[PATCH v2] tools/bootconfig: Fix integer overflow in size check

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

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;
    }

This would lead to load_xbc_fd() being called with a negative size,
resulting in an undersized allocation, heap overflow on read, and
out-of-bounds write on the terminating null byte.

Avoid the integer overflow by checking whether size is greater than
stat.st_size - BOOTCONFIG_FOOTER_SIZE. Since stat.st_size is already
verified to be at least BOOTCONFIG_FOOTER_SIZE earlier in the function,
this subtraction will never underflow.

Also reject size > XBC_DATA_MAX upfront in load_xbc_from_initrd() to
avoid allocating and reading oversized configurations into memory before
xbc_init() validation.

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/
Assisted-by: Antigravity:gemini-3.8-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 Changes in v2:
  - Add Cc: stable.
  - Also reject if "size > XBC_DATA_MAX", that is obviously wrong.
---
 tools/bootconfig/main.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/bootconfig/main.c b/tools/bootconfig/main.c
index 7dc9fff9b637..bf6a4e216315 100644
--- a/tools/bootconfig/main.c
+++ b/tools/bootconfig/main.c
@@ -218,7 +218,8 @@ static int load_xbc_from_initrd(int fd, char **buf)
 	csum = le32toh(csum);
 
 	/* Wrong size error  */
-	if (stat.st_size < size + BOOTCONFIG_FOOTER_SIZE) {
+	if (size > XBC_DATA_MAX ||
+	    size > stat.st_size - BOOTCONFIG_FOOTER_SIZE) {
 		pr_err("bootconfig size is too big\n");
 		return -E2BIG;
 	}