Re: [PATCH v5 08/14] module: Move authentication logic into dedicated new file

[Petr Pavlu <petr.pavlu@suse.com>]

On 5/5/26 11:05 AM, Thomas Weißschuh wrote:
> The module authentication functionality will also be used by the
> hash-based module authentication. To make it usable even if
> CONFIG_MODULE_SIG is disabled, move it to a new file.
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
> [...]
> diff --git a/kernel/module/auth.c b/kernel/module/auth.c
> index 956ac63d9d33..831a13eb0c9b 100644
> --- a/kernel/module/auth.c
> +++ b/kernel/module/auth.c
> @@ -5,10 +5,16 @@
>   * Written by David Howells (dhowells@redhat.com)
>   */
>  
> +#include <linux/errno.h>
>  #include <linux/export.h>
>  #include <linux/module.h>
> +#include <linux/module_signature.h>
>  #include <linux/moduleparam.h>
> +#include <linux/security.h>
> +#include <linux/string.h>
>  #include <linux/types.h>
> +#include <uapi/linux/module.h>
> +#include "internal.h"
>  
>  #undef MODULE_PARAM_PREFIX
>  #define MODULE_PARAM_PREFIX "module."
> @@ -30,3 +36,82 @@ void set_module_sig_enforced(void)
>  {
>  	sig_enforce = true;
>  }
> +
> +static int mod_verify_sig(const void *mod, struct load_info *info)
> +{
> +	struct module_signature ms;
> +	size_t sig_len, modlen = info->len;
> +	int ret;
> +
> +	if (modlen <= sizeof(ms))
> +		return -EBADMSG;
> +
> +	memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
> +
> +	ret = mod_check_sig(&ms, modlen, "module");
> +	if (ret)
> +		return ret;
> +
> +	sig_len = be32_to_cpu(ms.sig_len);
> +	modlen -= sig_len + sizeof(ms);
> +	info->len = modlen;
> +
> +	return module_sig_check(mod, modlen, mod + modlen, sig_len);
> +}
> +
> +int module_auth_check(struct load_info *info, int flags)
> +{
> +	int err = -ENODATA;
> +	const unsigned long markerlen = sizeof(MODULE_SIGNATURE_MARKER) - 1;
> +	const char *reason;
> +	const void *mod = info->hdr;
> +	bool mangled_module = flags & (MODULE_INIT_IGNORE_MODVERSIONS |
> +				       MODULE_INIT_IGNORE_VERMAGIC);
> +	/*
> +	 * Do not allow mangled modules as a module with version information
> +	 * removed is no longer the module that was signed.
> +	 */
> +	if (!mangled_module &&
> +	    info->len > markerlen &&
> +	    memcmp(mod + info->len - markerlen, MODULE_SIGNATURE_MARKER, markerlen) == 0) {
> +		/* We truncate the module to discard the signature */
> +		info->len -= markerlen;
> +		err = mod_verify_sig(mod, info);
> +		if (!err) {
> +			info->auth_ok = true;
> +			return 0;
> +		}
> +	}
> +
> +	/*
> +	 * We don't permit modules to be loaded into the trusted kernels
> +	 * without a valid signature on them, but if we're not enforcing,
> +	 * certain errors are non-fatal.
> +	 */
> +	switch (err) {
> +	case -ENODATA:
> +		reason = "unsigned module";
> +		break;
> +	case -ENOPKG:
> +		reason = "module with unsupported crypto";
> +		break;
> +	case -ENOKEY:
> +		reason = "module with unavailable key";
> +		break;
> +
> +	default:
> +		/*
> +		 * All other errors are fatal, including lack of memory,
> +		 * unparseable signatures, and signature check failures --
> +		 * even if signatures aren't required.
> +		 */
> +		return err;
> +	}
> +
> +	if (is_module_sig_enforced()) {
> +		pr_notice("Loading of %s is rejected\n", reason);
> +		return -EKEYREJECTED;
> +	}
> +
> +	return security_locked_down(LOCKDOWN_MODULE_SIGNATURE);
> +}

The resulting call chain of the module authentication/signature
functions is as follows:

ima_read_modsig() -----------------------------,
                                               v
module_auth_check() -> mod_verify_sig() -> mod_check_sig()
                             |
                             |-> module_sig_check()
                             '-> module_hash_check()

I think this logic is quite hard to follow because mod_verify_sig(),
mod_check_sig() and module_sig_check() have very similar names.

The naming of module_auth_check(), module_sig_check() and
module_hash_check() looks good to me, but I would prefer to rename
mod_check_sig() and mod_verify_sig(). Perhaps mod_check_sig() could be
renamed to mod_check_sig_header(), and mod_verify_sig() to
mod_dispatch_auth_check()?

Otherwise, the patch looks ok to me. Feel free to add:

Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>

-- 
Thanks,
Petr