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>

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: