Re: [U-Boot] [PATCH] vboot: Add FIT_SIGNATURE_MAX_SIZE protection

Hi Teddy,

On 3 June 2018 at 15:28, Teddy Reed teddy.reed@gmail.com wrote:

This adds a new config value FIT_SIGNATURE_MAX_SIZE, which controls the max size of a FIT header's totalsize field. The max size is checked before signature checks are applied to prevent reading past defined 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 sjg@chromium.org Signed-off-by: Teddy Reed teddy.reed@gmail.com

---

Kconfig | 10 ++++++++++ common/image-sig.c | 7 +++++++ test/py/tests/test_vboot.py | 31 +++++++++++++++++++++++++++++++ 3 files changed, 48 insertions(+)

Reviewed-by: Simon Glass sjg@chromium.org

Please see below.

diff --git a/Kconfig b/Kconfig index 5a82c95..c8b86cd 100644 --- a/Kconfig +++ b/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 diff --git a/common/image-sig.c b/common/image-sig.c index f65d883..e67d2a2 100644 --- a/common/image-sig.c +++ b/common/image-sig.c @@ -156,6 +156,13 @@ static int fit_image_setup_verify(struct image_sign_info *info, { char *algo_name;

+#ifdef CONFIG_FIT_SIGNATURE_MAX_SIZE

Is there a case where this CONFIG item is not present? If not, can we stop the #ifdef?

•   if (fdt_totalsize(fit) > CONFIG_FIT_SIGNATURE_MAX_SIZE) {
  

•           *err_msgp = "Total size too large";
  

•           return -1;
  

•   }
  

+#endif

•   if (fit_image_hash_get_algo(fit, noffset, &algo_name)) {
            *err_msgp = "Can't get hash algo property";
            return -1;
  

diff --git a/test/py/tests/test_vboot.py b/test/py/tests/test_vboot.py index ee939f2..7b986d2 100644 --- a/test/py/tests/test_vboot.py +++ b/test/py/tests/test_vboot.py @@ -26,6 +26,7 @@ Tests run with both SHA1 and SHA256 hashing.

import pytest import sys +import struct import u_boot_utils as util

@pytest.mark.boardspec('sandbox') @@ -105,6 +106,26 @@ def test_vboot(u_boot_console): util.run_and_log(cons, [mkimage, '-F', '-k', tmpdir, '-K', dtb, '-r', fit])

• def replace_fit_totalsize(size):

•    """Replace FIT header's totalsize.
  

•    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.
  

Please drop period for consistency - not really a sentence. Same below

•    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 +167,16 @@ def test_vboot(u_boot_console): util.run_and_log(cons, [fit_check_sign, '-f', fit, '-k', tmpdir, '-k', dtb])

•    # Replace header bytes
  

•    existing_size = replace_fit_totalsize(256 * 1024 * 1024 + 1)
  

Should this use the CONFIG? You should be able to read this. Here's an example:

bcfg = u_boot_console.config.buildconfig has_cmd_memory = bcfg.get('config_cmd_memory', 'n') == 'y'

•    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))
  

-- 2.7.4

Regards, Simon