Subject: Re: [PATCH v4 15/17] module: Introduce hash-based integrity checking

From: Nicolas Schier <[email-masked]>
Date: Mon, 23 Feb 2026 19:41:52 +0100

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-masked]>
> > > ---
> > >  .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-masked]>
> > > + * Copyright (C) 2025 Thomas Weißschuh <[email-masked]>
> > > + *
> > > + */
> > > +#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