From ae83f3b72621bd3187eb7956c7c2993a97d4b187 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Thu, 9 Oct 2025 20:06:09 -0700 Subject: module: Add compile-time check for embedded NUL characters Long ago, the kernel module license checks were bypassed by embedding a NUL character in the MODULE_LICENSE() string[1]. By using a string like "GPL\0proprietary text", the kernel would only read "GPL" due to C string termination at the NUL byte, allowing proprietary modules to avoid kernel tainting and access GPL-only symbols. The MODULE_INFO() macro stores these strings in the .modinfo ELF section, and get_next_modinfo() uses strcmp()-family functions which stop at the first NUL. This split the embedded string into two separate .modinfo entries, with only the first part being processed by license_is_gpl_compatible(). Add a compile-time check using static_assert that compares the full string length (sizeof - 1) against __builtin_strlen(), which stops at the first NUL. If they differ, compilation fails with a clear error message. While this check can still be circumvented by modifying the ELF binary post-compilation, it prevents accidental embedded NULs and forces intentional abuse to require deliberate binary manipulation rather than simple source-level tricks. Build tested with test modules containing both valid and invalid license strings. The check correctly rejects: MODULE_LICENSE("GPL\0proprietary") while accepting normal declarations: MODULE_LICENSE("GPL") Link: https://lwn.net/Articles/82305/ [1] Suggested-by: Rusty Russell Signed-off-by: Kees Cook Reviewed-by: Daniel Gomez Reviewed-by: Aaron Tomlin Reviewed-by: Petr Pavlu Tested-by: Daniel Gomez Signed-off-by: Daniel Gomez --- include/linux/moduleparam.h | 3 +++ 1 file changed, 3 insertions(+) (limited to 'include/linux/moduleparam.h') diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h index 6907aedc4f74..915f32f7d888 100644 --- a/include/linux/moduleparam.h +++ b/include/linux/moduleparam.h @@ -26,6 +26,9 @@ /* Generic info of form tag = "info" */ #define MODULE_INFO(tag, info) \ + static_assert( \ + sizeof(info) - 1 == __builtin_strlen(info), \ + "MODULE_INFO(" #tag ", ...) contains embedded NUL byte"); \ static const char __UNIQUE_ID(modinfo)[] \ __used __section(".modinfo") __aligned(1) \ = __MODULE_INFO_PREFIX __stringify(tag) "=" info -- cgit