[CC'ing Paul Moore]

On Fri, 2023-07-14 at 09:46 +0800, Coiby Xu wrote:

On Wed, Jul 12, 2023 at 02:31:43PM -0400, Mimi Zohar wrote:

quoted

[Cc'ing the LSM mailing list.]

On Tue, 2023-07-11 at 11:16 +0800, Coiby Xu wrote:

quoted

When IMA has verified the signature of the kernel image, kexec'ing this
kernel should be allowed.

Fixes: af16df54b89d ("ima: force signature verification when CONFIG_KEXEC_SIG is configured")
Signed-off-by: Coiby Xu <redacted>

The original commit  29d3c1c8dfe7 ("kexec: Allow kexec_file() with
appropriate IMA policy when locked down") was not in lieu of the PE-
COFF signature, but allowed using the IMA signature on other
architectures.

Currently on systems with both PE-COFF and IMA signatures, both
signatures are verified, assuming the file is in the IMA policy.  If
either signature verification fails, the kexec fails.

With this patch, only the IMA signature would be verified.

Thanks for correcting me! I thought it's already a consensus that we could use
either signature to verify a kernel image because that's what the code of
commit 29d3c1c8dfe7 has done and the code comment seems to confirm it.  But if
we just read the commit message, it indeed didn't give an answer on whether x86
and ARM are only allowed to use PE-COFF signature.

I'm not aware of any consensus one way or the other.  Commit
29d3c1c8dfe7 continued to fail the kexec on failure, when
CONFIG_KEXEC_SIG_FORCE was enabled.

As there isn't a lockdown maintainer, Paul are you ok with this change?

quoted

quoted

---
 kernel/kexec_file.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 881ba0d1714c..96fce001fbc0 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c

@@ -162,6 +162,13 @@ kimage_validate_signature(struct kimage *image)
 	ret = kexec_image_verify_sig(image, image->kernel_buf,
 				     image->kernel_buf_len);
 	if (ret) {
+		/*
+		 * If the kernel image already has its IMA signature verified, permit it.
+		 */
+		if (ima_appraise_signature(READING_KEXEC_IMAGE)) {
+			pr_notice("The kernel image already has its IMA signature verified.\n");
+			return 0;
+		}

 		if (sig_enforce) {
 			pr_notice("Enforced kernel signature verification failed (%d).\n", ret);

@@ -169,12 +176,9 @@ kimage_validate_signature(struct kimage *image)
 		}

 		/*
-		 * If IMA is guaranteed to appraise a signature on the kexec
-		 * image, permit it even if the kernel is otherwise locked
-		 * down.
+		 * When both IMA and KEXEC_SIG fail in lockdown mode, reject it.
 		 */
-		if (!ima_appraise_signature(READING_KEXEC_IMAGE) &&
-		    security_locked_down(LOCKDOWN_KEXEC))
+		if (security_locked_down(LOCKDOWN_KEXEC))
 			return -EPERM;

 		pr_debug("kernel signature verification failed (%d).\n", ret);