linux-kernel - Re: [PATCH 5.15 0/6] arm64: kexec_file: use more system keyrings to verify kernel image signature + dependencies

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Date:   Tue, 27 Sep 2022 11:39:52 +0900
From:   AKASHI Takahiro <takahiro.akashi@...aro.org>
To:     Michal Such??nek <msuchanek@...e.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Philipp Rudo <prudo@...hat.com>,
        Sasha Levin <sashal@...nel.org>, Baoquan He <bhe@...hat.com>,
        Alexander Egorenkov <egorenar@...ux.ibm.com>,
        "open list:S390" <linux-s390@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        "open list:KEXEC" <kexec@...ts.infradead.org>,
        Coiby Xu <coxu@...hat.com>, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        James Morse <james.morse@....com>
Subject: Re: [PATCH 5.15 0/6] arm64: kexec_file: use more system keyrings to
 verify kernel image signature + dependencies

On Mon, Sep 26, 2022 at 09:40:25AM +0200, Michal Such??nek wrote:
> On Mon, Sep 26, 2022 at 08:47:32AM +0200, Greg Kroah-Hartman wrote:
> > On Sat, Sep 24, 2022 at 01:55:23PM +0200, Michal Suchánek wrote:
> > > On Sat, Sep 24, 2022 at 12:13:34PM +0200, Greg Kroah-Hartman wrote:
> > > > On Sat, Sep 24, 2022 at 11:45:21AM +0200, Michal Suchánek wrote:
> > > > > On Sat, Sep 24, 2022 at 11:19:19AM +0200, Greg Kroah-Hartman wrote:
> > > > > > On Fri, Sep 23, 2022 at 07:10:28PM +0200, Michal Suchanek wrote:
> > > > > > > Hello,
> > > > > > > 
> > > > > > > this is backport of commit 0d519cadf751
> > > > > > > ("arm64: kexec_file: use more system keyrings to verify kernel image signature")
> > > > > > > to table 5.15 tree including the preparatory patches.
> > > > > > 
> > > > > > This feels to me like a new feature for arm64, one that has never worked
> > > > > > before and you are just making it feature-parity with x86, right?
> > > > > > 
> > > > > > Or is this a regression fix somewhere?  Why is this needed in 5.15.y and
> > > > > > why can't people who need this new feature just use a newer kernel
> > > > > > version (5.19?)
> > > > > 
> > > > > It's half-broken implementation of the kexec kernel verification. At the time
> > > > > it was implemented for arm64 we had the platform and secondary keyrings
> > > > > and x86 was using them but on arm64 the initial implementation ignores
> > > > > them.
> > > > 
> > > > Ok, so it's something that never worked.  Adding support to get it to
> > > > work doesn't really fall into the stable kernel rules, right?
> > > 
> > > Not sure. It was defective, not using the facilities available at the
> > > time correctly. Which translates to kernels that can be kexec'd on x86
> > > failing to kexec on arm64 without any explanation (signed with same key,
> > > built for the appropriate arch).
> > 
> > Feature parity across architectures is not a "regression", but rather a
> > "this feature is not implemented for this architecture yet" type of
> > thing.
> 
> That depends on the view - before kexec verification you could boot any
> kernel, now you can boot some kernels signed with a valid key, but not
> others - the initial implementation is buggy, probably because it
> is based on an old version of the x86 code.

Buggy?
The feature of supporting platform ring had been slipped in just before
I submitted the latest patch series which was eventually merged.
(I should have noticed it though.)

Looking at changes in the commit 278311e417be ("kexec, KEYS: Make use of platform
keyring for signature verify"), it seems to be obvious that it is a new feature
because it introduced a new Kconfig option, CONFIG_INTEGRITY_PLATFORM_KEYRING,
which allows for enabling/disabling platform ring support.

-Takahiro Akashi

> > 
> > > > Again, what's wrong with 5.19 for anyone who wants this?  Who does want
> > > > this?
> > > 
> > > Not sure, really.
> > > 
> > > The final patch was repeatedly backported to stable and failed to build
> > > because the prerequisites were missing.
> > 
> > That's because it was tagged, but now that you show the full set of
> > requirements, it's pretty obvious to me that this is not relevant for
> > going this far back.
> 
> That also works.
> 
> Thanks
> 
> Michal