linux-kernel - Re: [PATCH] lockdown: s390: kexec_file: don't skip signature verification when not secure IPLed

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Message-ID: <20221123005251.h4t2t2lv6tqb5nrp@Rk>
Date:   Wed, 23 Nov 2022 08:52:51 +0800
From:   Coiby Xu <coxu@...hat.com>
To:     Vasily Gorbik <gor@...ux.ibm.com>
Cc:     kexec@...ts.infradead.org, Matthew Garrett <mjg59@...f.ucam.org>,
        Jiri Bohac <jbohac@...e.cz>,
        David Howells <dhowells@...hat.com>,
        Philipp Rudo <prudo@...hat.com>, linux-s390@...r.kernel.org,
        Heiko Carstens <hca@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        James Morris <jmorris@...ei.org>,
        Matthew Garrett <mjg59@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>
Subject: Re: [PATCH] lockdown: s390: kexec_file: don't skip signature
 verification when not secure IPLed

On Tue, Nov 22, 2022 at 04:15:04PM +0100, Vasily Gorbik wrote:
>On Mon, Nov 21, 2022 at 03:27:15PM +0800, Coiby Xu wrote:
>> --- a/arch/s390/kernel/machine_kexec_file.c
>> +++ b/arch/s390/kernel/machine_kexec_file.c
>> @@ -33,10 +33,6 @@ int s390_verify_sig(const char *kernel, unsigned long kernel_len)
>>  	unsigned long sig_len;
>>  	int ret;
>>
>> -	/* Skip signature verification when not secure IPLed. */
>> -	if (!ipl_secure_flag)
>> -		return 0;
>
>Looking at s390_verify_sig() especially before commit 0828c4a39be5
>("kexec, KEYS, s390: Make use of built-in and secondary keyring for
>signature verification") I think this condition actually expresses
>2 things:
>1. the firmware is secure IPL capable and secure IPL keys are
>   provided and present in platform keyring.
>2. secure IPL is enabled.
>
>Wouldn't this change have implications for machines with older firmware
>which doesn't support secure IPL? In this case platform keyring won't
>have any secure IPL keys (since firmware doesn't provide them)
>and any properly signed kernels will be rejected for kexec in this
>function. Unless secure IPL keys are also present in built-in or secondary
>keyring (which is possible after commit 0828c4a39be5) - is that what
>distributions normally do?

Thanks for pointing me to the above commit and reminding me older
firmware doesn't support secure IPL! But I don't think this change will
break machines with older firmwares which doesn't support secure IPL.
Distributions like Fedora/RHEL have downstream-only patch that enable
lockdown automatically when secure boot is enabled. Since there is no
secure IPL, lockdown won't be enabled which means
kimage_validate_signature (kernel/kexec_file.c) doesn't enforce
signature verification (sorry I should change the commit subject which
is misleading). For the case where users manually enables lockdown, I
assume they know what lockdown means and expect signature verification
to be enforced instead to be silently bypassed.

-- 
Best regards,
Coiby