vboot: Add FIT_SIGNATURE_MAX_SIZE protection

This adds a new config value FIT_SIGNATURE_MAX_SIZE, which controls the
max size of a FIT header's totalsize field. The field is checked before
signature checks are applied to protect from reading past the intended
FIT regions.

This field is not part of the vboot signature so it should be sanity
checked. If the field is corrupted then the structure or string region
reads may have unintended behavior, such as reading from device memory.
A default value of 256MB is set and intended to support most max storage
sizes.

Suggested-by: Simon Glass <[email protected]>
Signed-off-by: Teddy Reed <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

Kconfig

@@ -267,6 +267,16 @@ config FIT_SIGNATURE
 	  format support in this case, enable it using
 	  CONFIG_IMAGE_FORMAT_LEGACY.
 
+config FIT_SIGNATURE_MAX_SIZE
+	hex "Max size of signed FIT structures"
+	depends on FIT_SIGNATURE
+	default 0x10000000
+	help
+	  This option sets a max size in bytes for verified FIT uImages.
+	  A sane value of 256MB protects corrupted DTB structures from overlapping
+	  device memory. Assure this size does not extend past expected storage
+	  space.
+
 config FIT_VERBOSE
 	bool "Show verbose messages when FIT images fail"
 	help

common/image-sig.c

@@ -156,6 +156,11 @@ static int fit_image_setup_verify(struct image_sign_info *info,
 {
 	char *algo_name;
 
+	if (fdt_totalsize(fit) > CONFIG_FIT_SIGNATURE_MAX_SIZE) {
+		*err_msgp = "Total size too large";
+		return 1;
+	}
+
 	if (fit_image_hash_get_algo(fit, noffset, &algo_name)) {
 		*err_msgp = "Can't get hash algo property";
 		return -1;

test/py/tests/test_vboot.py

@@ -26,6 +26,7 @@
 
 import pytest
 import sys
+import struct
 import u_boot_utils as util
 
 @pytest.mark.boardspec('sandbox')
@@ -105,6 +106,26 @@ def sign_fit(sha_algo):
         util.run_and_log(cons, [mkimage, '-F', '-k', tmpdir, '-K', dtb,
                                 '-r', fit])
 
+    def replace_fit_totalsize(size):
+        """Replace FIT header's totalsize with something greater.
+
+        The totalsize must be less than or equal to FIT_SIGNATURE_MAX_SIZE.
+        If the size is greater, the signature verification should return false.
+
+        Args:
+            size: The new totalsize of the header
+
+        Returns:
+            prev_size: The previous totalsize read from the header
+        """
+        total_size = 0
+        with open(fit, 'r+b') as handle:
+            handle.seek(4)
+            total_size = handle.read(4)
+            handle.seek(4)
+            handle.write(struct.pack(">I", size))
+        return struct.unpack(">I", total_size)[0]
+
     def test_with_algo(sha_algo):
         """Test verified boot with the given hash algorithm.
 
@@ -146,6 +167,18 @@ def test_with_algo(sha_algo):
         util.run_and_log(cons, [fit_check_sign, '-f', fit, '-k', tmpdir,
                                 '-k', dtb])
 
+        # Replace header bytes
+        bcfg = u_boot_console.config.buildconfig
+        max_size = int(bcfg.get('config_fit_signature_max_size', 0x10000000), 0)
+        existing_size = replace_fit_totalsize(max_size + 1)
+        run_bootm(sha_algo, 'Signed config with bad hash', 'Bad Data Hash', False)
+        cons.log.action('%s: Check overflowed FIT header totalsize' % sha_algo)
+
+        # Replace with existing header bytes
+        replace_fit_totalsize(existing_size)
+        run_bootm(sha_algo, 'signed config', 'dev+', True)
+        cons.log.action('%s: Check default FIT header totalsize' % sha_algo)
+
         # Increment the first byte of the signature, which should cause failure
         sig = util.run_and_log(cons, 'fdtget -t bx %s %s value' %
                                (fit, sig_node))

tools/Makefile

@@ -133,6 +133,7 @@ ifdef CONFIG_FIT_SIGNATURE
 # This affects include/image.h, but including the board config file
 # is tricky, so manually define this options here.
 HOST_EXTRACFLAGS	+= -DCONFIG_FIT_SIGNATURE
+HOST_EXTRACFLAGS	+= -DCONFIG_FIT_SIGNATURE_MAX_SIZE=$(CONFIG_FIT_SIGNATURE_MAX_SIZE)
 endif
 
 ifdef CONFIG_SYS_U_BOOT_OFFS