Re: [PATCH V34 04/29] Enforce module signatures if the kernel is locked down

Kees Cook
2019-06-22T23:48:21Z

On Fri, Jun 21, 2019 at 05:03:33PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@redhat.com>
>
> If the kernel is locked down, require that all modules have valid
> signatures that we can verify.
>
> I have adjusted the errors generated:
>
> (1) If there's no signature (ENODATA) or we can't check it (ENOPKG,
> ENOKEY), then:
>
> (a) If signatures are enforced then EKEYREJECTED is returned.
>
> (b) If there's no signature or we can't check it, but the kernel is
> locked down then EPERM is returned (this is then consistent with
> other lockdown cases).
>
> (2) If the signature is unparseable (EBADMSG, EINVAL), the signature fails
> the check (EKEYREJECTED) or a system error occurs (eg. ENOMEM), we
> return the error we got.
>
> Note that the X.509 code doesn't check for key expiry as the RTC might not
> be valid or might not have been transferred to the kernel's clock yet.
>
> [Modified by Matthew Garrett to remove the IMA integration. This will
> be replaced with integration with the IMA architecture policy
> patchset.]
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Matthew Garrett <matthewgarrett@google.com>
> Cc: Jessica Yu <jeyu@kernel.org>
> ---
> include/linux/security.h | 1 +
> kernel/module.c | 38 +++++++++++++++++++++++++++++-------
> security/lockdown/lockdown.c | 1 +
> 3 files changed, 33 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/security.h b/include/linux/security.h
> index c808d344ec75..46d85cd63b06 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -82,6 +82,7 @@ enum lsm_event {
> */
> enum lockdown_reason {
> LOCKDOWN_NONE,
> + LOCKDOWN_MODULE_SIGNATURE,
> LOCKDOWN_INTEGRITY_MAX,
> LOCKDOWN_CONFIDENTIALITY_MAX,
> };
> diff --git a/kernel/module.c b/kernel/module.c
> index 0b9aa8ab89f0..6aa681edd660 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2763,8 +2763,9 @@ static inline void kmemleak_load_module(const struct module *mod,
> #ifdef CONFIG_MODULE_SIG
> static int module_sig_check(struct load_info *info, int flags)
> {
> - int err = -ENOKEY;
> + int ret, err = -ENODATA;
> const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
> + const char *reason;
> const void *mod = info->hdr;
>
> /*
> @@ -2779,16 +2780,39 @@ static int module_sig_check(struct load_info *info, int flags)
> err = mod_verify_sig(mod, info);
> }
>
> - if (!err) {
> + switch (err) {
> + case 0:
> info->sig_ok = true;
> return 0;
> - }
>
> - /* Not having a signature is only an error if we're strict. */
> - if (err == -ENOKEY && !is_module_sig_enforced())
> - err = 0;
> + /* We don't permit modules to be loaded into trusted kernels
> + * without a valid signature on them, but if we're not
> + * enforcing, certain errors are non-fatal.
> + */
> + case -ENODATA:
> + reason = "Loading of unsigned module";
> + goto decide;
> + case -ENOPKG:
> + reason = "Loading of module with unsupported crypto";
> + goto decide;
> + case -ENOKEY:
> + reason = "Loading of module with unavailable key";
> + decide:
> + if (is_module_sig_enforced()) {
> + pr_notice("%s is rejected\n", reason);
> + return -EKEYREJECTED;
> + }
>
> - return err;
> + ret = security_locked_down(LOCKDOWN_MODULE_SIGNATURE);
> + return ret;

return security_locked_down(LOCKDOWN_MODULE_SIGNATURE); ? Means no need
to add "ret". Regardless:

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> +
> + /* All other errors are fatal, including nomem, unparseable
> + * signatures and signature check failures - even if signatures
> + * aren't required.
> + */
> + default:
> + return err;
> + }
> }
> #else /* !CONFIG_MODULE_SIG */
> static int module_sig_check(struct load_info *info, int flags)
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 8e39b36b8f33..25a3a5b0aa9c 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -18,6 +18,7 @@ static enum lockdown_reason kernel_locked_down;
>
> static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
> [LOCKDOWN_NONE] = "none",
> + [LOCKDOWN_MODULE_SIGNATURE] = "unsigned module loading",
> [LOCKDOWN_INTEGRITY_MAX] = "integrity",
> [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
> };
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

--
Kees Cook