bootm: add support for dynamically forcing signature verification

So far, secure booting systems statically configured
CONFIG_BOOTM_FORCE_SIGNED_IMAGES=y to restrict bootm to signed images.

This remains the recommended way, but some systems require the ability
to decide at runtime whether to enforce secure boot or to disable it,
e.g. after verifying a JSON web token with the appropriate claim.

For such systems, provide a bootm_force_signed_images() function.
There's intentionally no unforce counterpart as this is meant to be
non-reversible.

Signed-off-by: Ahmad Fatoum <[email protected]>
Signed-off-by: Marco Felsch <[email protected]>
Link: https://lore.barebox.org/[email protected]
Signed-off-by: Sascha Hauer <[email protected]>

Author: a3f

common/bootm.c

@@ -89,6 +89,22 @@ static const char * const bootm_verify_names[] = {
 
 static bool force_signed_images = IS_ENABLED(CONFIG_BOOTM_FORCE_SIGNED_IMAGES);
 
+void bootm_force_signed_images(void)
+{
+	static unsigned int verify_mode = 0;
+
+	if (force_signed_images)
+		return;
+
+	/* recreate bootm.verify with a single enumeration as option */
+	globalvar_remove("bootm.verify");
+	globalvar_add_simple_enum("bootm.verify", &verify_mode,
+				  &bootm_verify_names[BOOTM_VERIFY_SIGNATURE], 1);
+
+	bootm_verify_mode = BOOTM_VERIFY_SIGNATURE;
+	force_signed_images = true;
+}
+
 bool bootm_signed_images_are_forced(void)
 {
 	return force_signed_images;

include/bootm.h

@@ -153,6 +153,7 @@ enum bootm_verify bootm_get_verify_mode(void);
 void bootm_set_verify_mode(enum bootm_verify mode);
 
 bool bootm_signed_images_are_forced(void);
+void bootm_force_signed_images(void);
 
 #define UIMAGE_SOME_ADDRESS (UIMAGE_INVALID_ADDRESS - 1)