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

Matthew Garrett
2019-06-22T00:04:15Z

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;
+
+ /* 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