kexec: Do not verify the signature without the lockdown or mandatory signature

So, here, let's simplify the logic to improve code readability. If the KEXEC_SIG_FORCE enabled or kexec lockdown enabled, signature verification is mandated. Otherwise, we lift the bar for any kernel image.

Signed-off-by Lianbo Jiang <[email protected]>

Signature verification is an important security feature, to protect system from being attacked with a kernel of unknown origin. Kexec rebooting is a way to replace the running kernel, hence need be secured carefully.

In the current code of handling signature verification of kexec kernel, the logic is very twisted. It mixes signature verification, IMA signature appraising and kexec lockdown.

If there is no KEXEC_SIG_FORCE, kexec kernel image doesn't have one of signature, the supported crypto, and key, we don't think this is wrong, Unless kexec lockdown is executed. IMA is considered as another kind of signature appraising method.

If kexec kernel image has signature/crypto/key, it has to go through the signature verification and pass. Otherwise it's seen as verification failure, and won't be loaded.

Seems kexec kernel image with an unqualified signature is even worse than those w/o signature at all, this sounds very unreasonable. E.g. If people get a unsigned kernel to load, or a kernel signed with expired key, which one is more dangerous?

So, here, let's simplify the logic to improve code readability. If the KEXEC_SIG_FORCE enabled or kexec lockdown enabled, signature verification is mandated. Otherwise, we lift the bar for any kernel image.

Signed-off-by Lianbo Jiang <[email protected]>

---
Changes since v1:
[1] Modify the log level(suggested by Jiri Bohac)

 kernel/kexec_file.c | 34 ++++++----------------------------
 1 file changed, 6 insertions(+), 28 deletions(-)

kernel/kexec_file.c +6 -28

index faa74d5f6941..fae496958a68 100644
@@ -181,34 +181,19 @@ void kimage_file_post_load_cleanup(struct kimage *image)
 static int
 kimage_validate_signature(struct kimage *image)
 {
-	const char *reason;
 	int ret;
 
 	ret = arch_kexec_kernel_verify_sig(image, image->kernel_buf,
 					   image->kernel_buf_len);
-	switch (ret) {
-	case 0:
-		break;
+	if (ret) {
 
-		/* Certain verification errors are non-fatal if we're not
-		 * checking errors, provided we aren't mandating that there
-		 * must be a valid signature.
-		 */
-	case -ENODATA:
-		reason = "kexec of unsigned image";
-		goto decide;
-	case -ENOPKG:
-		reason = "kexec of image with unsupported crypto";
-		goto decide;
-	case -ENOKEY:
-		reason = "kexec of image with unavailable key";
-	decide:
 		if (IS_ENABLED(CONFIG_KEXEC_SIG_FORCE)) {
-			pr_notice("%s rejected\n", reason);
+			pr_notice("Enforced kernel signature verification failed (%d).\n", ret);
 			return ret;
 		}
 
-		/* If IMA is guaranteed to appraise a signature on the kexec
+		/*
+		 * If IMA is guaranteed to appraise a signature on the kexec
 		 * image, permit it even if the kernel is otherwise locked
 		 * down.
 		 */

@@ -216,17 +201,10 @@ kimage_validate_signature(struct kimage *image)
 		    security_locked_down(LOCKDOWN_KEXEC))
 			return -EPERM;
 
-		return 0;
-
-		/* All other errors are fatal, including nomem, unparseable
-		 * signatures and signature check failures - even if signatures
-		 * aren't required.
-		 */
-	default:
-		pr_notice("kernel signature verification failed (%d).\n", ret);
+		pr_debug("kernel signature verification failed (%d).\n", ret);
 	}
 
-	return ret;
+	return 0;
 }
 #endif
 

Signature · 1 line

2.17.1

Signature verification is an important security feature, to protect system from being attacked with a kernel of unknown origin. Kexec rebooting is a way to replace the running kernel, hence need be secured carefully.

In the current code of handling signature verification of kexec kernel, the logic is very twisted. It mixes signature verification, IMA signature appraising and kexec lockdown.

If there is no KEXEC_SIG_FORCE, kexec kernel image doesn't have one of signature, the supported crypto, and key, we don't think this is wrong, Unless kexec lockdown is executed. IMA is considered as another kind of signature appraising method.

If kexec kernel image has signature/crypto/key, it has to go through the signature verification and pass. Otherwise it's seen as verification failure, and won't be loaded.

Seems kexec kernel image with an unqualified signature is even worse than those w/o signature at all, this sounds very unreasonable. E.g. If people get a unsigned kernel to load, or a kernel signed with expired key, which one is more dangerous?

So, here, let's simplify the logic to improve code readability. If the KEXEC_SIG_FORCE enabled or kexec lockdown enabled, signature verification is mandated. Otherwise, we lift the bar for any kernel image.

Signed-off-by Lianbo Jiang <[email protected]>

---
Changes since v1:
[1] Modify the log level(suggested by Jiri Bohac)

 kernel/kexec_file.c | 34 ++++++----------------------------
 1 file changed, 6 insertions(+), 28 deletions(-)

kernel/kexec_file.c +6 -28

index faa74d5f6941..fae496958a68 100644
@@ -181,34 +181,19 @@ void kimage_file_post_load_cleanup(struct kimage *image)
 static int
 kimage_validate_signature(struct kimage *image)
 {
-	const char *reason;
 	int ret;
 
 	ret = arch_kexec_kernel_verify_sig(image, image->kernel_buf,
 					   image->kernel_buf_len);
-	switch (ret) {
-	case 0:
-		break;
+	if (ret) {
 
-		/* Certain verification errors are non-fatal if we're not
-		 * checking errors, provided we aren't mandating that there
-		 * must be a valid signature.
-		 */
-	case -ENODATA:
-		reason = "kexec of unsigned image";
-		goto decide;
-	case -ENOPKG:
-		reason = "kexec of image with unsupported crypto";
-		goto decide;
-	case -ENOKEY:
-		reason = "kexec of image with unavailable key";
-	decide:
 		if (IS_ENABLED(CONFIG_KEXEC_SIG_FORCE)) {
-			pr_notice("%s rejected\n", reason);
+			pr_notice("Enforced kernel signature verification failed (%d).\n", ret);
 			return ret;
 		}
 
-		/* If IMA is guaranteed to appraise a signature on the kexec
+		/*
+		 * If IMA is guaranteed to appraise a signature on the kexec
 		 * image, permit it even if the kernel is otherwise locked
 		 * down.
 		 */

@@ -216,17 +201,10 @@ kimage_validate_signature(struct kimage *image)
 		    security_locked_down(LOCKDOWN_KEXEC))
 			return -EPERM;
 
-		return 0;
-
-		/* All other errors are fatal, including nomem, unparseable
-		 * signatures and signature check failures - even if signatures
-		 * aren't required.
-		 */
-	default:
-		pr_notice("kernel signature verification failed (%d).\n", ret);
+		pr_debug("kernel signature verification failed (%d).\n", ret);
 	}
 
-	return ret;
+	return 0;
 }
 #endif
 

Signature · 1 line

2.17.1

Signature verification is an important security feature, to protect system from being attacked with a kernel of unknown origin. Kexec rebooting is a way to replace the running kernel, hence need be secured carefully.

In the current code of handling signature verification of kexec kernel, the logic is very twisted. It mixes signature verification, IMA signature appraising and kexec lockdown.

If there is no KEXEC_SIG_FORCE, kexec kernel image doesn't have one of signature, the supported crypto, and key, we don't think this is wrong, Unless kexec lockdown is executed. IMA is considered as another kind of signature appraising method.

If kexec kernel image has signature/crypto/key, it has to go through the signature verification and pass. Otherwise it's seen as verification failure, and won't be loaded.

Seems kexec kernel image with an unqualified signature is even worse than those w/o signature at all, this sounds very unreasonable. E.g. If people get a unsigned kernel to load, or a kernel signed with expired key, which one is more dangerous?

So, here, let's simplify the logic to improve code readability. If the KEXEC_SIG_FORCE enabled or kexec lockdown enabled, signature verification is mandated. Otherwise, we lift the bar for any kernel image.

Signed-off-by Lianbo Jiang <[email protected]>

---
Changes since v1:
[1] Modify the log level(suggested by Jiri Bohac)

 kernel/kexec_file.c | 34 ++++++----------------------------
 1 file changed, 6 insertions(+), 28 deletions(-)

kernel/kexec_file.c +4 -25

index faa74d5f6941..fae496958a68 100644
@@ -181,34 +181,19 @@ void kimage_file_post_load_cleanup(struct kimage *image)
 static int
 kimage_validate_signature(struct kimage *image)
 {
-	const char *reason;
 	int ret;
 
 	ret = arch_kexec_kernel_verify_sig(image, image->kernel_buf,
 					   image->kernel_buf_len);
-	switch (ret) {
-	case 0:
-		break;
+	if (ret) {
 
-		/* Certain verification errors are non-fatal if we're not
-		 * checking errors, provided we aren't mandating that there
-		 * must be a valid signature.
-		 */
-	case -ENODATA:
-		reason = "kexec of unsigned image";
-		goto decide;
-	case -ENOPKG:
-		reason = "kexec of image with unsupported crypto";
-		goto decide;
-	case -ENOKEY:
-		reason = "kexec of image with unavailable key";
-	decide:
 		if (IS_ENABLED(CONFIG_KEXEC_SIG_FORCE)) {
-			pr_notice("%s rejected\n", reason);
+			pr_notice("Enforced kernel signature verification failed (%d).\n", ret);
 			return ret;
 		}
 
-		/* If IMA is guaranteed to appraise a signature on the kexec
+		/*
+		 * If IMA is guaranteed to appraise a signature on the kexec
 		 * image, permit it even if the kernel is otherwise locked
 		 * down.
 		 */

@@ -216,17 +201,10 @@ kimage_validate_signature(struct kimage *image)
 		    security_locked_down(LOCKDOWN_KEXEC))
 			return -EPERM;
 
-		return 0;
-
-		/* All other errors are fatal, including nomem, unparseable
-		 * signatures and signature check failures - even if signatures
-		 * aren't required.
-		 */

Signature verification is an important security feature, to protect system from being attacked with a kernel of unknown origin. Kexec rebooting is a way to replace the running kernel, hence need be secured carefully.

In the current code of handling signature verification of kexec kernel, the logic is very twisted. It mixes signature verification, IMA signature appraising and kexec lockdown.

If there is no KEXEC_SIG_FORCE, kexec kernel image doesn't have one of signature, the supported crypto, and key, we don't think this is wrong,

Unless kexec lockdown is executed. IMA is considered as another kind of signature appraising method.

If kexec kernel image has signature/crypto/key, it has to go through the signature verification and pass. Otherwise it's seen as verification failure, and won't be loaded.

Seems kexec kernel image with an unqualified signature is even worse than those w/o signature at all, this sounds very unreasonable. E.g. If people get a unsigned kernel to load, or a kernel signed with expired key, which one is more dangerous?

So, here, let's simplify the logic to improve code readability. If the KEXEC_SIG_FORCE enabled or kexec lockdown enabled, signature verification is mandated. Otherwise, we lift the bar for any kernel image.

Quoted 4 lines · On Tue, 2 Jun 2020 12:59:52 +0800 Lianbo Jiang <[email protected]> wrote:

Signature verification is an important security feature, to protect system from being attacked with a kernel of unknown origin. Kexec rebooting is a way to replace the running kernel, hence need be secured carefully.

I'm finding this changelog quite hard to understand,

Quoted 6 lines

In the current code of handling signature verification of kexec kernel, the logic is very twisted. It mixes signature verification, IMA signature appraising and kexec lockdown.

If there is no KEXEC_SIG_FORCE, kexec kernel image doesn't have one of signature, the supported crypto, and key, we don't think this is wrong,

I think this is saying that in the absence of KEXEC_SIG_FORCE and if the signature/crypto/key are all incorrect, the kexec still succeeds, but it should not.

Quoted 6 lines

Unless kexec lockdown is executed. IMA is considered as another kind of signature appraising method.

If kexec kernel image has signature/crypto/key, it has to go through the signature verification and pass. Otherwise it's seen as verification failure, and won't be loaded.

I don't know if this is describing the current situation or the post-patch situation.

Quoted 8 lines

Seems kexec kernel image with an unqualified signature is even worse than those w/o signature at all, this sounds very unreasonable. E.g. If people get a unsigned kernel to load, or a kernel signed with expired key, which one is more dangerous?

So, here, let's simplify the logic to improve code readability. If the KEXEC_SIG_FORCE enabled or kexec lockdown enabled, signature verification is mandated. Otherwise, we lift the bar for any kernel image.

I think the whole thing needs a rewrite. Start out by fully describing the current situation. THen describe what is wrong with it, and why. Then describe the proposed change. Or something along these lines.

The changelog should also make clear the end-user impact of the patch. In sufficient detail for others to decide which kernel version(s) should be patched. Your recommendations will also be valuable - which kernel version(s) do you think should be patched, and why?