Hi Kees, Dan,
Sorry to react on an oldish patch, but I have a question about it, see
below.
On 10/10/2025 05:06, Kees Cook wrote:
> 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 <[email protected]>
> Signed-off-by: Kees Cook <[email protected]>
> ---
> Cc: Luis Chamberlain <[email protected]>
> Cc: Petr Pavlu <[email protected]>
> Cc: Daniel Gomez <[email protected]>
> Cc: Sami Tolvanen <[email protected]>
> Cc: <[email protected]>
> ---
> include/linux/moduleparam.h | 3 +++
> 1 file changed, 3 insertions(+)
>
> 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"); \
When checking MPTCP code on top of Linus tree, I get this new warning
with all MPTCP KUnit tests (net/mptcp/*_test.c), e.g.
$ touch net/mptcp/crypto_test.c && make C=1 net/mptcp/crypto_test.o
CC [M] net/mptcp/crypto_test.o
CHECK net/mptcp/crypto_test.c
net/mptcp/crypto_test.c:72:1: error: bad integer constant expression
net/mptcp/crypto_test.c:72:1: error: static assertion failed:
"MODULE_INFO(license, ...) contains embedded NUL byte"
net/mptcp/crypto_test.c:73:1: error: bad integer constant expression
net/mptcp/crypto_test.c:73:1: error: static assertion failed:
"MODULE_INFO(description, ...) contains embedded NUL byte"
I'm using Sparse last development version with Dan's commit:
$ sparse --version
v0.6.4-73-gfbdde312
=> fbdde312 ("builtin: implement __builtin_strlen() for constants")
And the two lines causing the warnings don't have "\0":
72 MODULE_LICENSE("GPL");
73 MODULE_DESCRIPTION("KUnit tests for MPTCP Crypto");
Am I missing something?
Cheers,
Matt
--
Sponsored by the NGI0 Core fund.
