bootm: move OS index bound check into the legacy path

Commit 103b1e7ce8 ("bootm: bound-check OS index in
bootm_os_get_boot_func()") added a range check to the shared accessor so
an out-of-range OS id can no longer drive an out-of-bounds read of
boot_os[]. That accessor is reached by every image format, but only a
legacy uImage can deliver an unchecked value. bootm_find_os() takes the
raw 8-bit ih_os byte straight from image_get_os() for legacy images,
whereas the FIT path reaches the accessor only after fit_image_load()
has rejected any image whose os is not one of the supported types, and
the Android path hardcodes IH_OS_LINUX. The check can therefore never
fail for FIT, where it only adds confusion and code.

Move the test to the legacy branch of bootm_find_os(), rejecting an
out-of-range OS where the untrusted byte enters. This keeps the FIT path
clear and lets the check be compiled out when CONFIG_LEGACY_IMAGE_FORMAT
is disabled. A valid OS id that has no handler is still reported by the
existing NULL return path in bootm_run_states().

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

boot/bootm.c

@@ -330,6 +330,10 @@ static int bootm_find_os(const char *cmd_name, const char *addr_fit)
 		images.os.type = image_get_type(os_hdr);
 		images.os.comp = image_get_comp(os_hdr);
 		images.os.os = image_get_os(os_hdr);
+		if (images.os.os >= IH_OS_COUNT) {
+			printf("Unsupported OS type %d\n", images.os.os);
+			return 1;
+		}
 
 		images.os.end = image_get_image_end(os_hdr);
 		images.os.load = image_get_load(os_hdr);

boot/bootm_os.c

@@ -599,7 +599,5 @@ int boot_selected_os(int state, struct bootm_info *bmi, boot_os_fn *boot_fn)
 
 boot_os_fn *bootm_os_get_boot_func(int os)
 {
-	if (os < 0 || os >= ARRAY_SIZE(boot_os))
-		return NULL;
 	return boot_os[os];
 }