Re: [PATCH v8 0/4] use more system keyrings to verify arm64 and s390 kexec kernel image signature

From: Coiby Xu <coxu@redhat.com>
To: Mimi Zohar <zohar@linux.ibm.com>
Cc: kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org,
	Michal Suchanek <msuchanek@suse.de>, Baoquan He <bhe@redhat.com>,
	Dave Young <dyoung@redhat.com>, Will Deacon <will@kernel.org>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Chun-Yi Lee <jlee@suse.com>
Subject: Re: [PATCH v8 0/4] use more system keyrings to verify arm64 and s390 kexec kernel image signature
Date: Mon, 20 Jun 2022 21:14:55 +0800	[thread overview]
Message-ID: <20220620131455.lo2yzumr6ugmofuw@Rk> (raw)
In-Reply-To: <dc23f46380e6fb218181e685ad2f0a1db34500fe.camel@linux.ibm.com>

On Fri, Jun 17, 2022 at 07:58:37AM -0400, Mimi Zohar wrote:
>On Fri, 2022-06-17 at 11:57 +0800, Coiby Xu wrote:
>> >Thanks for explaining IMA to me! There is still the question of what's
>> >the root of trust for .builtin_trusted_keys when there is no real
>> >signature verification. For example, when CONFIG_KEXEC_SIG is enabled,
>> >the default IMA policy is to not appraise kexec image. Since lockdown is
>> >not enabled by default, there is no real verification as
>> >kimage_validate_signature succeeds even when kexec_image_verify_sig
>> >fails.
>>
>> I realize my reasoning is incorrect. Actually the signature
>> verification which establishes the trust on the keys happens in the
>> bootloader. So IMA appraisal or kimage_validate_signature is irrelevant
>> to the question of the root of trust of .builtin_trusted_key. For GRUB,
>> it won't verify the signature by default when secure boot is not enabled.
>> Thus the question of what's root of trust when there is no signature
>> verification is still valid.
>
>We're saying the same thing, just differently.  Your wording describes
>secure boot, how it is established, and who/what is responsible for it.
>I don't think those details are needed.  I originally said,

I think I'm addressing a different concern or case. If kexec_file_load is going
to verify a kernel image signature, what keys is it going to trust and why? I
believe explaining the root trust for different keyrings is to answer this
question. When a bootloader verifies a kernel image signature, the trust is
based on verification of the kernel image signature which we both agree. But
what if a bootloader doesn't do the verification? 

>
>.builtin_trusted_keys:
>
>For example,
>
>Keys may be built into the kernel during build or inserted into memory
>reserved for keys post build.  In both of these cases, trust is based
>on verification of the kernel image signature.  On a physical system in
>a secure boot environment, this trust is rooted in HW.
>
>The last line should have said, "For example, on a physical system in a
>...".
>
>thanks,
>
>Mimi
>

-- 
Best regards,
Coiby

_______________________________________________
kexec mailing list
kexec@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/kexec