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

[Simon Glass]

Hi Teddy,

On 3 June 2018 at 15:28, Teddy Reed <teddy.reed at 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 at chromium.org>
> Signed-off-by: Teddy Reed <teddy.reed at 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 at 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