Re: [PATCH v3 1/2] tools/bootconfig: Fix integer overflow and truncation in size checks

On Thu, Sep 10, 2026 at 10:59:24AM +0900, Masami Hiramatsu (Google) wrote:

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Sashiko reported that on 32-bit systems, if an attacker crafts size in
the bootconfig footer such that adding BOOTCONFIG_FOOTER_SIZE wraps around
(for instance, if size is 0xFFFFFFFF), the size check in
load_xbc_from_initrd() can be bypassed:

    if (stat.st_size < size + BOOTCONFIG_FOOTER_SIZE) {
        pr_err("bootconfig size is too big\n");
        return -E2BIG;
    }

Furthermore, on 64-bit systems with an initrd > 4.29 GB, comparing a
corrupted 32-bit size (e.g. 0xFFFFFFFF) against
stat.st_size - BOOTCONFIG_FOOTER_SIZE can also bypass the check if
size is not bounded. Similarly, load_xbc_file() passes 64-bit stat.st_size
directly into the 32-bit int size parameter of load_xbc_fd(), truncating
large standalone files (>= 2GB).

In both cases, passing 0xFFFFFFFF to load_xbc_fd() truncates to -1,
resulting in malloc(0), an integer overflow in read(), and an
out-of-bounds null-byte write.

Fix this by:
1. Rejecting size > XBC_DATA_MAX or
   size > stat.st_size - BOOTCONFIG_FOOTER_SIZE in load_xbc_from_initrd().
2. Rejecting stat.st_size > XBC_DATA_MAX in load_xbc_file() before passing
   it to load_xbc_fd().
3. Checking size < 0 || size > XBC_DATA_MAX defensively in load_xbc_fd().

Fixes: 950313ebf79c ("tools: bootconfig: Add bootconfig command")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260909161113.16C691F00A3A@smtp.kernel.org/ (local)
Closes: https://lore.kernel.org/all/20260910010137.EE0431F000FF@smtp.kernel.org/ (local)
Assisted-by: Antigravity:gemini-3.8-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Reviewed-by: Breno Leitao <leitao@debian.org>