linux-kernel - Re: [PATCH 0/5] Support kdump with LUKS encryption by reusing LUKS volume key

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Message-ID: <f7a1a20c-bee8-c7a4-4c49-b66415f556f9@gmail.com>
Date:   Thu, 8 Jun 2023 12:39:26 +0200
From:   Milan Broz <gmazyland@...il.com>
To:     Coiby Xu <coxu@...hat.com>
Cc:     Eric Biggers <ebiggers@...nel.org>, kexec@...ts.infradead.org,
        Baoquan He <bhe@...hat.com>, x86@...nel.org,
        dm-devel@...hat.com, Pingfan Liu <kernelfans@...il.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Kairui Song <ryncsn@...il.com>,
        Jan Pazdziora <jpazdziora@...hat.com>,
        Thomas Staudt <tstaudt@...ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Ondrej Kozina <okozina@...hat.com>
Subject: Re: [PATCH 0/5] Support kdump with LUKS encryption by reusing LUKS
 volume key

On 6/7/23 14:39, Coiby Xu wrote:
...
>> I do not think you need any cryptsetup patches, all you need is to write
>> decrypted volume key from LUKS metadata with
>>   cryptsetup luksDump ---dump-volume-key -volume-key-file <out> <device>
>> (or any code equivalent with libcryptsetup), am I correct?
> 
> Correct me if I'm wrong, but I don't think there will be a safer way to
> preserve key without patching cryptsetup. Actually the --dump-volume-key
> approach has been proposed before and I agree with your conclusion [1]
> on that approach i.e. "passing volume key this way is quite insecure".
> Without patching cryptsetup, even if I save the volume key in the memory
> reserved for the kdump kernel, I need to retrieve this key in the
> userspace to unlock the LUKS device which may lead to quite a security
> vulnerability.

Hm, where are the patches for cryptsetup, then? I am afraid we do not want
to add such specific things there.

But we are just going to merge a patchset that changes how we use keyring
where you can tell cryptsetup to store/link key under some specific name
and to specific keyring
(see https://gitlab.com/cryptsetup/cryptsetup/-/merge_requests/492)
(Please talk to Red Hat cryptsetup maintainers for more info,
I just mentioned this mail to them today.)

> I respect the efforts from you and the cryptsetup community to make LUKS
> as secure as possible. And kdump is used in product environment. Kdump
> is to a server as a black box is to an aircraft. So by no means I want
> to reverse the used security measures and patching cryptsetup can allow
> to keep the security measures. One concern raised by you against "FRC
> v1" was a copy of the LUKS volume key for the kdump kernel creates an
> attack vector. I took this feedback seriously and have sought advice
> from my colleagues to implement the countermeasures ([PATCH 1/5] and
> [Patch 4/5]).
> 
> [1] https://yhbt.net/lore/all/e5abd089-3398-fdb4-7991-0019be434b79@gmail.com/

Yes, I appreciate that. And it is perfectly ok if your customers accept
the trade-off and security risk of handling the key this way.

Anyway, I feel we are going in circles here, and as it seems to be my fault,
I do not want to sound grumpy as I am perhaps missing some context.

Could you please talk to internal RH cryptsetup maintainers first and discuss
your solution? They know what we can do here can help to find an acceptable
solution. (I added cc to Ondra.)

Thanks,
Milan