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Bug 1830280 - Please enable CONFIG_RANDOM_TRUST_CPU
Summary: Please enable CONFIG_RANDOM_TRUST_CPU
Keywords:
Status: CLOSED ERRATA
Alias: None
Product: Red Hat Enterprise Linux 8
Classification: Red Hat
Component: kernel
Version: 8.3
Hardware: Unspecified
OS: Unspecified
unspecified
unspecified
Target Milestone: rc
: 8.0
Assignee: Chris von Recklinghausen
QA Contact: Chunyu Hu
URL:
Whiteboard:
Depends On:
Blocks: 1807069 1928027
TreeView+ depends on / blocked
 
Reported: 2020-05-01 13:05 UTC by Colin Walters
Modified: 2023-12-15 17:48 UTC (History)
9 users (show)

Fixed In Version: kernel-4.18.0-201.el8
Doc Type: If docs needed, set a value
Doc Text:
Clone Of:
: 1928027 1976877 (view as bug list)
Environment:
Last Closed: 2020-11-04 01:16:48 UTC
Type: Bug
Target Upstream Version:
Embargoed:

Attachments (Terms of Use)

Description Colin Walters 2020-05-01 13:05:48 UTC 
Split out from discussions in this bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1778762#c23

The high level here is "synchronize with the current state of Fedora" for how entropy (/dev/random) works.  The two bigger recent changes in Linux are the builtin kernel jitter entropy (covered by the above bug) as well as CONFIG_RANDOM_TRUST_CPU (this bug).

Modern AWS instance types have hardware with RDRAND, but there's still no hypervisor entropy (unlike GCP which offers virtio-rng).

Booting a m5d.4xlarge instance for example, firstboot has:

[   19.887867] random: crng init done

That's quite slow; now obviously there are projects like rng-tools that try to address this but the problem is that particularly for CoreOS, we do a whole lot inside the initial ramdisk, so any userspace solution like rng-tools basically needs to be nearly the first process started - we want to support things like dm-crypt for the root filesystem.  

And not to mention a lot of parts of the kernel want entropy even before userspace is launched.

Whereas instead if I do:

# rpm-ostree kargs --append=random.trust_cpu=on
# systemctl reboot

I see the same thing we have with FCOS:

[    0.001000] random: crng done (trusting CPU's manufacturer)

i.e. basically immediately after kernel boot we aren't going to block on strong entropy.

Now, we *could* enable this kernel argument just for RHCOS but I'd want to  see an argument for why we don't have it enabled by default across the board (like Fedora has).
Comment 1 Herbert Xu 2020-05-01 23:55:25 UTC 
The random device is not part of the kernel Crypto API.
Comment 2 Colin Walters 2020-05-04 17:21:29 UTC 
There's no reason for this bug to be private (did something in Bugzilla change to default to private?), and I needed to duplicate some public bugs against related bugs.
Comment 3 Colin Walters 2020-05-12 13:02:27 UTC 
I'd hoped to at least get a "yeah seems sane" or "no we'll never do that because $reasons" here.

For now: https://gitlab.cee.redhat.com/coreos/redhat-coreos/merge_requests/937
Comment 4 Chris von Recklinghausen 2020-05-12 13:28:12 UTC 
I don't know of any reason to not to. Fedora doesn't enable it for aarch64. I'll put out a change with it enabled just like kernel-ark does and see if there are any objections from reviewers.

I'm currently preempted by some high priority work, so it may be a week or so before I'm able to see if there are any objections to enabling it.
Comment 5 Colin Walters 2020-05-12 14:25:18 UTC 
For what it's worth I checked Debian stable:

# dpkg -l linux*
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                                Version           Architecture Description
+++-===================================-=================-============-==================================
ii  linux-base                          4.6               all          Linux image base package
un  linux-doc-4.19                      <none>            <none>       (no description available)
ii  linux-image-4.19.0-6-amd64          4.19.67-2+deb10u2 amd64        Linux 4.19 for 64-bit PCs (signed)
un  linux-image-4.19.0-6-amd64-unsigned <none>            <none>       (no description available)
un  linux-initramfs-tool                <none>            <none>       (no description available)
un  linux-show-player                   <none>            <none>       (no description available)
# grep RANDOM_TRUST_CPU /boot/config-4.19.0-6-amd64 
CONFIG_RANDOM_TRUST_CPU=y
#

OpenSUSE leap apparently doesn't:

# rpm -qa|grep kernel
kernel-vanilla-base-4.12.14-lp151.28.48.1.x86_64
# grep RANDOM_TRUST /boot/config-4.12.14-lp151.28.48-vanilla 
# 

But tumbleweed does:

$ podman run --rm -ti docker.io/opensuse/tumbleweed
...
0900b096880b:/ # grep CONFIG_RANDOM_TRUST /boot/config-5.6.11-1-vanilla 
CONFIG_RANDOM_TRUST_CPU=y
CONFIG_RANDOM_TRUST_BOOTLOADER=y
0900b096880b:/ #
Comment 7 Chris von Recklinghausen 2020-05-14 18:17:15 UTC 
Patches have been posted for review. FYI we only enable it for x86_64, ppc64le and s390x. aarch64 does not have it enabled. This is identical to how Fedora configures it.

FYI Fedora does not enable CONFIG_RANDOM_TRUST_BOOTLOADER
Comment 8 Colin Walters 2020-05-14 18:55:41 UTC 
> Patches have been posted for review.

Thank you very much!

> FYI we only enable it for x86_64, ppc64le and s390x. aarch64 does not have it enabled. This is identical to how Fedora configures it.

That seems OK I guess, though we will probably want to chase down whether aarch64 actually needs it or not at some point.

> FYI Fedora does not enable CONFIG_RANDOM_TRUST_BOOTLOADER

I saw that one and briefly looked into it - it seems like that comes into play mostly for architectures that don't have hardware entropy - the people deploying these systems want to basically have /var/lib/systemd/random-seed handled by the bootloader, which makes total sense.  I was talking with Lennart about this once and he pointed out that OpenBSD has done it this way for a while.  But in the Linux world we have a much more...variety...in userspace and bootloaders.
He ended up implementing support for this in systemd/sd-boot: https://github.com/systemd/systemd/pull/13137/

But I don't think anyone looked at it for other bootloaders like grub.

I don't think it's important to enable right now, but (architecture astronauting here) eventually a good flow would be:

Firstboot: use hardware entropy up to early userspace
  Gather entropy from outside the CPU (e.g. we can talk to network services like https://blog.cloudflare.com/league-of-entropy/ or whatever)
  Save that strong entropy in the bootloader config

Subsequent boots: Don't trust the CPU entropy if we have bootloader entropy

But anyways, not critical right now.
Comment 10 Frantisek Hrbata 2020-05-19 12:25:45 UTC 
Patch(es) available on kernel-4.18.0-201.el8
Comment 19 errata-xmlrpc 2020-11-04 01:16:48 UTC 
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: kernel security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2020:4431
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