEG */
+	u32 smm_base = SMM_BASE; /* ASEG */
 
 	/* Are we ok to execute the handler? */
 	if (!smi_obtain_lock()) {

src/cpu/x86/smm/smm_module_handler.c

@@ -2,6 +2,7 @@
 
 #include <arch/io.h>
 #include <console/console.h>
+#include <commonlib/region.h>
 #include <cpu/x86/smm.h>
 #include <rmodule.h>
 
@@ -103,6 +104,14 @@ void *smm_get_save_state(int cpu)
 	return base;
 }
 
+bool smm_region_overlaps_handler(const struct region *r)
+{
+	const struct region r_smm = {smm_runtime->smbase, smm_runtime->smm_size};
+	const struct region r_aseg = {SMM_BASE, SMM_DEFAULT_SIZE};
+
+	return region_overlap(&r_smm, r) || region_overlap(&r_aseg, r);
+}
+
 asmlinkage void smm_handler_start(void *arg)
 {
 	const struct smm_module_params *p;

src/include/cpu/x86/smm.h

@@ -4,6 +4,7 @@
 #define CPU_X86_SMM_H
 
 #include <arch/cpu.h>
+#include <commonlib/region.h>
 #include <types.h>
 
 #define SMM_DEFAULT_BASE 0x30000
@@ -93,6 +94,17 @@ asmlinkage void smm_handler_start(void *params);
  * account CPUs which are configured to not save their state to RAM. */
 void *smm_get_save_state(int cpu);
 
+/* Returns true if the region overlaps with the SMM */
+bool smm_region_overlaps_handler(const struct region *r);
+
+/* Returns true if the memory pointed to overlaps with SMM reserved memory. */
+static inline bool smm_points_to_smram(const void *ptr, const size_t len)
+{
+	const struct region r = {(uintptr_t)ptr, len};
+
+	return smm_region_overlaps_handler(&r);
+}
+
 /* SMM Module Loading API */
 
 /* The smm_loader_params structure provides direction to the SMM loader: