On Mon, Feb 23, 2026 at 08:53:29AM +0100, Thomas Weißschuh wrote: > On 2026-02-21 22:38:29+0100, Nicolas Schier wrote: > > On Tue, Jan 13, 2026 at 01:28:59PM +0100, Thomas Weißschuh wrote: > > > The current signature-based module integrity checking has some drawbacks > > > in combination with reproducible builds. Either the module signing key > > > is generated at build time, which makes the build unreproducible, or a > > > static signing key is used, which precludes rebuilds by third parties > > > and makes the whole build and packaging process much more complicated. > > > > > > The goal is to reach bit-for-bit reproducibility. Excluding certain > > > parts of the build output from the reproducibility analysis would be > > > error-prone and force each downstream consumer to introduce new tooling. > > > > > > Introduce a new mechanism to ensure only well-known modules are loaded > > > by embedding a merkle tree root of all modules built as part of the full > > > kernel build into vmlinux. > > > > > > Non-builtin modules can be validated as before through signatures. > > > > > > Normally the .ko module files depend on a fully built vmlinux to be > > > available for modpost validation and BTF generation. With > > > CONFIG_MODULE_HASHES, vmlinux now depends on the modules > > > to build a merkle tree. This introduces a dependency cycle which is > > > impossible to satisfy. Work around this by building the modules during > > > link-vmlinux.sh, after vmlinux is complete enough for modpost and BTF > > > but before the final module hashes are > > > > > > The PKCS7 format which is used for regular module signatures can not > > > represent Merkle proofs, so a new kind of module signature is > > > introduced. As this signature type is only ever used for builtin > > > modules, no compatibility issues can arise. > > > > > > Signed-off-by: Thomas Weißschuh <[email protected]> > > > --- > > > .gitignore | 1 + > > > Documentation/kbuild/reproducible-builds.rst | 5 +- > > > Makefile | 8 +- > > > include/asm-generic/vmlinux.lds.h | 11 + > > > include/linux/module_hashes.h | 25 ++ > > > include/linux/module_signature.h | 1 + > > > kernel/module/Kconfig | 21 +- > > > kernel/module/Makefile | 1 + > > > kernel/module/hashes.c | 92 ++++++ > > > kernel/module/hashes_root.c | 6 + > > > kernel/module/internal.h | 1 + > > > kernel/module/main.c | 4 +- > > > scripts/.gitignore | 1 + > > > scripts/Makefile | 3 + > > > scripts/Makefile.modfinal | 11 + > > > scripts/Makefile.modinst | 13 + > > > scripts/Makefile.vmlinux | 5 + > > > scripts/link-vmlinux.sh | 14 +- > > > scripts/modules-merkle-tree.c | 467 > > > +++++++++++++++++++++++++++ > > > security/lockdown/Kconfig | 2 +- > > > 20 files changed, 685 insertions(+), 7 deletions(-) > > > > > [...] > > > > > diff --git a/kernel/module/hashes_root.c b/kernel/module/hashes_root.c > > > new file mode 100644 > > > index 000000000000..1abfcd3aa679 > > > --- /dev/null > > > +++ b/kernel/module/hashes_root.c > > > @@ -0,0 +1,6 @@ > > > +// SPDX-License-Identifier: GPL-2.0-or-later > > > + > > > +#include <linux/module_hashes.h> > > > + > > > +/* Blank dummy data. Will be overridden by link-vmlinux.sh */ > > > +const struct module_hashes_root module_hashes_root > > > __module_hashes_section = {}; > > > diff --git a/kernel/module/internal.h b/kernel/module/internal.h > > > index e2d49122c2a1..e22837d3ac76 100644 > > > --- a/kernel/module/internal.h > > > +++ b/kernel/module/internal.h > > > @@ -338,6 +338,7 @@ void module_mark_ro_after_init(const Elf_Ehdr *hdr, > > > Elf_Shdr *sechdrs, > > > const char *secstrings); > > > > > > int module_sig_check(struct load_info *info, const u8 *sig, size_t > > > sig_len); > > > +int module_hash_check(struct load_info *info, const u8 *sig, size_t > > > sig_len); > > > > > > #ifdef CONFIG_DEBUG_KMEMLEAK > > > void kmemleak_load_module(const struct module *mod, const struct > > > load_info *info); > > > diff --git a/kernel/module/main.c b/kernel/module/main.c > > > index 2a28a0ece809..fa30b6387936 100644 > > > --- a/kernel/module/main.c > > > +++ b/kernel/module/main.c > > > @@ -3362,8 +3362,10 @@ static int module_integrity_check(struct load_info > > > *info, int flags) > > > > > > if (IS_ENABLED(CONFIG_MODULE_SIG) && sig_type == PKEY_ID_PKCS7) { > > > err = module_sig_check(info, sig, sig_len); > > > + } else if (IS_ENABLED(CONFIG_MODULE_HASHES) && sig_type == > > > PKEY_ID_MERKLE) { > > > + err = module_hash_check(info, sig, sig_len); > > > } else { > > > - pr_err("module: not signed with expected PKCS#7 message\n"); > > > + pr_err("module: not signed with signature mechanism\n"); > > > err = -ENOPKG; > > > > To prevent others from running into the same issue: > > > > My first test got stuck here, as I tested with virtme-ng, which symlinks > > modules from build tree to /lib/modules/$(uname -r)/..., resulting in > > > > [ 15.956855] module: not signed with signature mechanism > > modprobe: ERROR: could not insert 'efivarfs': Package not installed > > > > As the modules_install step was missing, modules were not being signed. > > Currently the signing is deferred to installation time to keep in sync > with regular module signing and to keep the logic simpler by not having > to gracefully handle previously-signed files. > But this could be changed.
I did not want to suggest changing the behaviour, that would make things more complicated to prevent needless rebuilds. I just wanted to mention it here to prevent others from burning time. > > [...] > > > diff --git a/scripts/modules-merkle-tree.c b/scripts/modules-merkle-tree.c > > > new file mode 100644 > > > index 000000000000..a6ec0e21213b > > > --- /dev/null > > > +++ b/scripts/modules-merkle-tree.c > > > @@ -0,0 +1,467 @@ > > > +// SPDX-License-Identifier: GPL-2.0-or-later > > > +/* > > > + * Compute hashes for modules files and build a merkle tree. > > > + * > > > + * Copyright (C) 2025 Sebastian Andrzej Siewior <[email protected]> > > > + * Copyright (C) 2025 Thomas Weißschuh <[email protected]> > > > + * > > > + */ > > > +#define _GNU_SOURCE 1 > > > +#include <arpa/inet.h> > > > +#include <err.h> > > > +#include <unistd.h> > > > +#include <fcntl.h> > > > +#include <stdarg.h> > > > +#include <stdio.h> > > > +#include <string.h> > > > +#include <stdbool.h> > > > +#include <stdlib.h> > > > + > > > +#include <sys/stat.h> > > > +#include <sys/mman.h> > > > + > > > +#include <openssl/evp.h> > > > +#include <openssl/err.h> > > > + > > > +#include "ssl-common.h" > > > + > > > +static int hash_size; > > > +static EVP_MD_CTX *ctx; > > > + > > > +struct module_signature { > > > + uint8_t algo; /* Public-key crypto algorithm [0] */ > > > + uint8_t hash; /* Digest algorithm [0] */ > > > + uint8_t id_type; /* Key identifier type [PKEY_ID_PKCS7] > > > */ > > > + uint8_t signer_len; /* Length of signer's name [0] */ > > > + uint8_t key_id_len; /* Length of key identifier [0] */ > > > + uint8_t __pad[3]; > > > + uint32_t sig_len; /* Length of signature data */ > > > +}; > > > + > > > +#define PKEY_ID_MERKLE 3 > > > + > > > +static const char magic_number[] = "~Module signature appended~\n"; > > > > This here will be the forth definition of struct module_signature, > > increasing the risk of unwanted diversion. I second Petr's suggestion > > to reuse a _common_ definition instead. > > Ack. > > > (Here, even include/linux/module_signature.h could be included itself.) > > I'd like to avoid including internal headers from other components. > We could move it to an UAPI header. Various other subsystems use those > for not-really-UAPI but still ABI definitions. Yeah, ack. > (...) > > > > +static inline char *xasprintf(const char *fmt, ...) > > > +{ > > > + va_list ap; > > > + char *strp; > > > + int ret; > > > + > > > + va_start(ap, fmt); > > > + ret = vasprintf(&strp, fmt, ap); > > > + va_end(ap); > > > + if (ret == -1) > > > + err(1, "Memory allocation failed"); > > > + > > > + return strp; > > > +} > > > > Please consider moving these x* functions into scripts/include/xalloc.h > > for reuse. (I am sure someone else wrote this already, but I can't find > > it...) > > Petr suggested it somewhere, it is done for the next revision. > > > thanks for all your efforts for reproducibility! > > > > As I have no clue about that: Is the patent for merkle trees [1] a > > problem when integrating that here? > > That should have expired a long time ago [2]. > And fs-verity is also using merkle trees. Great, thanks. > > Can you verify if I get the mechanics roughly correct? > > > > * Modules are merkle tree leaves. Modules are built and logically > > paired by the order from modules.order; a single left-over module is > > paired with itself. > > > > * Hashes of paired modules are hashed again (branch node hash); > > hashes of pairs of branch nodes' hashes are hashed again; > > repeat until we reach the single merkle tree root hash > > > > * The final merkle tree root hash (and the count of tree levels) is > > included in vmlinux > > The merkle tree code was written by Sebastian so he will have the best > knowledge about it. But this is also my understanding. I'd like to see some (rough) description in Documentation or in a commit message at least, otherwise future me will have to ask that again. > > 'make && find . -name '*.ko' -exec rm {} \; && make' does not rebuild > > the in-tree modules. Shifting the module-hashes support from > > scripts/link-vmlinux.sh to scripts/Makefile.vmlinux might (make it > > easier) to fix this again. > > I'll take a look at it. Thanks! Kind regards, Nicolas > > [1]: > > https://worldwide.espacenet.com/patent/search/family/022107098/publication/US4309569A?q=pn%3DUS4309569 > > [2] > https://patents.stackexchange.com/questions/17901/validity-of-patent-on-merkle-trees > > > Thomas -- Nicolas
