[PATCH] kexec_file: ima: allow loading a kernel with its IMA signature verified

[Coiby Xu]

On Wed, Jul 12, 2023 at 02:31:43PM -0400, Mimi Zohar wrote:
>[Cc'ing the LSM mailing list.]
>
>On Tue, 2023-07-11 at 11:16 +0800, Coiby Xu wrote:
>> 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 <coxu at redhat.com>
>
>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.

>
>> ---
>>  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);
>
>

-- 
Best regards,
Coiby