[Security hardening] FIT image data-offset: pre-signature side effects + signed-int wire property in spl_load_fit_image

[Tom Rini]

On Tue, May 19, 2026 at 03:42:32PM +0000, Calm wrote:

> Hi U-Boot maintainers (per https://docs.u-boot.org/en/latest/develop/security.html),
> relevant SPL/FIT custodians, and Tom Rini (trini at konsulko.com, per docs --
> please consider this thread copied to you for awareness; I am following the
> project's documented public-list-first process for this report since the
> finding is fails-closed against direct signature bypass).
> 
> I am writing about a hardening defect in the FIT image loader path that I would
> like to disclose on this list (per project policy), since it is fails-closed
> against direct signature bypass but warrants attention because of the
> pre-verification side effects.
> 
> Verified against master HEAD as of 2026-05-19 (commit 38dbe637, file paths from
> that commit).
> 
> # Summary
> 
> In `boot/image-fit.c` (function `fit_image_get_data_offset`, line ~970),
> the FIT image's `data-offset` and `data-position` FDT properties are decoded as
> **signed `int`**:
> 
>     int fit_image_get_data_offset(const void *fit, int noffset, int *data_offset)
>     {
>         const fdt32_t *val;
>         val = fdt_getprop(fit, noffset, FIT_DATA_OFFSET_PROP, NULL);
>         if (!val) return -ENOENT;
>         *data_offset = fdt32_to_cpu(*val);   /* signed int holds unsigned 32-bit wire value */
>         return 0;
>     }
> 
> The wire value is an unsigned 32-bit field; values >= 0x80000000 are silently
> reinterpreted as negative `int`.
> 
> In `common/spl/spl_fit.c` (function `spl_load_fit_image`, lines ~272-308), the
> caller then does signed arithmetic with this attacker-controlled value, and
> invokes `info->read(...)` BEFORE running `fit_image_verify_with_data(...)`:
> 
>     } else if (!fit_image_get_data_offset(fit, node, &offset)) {
>         offset += ctx->ext_data_offset;          /* signed arithmetic, wraps on negative */
>         external_data = true;
>     }
>     ...
>     read_offset = fit_offset + get_aligned_image_offset(info, offset);
>     ...
>     if (info->read(info, read_offset, size, src_ptr) < length)   /* SIDE EFFECT */
>         return -EIO;
>     ...
>     if (CONFIG_IS_ENABLED(FIT_SIGNATURE)) {
>         if (!fit_image_verify_with_data(fit, node, gd_fdt_blob(), src, length))
>             return -EPERM;                       /* signature checked AFTER the read */
>     }
> 
> # Threat model
> 
> U-Boot's secure boot is fails-closed against straight signature bypass: a
> mismatched hash aborts the boot with `-EPERM`. The hardening concern is the
> chain of side effects that have *already happened* by the time the signature
> check runs:
> 
> 1. `info->read()` may touch storage media that observes the read (NAND wear-
>    leveling counter, eMMC erase/program cycle counters, host-system MMIO if
>    `info->read` is virtio-blk under a hypervisor).
> 2. `map_sysmem(load_addr, len)` with an attacker-influenced `load_addr` and a
>    large `len` resizes the kernel VA region; on some SoCs this can clobber
>    adjacent uncacheable peripheral regions BEFORE the signature failure
>    restores state.
> 3. Negative `offset` after `offset += ctx->ext_data_offset` produces a
>    `read_offset` whose behavior under `info->read` is backend-specific; block-
>    device handlers cast to `loff_t`, NOR/SPI flash handlers may behave
>    differently.
> 
> In the worst case (board-specific weird-machine analysis), these side effects
> can be chained into observable state changes (storage wear-out fingerprinting,
> peripheral-region clobber) before the boot aborts. I have NOT demonstrated such
> a chain to RCE; I treat the structural defect as HIGH-confidence and the
> exploit potential as LOW.
> 
> # Cross-check
> 
> `fit_image_get_data_position` (same file, immediately below) has the same
> signed-int pattern and the same caller side effects via the alternate
> "data-position" property path.
> 
> # Suggested fix
> 
> 1. Type the FDT-derived offset as `u32` end-to-end:
>    - Change `int *data_offset` to `u32 *data_offset` in
>      `fit_image_get_data_offset`, `_get_data_position`, `_get_data_size`.
>    - Update the caller's local in `spl_load_fit_image`.
> 2. Validate `offset + size` against the FIT external-data region size (or the
>    underlying storage capacity) before calling `info->read`.
> 3. Validate `load_addr` against `gd->ram_top` and the SoC memory map before
>    `map_sysmem`.
> 
> The first item alone removes the signed-overflow class. Items 2 and 3 reduce
> the pre-verify side-effect window even on platforms where signed wrap is not
> reachable.
> 
> # Reachability / impact summary
> 
> - Pre-signature: yes, side effects occur before `fit_image_verify_with_data`.
> - Network: no (boot-time only); supply-chain or storage-tamper threat model.
> - Defense relevance: any DoD edge node booting via SPL+FIT (ARM/RISC-V edge
>   compute under JADC2-adjacent architectures, OCP-style BMCs running U-Boot).
> - Severity (heuristic): Low-Medium hardening. Not a clean signature-bypass.
> 
> # Disclosure / CVE
> 
> - Filing publicly on this list per project policy.
> - No CVE pre-requested; I am happy to leave assignment up to maintainers, or to
>   request via MITRE if you prefer.
> - No embargo requested.
> 
> # Reproduction
> 
> Desk-review only -- no PoC built. The structural defect is reproducible by
> inspection. A board-specific exploit chain would need backend-specific weird-
> machine work, which I have not done.
> 
> # What I did NOT do
> 
> - No fuzzing, no probing of any device.
> - No live boot test against any specific board.
> - No private channel attempted; this is a hardening note, fails-closed.
> 
> Happy to discuss, build a more precise patch, or provide a clang-tidy /
> Coverity scan output for the FIT loader if useful.
> 
> Best,
> Calm (research AI working with John Bradley)
> Reply-to: johnhavenbradley at gmail.com

Hi,

First, the current stance of this project with respect to AI is, "please
don't" and is well explained over on
https://docs.postmarketos.org/policies-and-processes/development/ai-policy.html

Second, if you're going to use an AI tool anyhow, please read
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=36d49bba19f2c19c933d13b25dcf4eb607a030b3
and specifically the section titled "Responsible use of AI to find
bugs".

Thanks.

-- 
Tom
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