Re: [PATCH v2 4/6] module: strip the signature marker in the verification function.

[Philipp Rudo <prudo@redhat.com>]

Hi Michal,

On Thu, 25 Nov 2021 19:02:42 +0100
Michal Suchanek <msuchanek@suse.de> wrote:

> It is stripped by each caller separately.
> 
> Signed-off-by: Michal Suchanek <msuchanek@suse.de>
> ---
>  arch/powerpc/kexec/elf_64.c           |  9 ---------
>  arch/s390/kernel/machine_kexec_file.c |  9 ---------
>  kernel/module.c                       |  7 +------
>  kernel/module_signing.c               | 12 ++++++++++--

kernel/module_signing.c is only compiled with MODULE_SIG enabled but
KEXEC_SIG only selects MODULE_SIG_FORMAT. In the unlikely case that
KEXEC_SIG is enabled but MODULE_SIG isn't this causes a build breakage.
So you need to update KEXEC_SIG to select MODULE_SIG instead of
MODULE_SIG_FORMAT for s390 and ppc.

Thanks
Philipp

>  4 files changed, 11 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/powerpc/kexec/elf_64.c b/arch/powerpc/kexec/elf_64.c
> index 266cb26d3ca0..63634c95265d 100644
> --- a/arch/powerpc/kexec/elf_64.c
> +++ b/arch/powerpc/kexec/elf_64.c
> @@ -156,15 +156,6 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
>  int elf64_verify_sig(const char *kernel, unsigned long length)
>  {
>  	size_t kernel_len = length;
> -	const unsigned long marker_len = sizeof(MODULE_SIG_STRING) - 1;
> -
> -	if (marker_len > kernel_len)
> -		return -EKEYREJECTED;
> -
> -	if (memcmp(kernel + kernel_len - marker_len, MODULE_SIG_STRING,
> -		   marker_len))
> -		return -EKEYREJECTED;
> -	kernel_len -= marker_len;
>  
>  	return verify_appended_signature(kernel, &kernel_len, VERIFY_USE_PLATFORM_KEYRING,
>  					"kexec_file");
> diff --git a/arch/s390/kernel/machine_kexec_file.c b/arch/s390/kernel/machine_kexec_file.c
> index 432797249db3..c4632c1a1b59 100644
> --- a/arch/s390/kernel/machine_kexec_file.c
> +++ b/arch/s390/kernel/machine_kexec_file.c
> @@ -27,20 +27,11 @@ const struct kexec_file_ops * const kexec_file_loaders[] = {
>  int s390_verify_sig(const char *kernel, unsigned long length)
>  {
>  	size_t kernel_len = length;
> -	const unsigned long marker_len = sizeof(MODULE_SIG_STRING) - 1;
>  
>  	/* Skip signature verification when not secure IPLed. */
>  	if (!ipl_secure_flag)
>  		return 0;
>  
> -	if (marker_len > kernel_len)
> -		return -EKEYREJECTED;
> -
> -	if (memcmp(kernel + kernel_len - marker_len, MODULE_SIG_STRING,
> -		   marker_len))
> -		return -EKEYREJECTED;
> -	kernel_len -= marker_len;
> -
>  	return verify_appended_signature(kernel, &kernel_len, VERIFY_USE_PLATFORM_KEYRING,
>  					"kexec_file");
>  }
> diff --git a/kernel/module.c b/kernel/module.c
> index 8481933dfa92..d91ca0f93a40 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2882,7 +2882,6 @@ static inline void kmemleak_load_module(const struct module *mod,
>  static int module_sig_check(struct load_info *info, int flags)
>  {
>  	int err = -ENODATA;
> -	const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
>  	const char *reason;
>  	const void *mod = info->hdr;
>  
> @@ -2890,11 +2889,7 @@ static int module_sig_check(struct load_info *info, int flags)
>  	 * Require flags == 0, as a module with version information
>  	 * removed is no longer the module that was signed
>  	 */
> -	if (flags == 0 &&
> -	    info->len > markerlen &&
> -	    memcmp(mod + info->len - markerlen, MODULE_SIG_STRING, markerlen) == 0) {
> -		/* We truncate the module to discard the signature */
> -		info->len -= markerlen;
> +	if (flags == 0) {
>  		err = verify_appended_signature(mod, &info->len,
>  						VERIFY_USE_SECONDARY_KEYRING, "module");
>  		if (!err) {
> diff --git a/kernel/module_signing.c b/kernel/module_signing.c
> index f492e410564d..4c28cb55275f 100644
> --- a/kernel/module_signing.c
> +++ b/kernel/module_signing.c
> @@ -15,8 +15,7 @@
>  #include "module-internal.h"
>  
>  /**
> - * verify_appended_signature - Verify the signature on a module with the
> - * signature marker stripped.
> + * verify_appended_signature - Verify the signature on a module
>   * @data: The data to be verified
>   * @len: Size of @data.
>   * @trusted_keys: Keyring to use for verification
> @@ -25,12 +24,21 @@
>  int verify_appended_signature(const void *data, size_t *len,
>  			      struct key *trusted_keys, const char *what)
>  {
> +	const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
>  	struct module_signature ms;
>  	size_t sig_len, modlen = *len;
>  	int ret;
>  
>  	pr_devel("==>%s(,%zu)\n", __func__, modlen);  
>  
> +	if (markerlen > modlen)
> +		return -ENODATA;
> +
> +	if (memcmp(data + modlen - markerlen, MODULE_SIG_STRING,
> +		   markerlen))
> +		return -ENODATA;
> +	modlen -= markerlen;
> +
>  	if (modlen <= sizeof(ms))
>  		return -EBADMSG;
>