[03/27] Enforce module signatures if the kernel is locked down

Commit Message

Matthew Garrett March 6, 2019, 11:58 p.m. UTC

From: David Howells <[e-mail]>

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 <[e-mail]>
Reviewed-by: Jiri Bohac <[e-mail]>
cc: "Lee, Chun-Yi" <[e-mail]>
cc: James Morris <[e-mail]>
Signed-off-by: Matthew Garrett <[e-mail]>
---
 kernel/module.c | 39 ++++++++++++++++++++++++++++++++-------
 1 file changed, 32 insertions(+), 7 deletions(-)

Comments

James Morris March 8, 2019, 11 p.m. UTC | #1

On Wed, 6 Mar 2019, Matthew Garrett wrote:

> From: David Howells <[e-mail]>
> 
> If the kernel is locked down, require that all modules have valid
> signatures that we can verify.

Perhaps note that this won't cover the case where folk are using DM-Verity 
with a signed root hash for verifying kernel modules.

Matthew Garrett March 8, 2019, 11:30 p.m. UTC | #2

On Fri, Mar 8, 2019 at 3:00 PM James Morris <[e-mail]> wrote:
>
> On Wed, 6 Mar 2019, Matthew Garrett wrote:
>
> > From: David Howells <[e-mail]>
> >
> > If the kernel is locked down, require that all modules have valid
> > signatures that we can verify.
>
> Perhaps note that this won't cover the case where folk are using DM-Verity
> with a signed root hash for verifying kernel modules.

Mm. I can't see a terribly good way of doing this generically -
loadpin gives no indication to the module loading code that it comes
from a trusted source. Would making the lockdown/module signature
enforcement a separate config option be reasonable?

James Morris March 9, 2019, 4:45 a.m. UTC | #3

On Fri, 8 Mar 2019, Matthew Garrett wrote:

> On Fri, Mar 8, 2019 at 3:00 PM James Morris <[e-mail]> wrote:
> >
> > On Wed, 6 Mar 2019, Matthew Garrett wrote:
> >
> > > From: David Howells <[e-mail]>
> > >
> > > If the kernel is locked down, require that all modules have valid
> > > signatures that we can verify.
> >
> > Perhaps note that this won't cover the case where folk are using DM-Verity
> > with a signed root hash for verifying kernel modules.
> 
> Mm. I can't see a terribly good way of doing this generically -
> loadpin gives no indication to the module loading code that it comes
> from a trusted source. Would making the lockdown/module signature
> enforcement a separate config option be reasonable?

I was just suggest documenting this.

Patch

diff --git a/kernel/module.c b/kernel/module.c
index 2ad1b5239910..9a377c6ea200 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2767,8 +2767,9 @@  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 err = -ENODATA;
 	const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
+	const char *reason;
 	const void *mod = info->hdr;
 
 	/*
@@ -2783,16 +2784,40 @@  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;
+		if (kernel_is_locked_down(reason))
+			return -EPERM;
+		return 0;
+
+		/* 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)