cpu/x86/smm: Add overflow check

Rather bail out than run into undefined behavior.

Change-Id: Ife26a0abed0ce6bcafe1e7cd8f499618631c4df4
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38763
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: <cedarhouse1@comcast.net>

src/cpu/x86/smm/smm_module_loader.c

@@ -202,6 +202,8 @@ static int smm_module_setup_stub(void *smbase, struct smm_loader_params *params,
 	/* Adjust remaining size to account for save state. */
 	total_save_state_size = params->per_cpu_save_state_size *
 				params->num_concurrent_save_states;
+	if (total_save_state_size > size)
+		return -1;
 	size -= total_save_state_size;
 
 	/* The save state size encroached over the first SMM entry point. */