Coiby Xu coxu@redhat.com 于2021年11月5日周五 下午6:10写道：
Hi Kairui,
On Thu, Nov 04, 2021 at 11:23:14PM +0800, Kairui Song wrote:
Hi Coiby,
So sorry to say that I forgot to back up some old emails so I lost them...
That's fine.
Plan 3 is not a good idea either. Making things too complicated and not safe.
Plan 4 is definitely the right choice, and no one was against it, and I vaguely remember it is doable as we were discussing. As the cryptsetup tool stores the device key as a logon key in kernel (logon so userspace can't access the key content), the idea to keep the logon key used by dm-crypt consistent in kernel over kexec reboot, just keep the reference in userspace (eg. pass a key-reference in kernel cmdline). cryptsetup tool in kdump kernel can just use that key reference to setup the dm-crypt device again. The only problem was that key have KEY_SPEC_THREAD_KEYRING flag, which means it's reference was no longer available in user space too after the initial cryptsetup thread exited. May need to loose this security measure a little bit, eg. to KEY_SPEC_GROUP_KEYRING?
According to [1], "using this feature dm-crypt no longer maintains a direct key copy (but there's always at least one copy in kernel crypto layer).". So maybe KEY_SPEC_THREAD_KEYRING isn't a problem since the master key is always there.
Yes, it is still there, but the reference is dropped, so userspace no longer have the reference, and can't re-setup the dm-crypt. Kernel can't setup dm-crypt device by itself.
Another question seems to be more concerning is how we can retrieve the master key in the 2nd kernel? It seems currently there is no way to pass data from the 1st second to 2nd second kernel. I'm aware there is work using the so called "kexec buffer" to carrying the IMA measurement list across kexec. But the patches haven't been merged.
You can check "security/integrity/ima/ima_kexec.c" in kernel and also check git log of that file, this should be the IMA measure list carrying feature you are talking about? It's merged, the patch may get some other update before being merged, so it looks a bit different from that mail.
[1] https://gitlab.com/cryptsetup/cryptsetup/-/blob/master/docs/Keyring.txt
[2] https://lists.ozlabs.org/pipermail/linuxppc-dev/2016-November/150790.html
You can definitely reboot the discussion in the cryptsetup mail list.
Coiby Xu coxu@redhat.com 于2021年11月4日周四 下午6:05写道：
Hi Kairui,
On Tue, Apr 20, 2021 at 05:23:49PM +0800, Kairui Song wrote:
Hi,
Thanks a lot, these info are very helpful. Better to keep it for debugging for now, and ask users to use it very carefully.
On Tue, Apr 20, 2021 at 3:54 PM Milan Broz gmazyland@gmail.com wrote:
Hi,
TL;DR what you are trying to do is to actually reverse many security measures we added. It is perhaps acceptable for debugging but hardly for real generic system.
- using memory-hard function increases cost of dictionary and brute-force
attacks You can always decrease amount of memory needed, but you should do it only if you know that security margin is ok (like password is randomly generated with enough entropy).
- key is in keyring to remove possibility for normal userspace to receive
the key from kernel. Moreover, there is no need to retain kernel in keyring once dm-crypt device is activated. (It is still in kernel memory but only in crypto functions context). (Systemd also uses keyring to cache passphrase but that's different thing.)
You can still use old way for activation with --disable-keyring activation, but then you disable this possibility.
More comments below.
How about plan 3 and 4?
Have you got any reply from Milan about his input on plan 3 and 4? If so, can you forward his email(s) to me? Thanks!
- Let kdump use some custom helper/service to load all needed
resources in the early initrd boot stage, prior to systemd-cryptsetup@%s.service. It will ask the password / parse the keyfile and load kdump, then provide info for systemd-cryptsetup or just do the setup. Or maybe let systemd-cryptsetup support some kind of "plugins" so other tools can use it.
Some details could be changed/improved, but systemd-cryptsetup@%s.service will prompt for a password or use a keyfile anyway. So I think at this point, loading kdump with the volume key should be safe? At least long as the kdump kernel/environment itself isn't compromised. Loaded kdump resources can be restricted to be only accessible from the kernel side. After panic, kernel kexec jumps to kdump kernel, and that's an minimized emergency environment that only lives for a very short period.
- A better and safer solution seems to keep a consistent key ring
between kexec boots but also more complex and difficult as different arch implements kexec differently.
Maybe plan 4 will be a good idea if doable? Since that keeps the key consistent in the kernel between kexec boots, and cryptsetup can just reuse it.
-- Best Regards, Kairui Song
-- Best regards, Coiby
-- Best regards, Coiby