[PATCH v4] ref-manual: variable FIT_SIGN_INDIVIDUAL mix-and-match attacks

[PATCH v4] ref-manual: variable FIT_SIGN_INDIVIDUAL mix-and-match attacks

[Adrian Freihofer]

Incorporate the lessons learned from a regression introduced with commit
  OE-Core rev: 259bfa86f384206f0d0a96a5b84887186c5f689e
               u-boot: kernel-fitimage: Fix dependency loop if
               UBOOT_SIGN_ENABLE and UBOOT_ENV enabled
and fixed with commit
  OE-Core rev: 0106e5efab99c8016836a2ab71e2327ce58a9a9d
               u-boot: kernel-fitimage: Restore FIT_SIGN_INDIVIDUAL="1"
               behavior
into the documentation.

The use of the variable FIT_SIGN_INDIVIDUAL is explicitly discouraged.
It is also noted that this variable may be removed. It is important that
we try to simplify the implementation of the FIT image as much as
possible. Adding appropriate notes to the documentation is a first step
towards this direction.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
---
 documentation/ref-manual/variables.rst | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index 861b04eaab1..aa8a894bfd2 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -3174,9 +3174,27 @@ system and gives an overview of their function and contents.
       class will sign the kernel, dtb and ramdisk images individually in addition
       to signing the FIT image itself. This could be useful if you are
       intending to verify signatures in another context than booting via
-      U-Boot.
+      U-Boot. This variable is set to "0" by default.
 
-      This variable is set to "0" by default.
+      If :term:`UBOOT_SIGN_ENABLE` is set to "1" and :term:`FIT_SIGN_INDIVIDUAL`
+      is left at its default value of "0", only the configurations are signed.
+      However, the configuration signatures include the hashes of the referenced
+      nodes. This means that the integrity of the entire FIT image is ensured
+      because each hash is compared against a runtime-computed hash for each
+      node.
+      Further information can be found in the U-Boot documentation:
+      `U-Boot fit signature <https://docs.u-boot.org/en/latest/usage/fit/signature.html>`__
+      and more specifically at:
+      `U-Boot signed configurations <https://docs.u-boot.org/en/latest/usage/fit/signature.html#signed-configurations>`__.
+
+      If :term:`UBOOT_SIGN_ENABLE` is set to "1" and :term:`FIT_SIGN_INDIVIDUAL`
+      is set to "1", then the FIT image is signed twice, which is redundant.
+      As this leads to additional complexity without providing any obvious
+      advantage, this feature will likely be removed in a future version.
+
+      Signing only the image nodes is intentionally not implemented by
+      :term:`OpenEmbedded-Core (OE-Core)`, as it is vulnerable to mix-and-match
+      attacks.
 
    :term:`FIT_SIGN_NUMBITS`
       Size of the private key used in the FIT image, in number of bits.
-- 
2.48.1

Re: [docs] [PATCH v4] ref-manual: variable FIT_SIGN_INDIVIDUAL mix-and-match attacks

[Antonin Godard]

Hi Adrian,

On Mon Mar 10, 2025 at 9:56 PM CET, Adrian Freihofer via lists.yoctoproject.org wrote:
> Incorporate the lessons learned from a regression introduced with commit
>   OE-Core rev: 259bfa86f384206f0d0a96a5b84887186c5f689e
>                u-boot: kernel-fitimage: Fix dependency loop if
>                UBOOT_SIGN_ENABLE and UBOOT_ENV enabled
> and fixed with commit
>   OE-Core rev: 0106e5efab99c8016836a2ab71e2327ce58a9a9d
>                u-boot: kernel-fitimage: Restore FIT_SIGN_INDIVIDUAL="1"
>                behavior
> into the documentation.
>
> The use of the variable FIT_SIGN_INDIVIDUAL is explicitly discouraged.
> It is also noted that this variable may be removed. It is important that
> we try to simplify the implementation of the FIT image as much as
> possible. Adding appropriate notes to the documentation is a first step
> towards this direction.
>
> Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
> ---
>  documentation/ref-manual/variables.rst | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
> index 861b04eaab1..aa8a894bfd2 100644
> --- a/documentation/ref-manual/variables.rst
> +++ b/documentation/ref-manual/variables.rst
> @@ -3174,9 +3174,27 @@ system and gives an overview of their function and contents.
>        class will sign the kernel, dtb and ramdisk images individually in addition
>        to signing the FIT image itself. This could be useful if you are
>        intending to verify signatures in another context than booting via
> -      U-Boot.
> +      U-Boot. This variable is set to "0" by default.
>  
> -      This variable is set to "0" by default.
> +      If :term:`UBOOT_SIGN_ENABLE` is set to "1" and :term:`FIT_SIGN_INDIVIDUAL`
> +      is left at its default value of "0", only the configurations are signed.
> +      However, the configuration signatures include the hashes of the referenced
> +      nodes. This means that the integrity of the entire FIT image is ensured
> +      because each hash is compared against a runtime-computed hash for each
> +      node.
> +      Further information can be found in the U-Boot documentation:
> +      `U-Boot fit signature <https://docs.u-boot.org/en/latest/usage/fit/signature.html>`__
> +      and more specifically at:
> +      `U-Boot signed configurations <https://docs.u-boot.org/en/latest/usage/fit/signature.html#signed-configurations>`__.
> +
> +      If :term:`UBOOT_SIGN_ENABLE` is set to "1" and :term:`FIT_SIGN_INDIVIDUAL`
> +      is set to "1", then the FIT image is signed twice, which is redundant.
> +      As this leads to additional complexity without providing any obvious
> +      advantage, this feature will likely be removed in a future version.
> +
> +      Signing only the image nodes is intentionally not implemented by
> +      :term:`OpenEmbedded-Core (OE-Core)`, as it is vulnerable to mix-and-match
> +      attacks.
>  
>     :term:`FIT_SIGN_NUMBITS`
>        Size of the private key used in the FIT image, in number of bits.

Looking good, thanks

Reviewed-by: Antonin Godard <antonin.godard@bootlin.com>

Antonin

-- 
Antonin Godard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com