Bug 1933036

Description of problem:
Hi,
Reading crypto-policies and update-crypto-policies, we tried to understand and encountered several difficulties.

1) It's missing the crucial information, which is to indicate the potential values a parameter can have.
For example:
- The possible values for "cipher = " are listed here:
/usr/share/crypto-policies/policies/DEFAULT.pol


2) The values for ssh_cipher = are indicated nowhere.
See for example:
 sudo less /usr/share/crypto-policies/policies/DEFAULT.pol | grep ssh

So we assumed some values, which I even think they are incomplete/incorrect.
See:
ssh_cipher = -AES-128-CBC -AES-256-CBC -CAMELLIA-256-CBC -CAMELLIA-128-CBC

I think, based on the output of this: man ssh_config | grep -i cbc
                   3des-cbc
                   aes128-cbc
                   aes192-cbc
                   aes256-cbc

We only have aes128-cbc, aes256-cbc which can be relevant for ssh_cipher.

Therefore, I provided additional useless -CAMELLIA-256-CBC -CAMELLIA-128-CBC
(the update-crypto-policies did not complain about that)


3) There is something very confusing with instructions.
It's clear there are some requirements of users to "remove CBC ciphers".
See: https://access.redhat.com/solutions/420283

We have to distinguish two type:
3.1 From the server
3.2 From SSHD process only


When reading the instructions, for example man crypto-policies, it shows:

tls_cipher: Optional; list of allowed symmetric encryption algorithms (including the modes) for use with the TLS protocol. If absent, the value is
           derived from cipher.

       ·   ssh_cipher: Optional; list of allowed symmetric encryption algorithms (including the modes) for use with the SSH protocol. If absent, the value is
           derived from cipher.

       ·   cipher: List of allowed symmetric encryption algorithms (including the modes) for use with other protocols


We understood incorrectly that it would be sufficient to create a custom policy file, modifying the DEFAULT, explicitely removing CBC related from the server.
That was not the case, it did not work.

See "How reproducible" for the details.



4) When doing this command: 
sudo update-crypto-policies --set DEFAULT:MINUSCBCCIPHER
We can verify: sudo update-crypto-policies --show 
DEFAULT:MINUSCBCCIPHER
We then modify the actual file:/etc/crypto-policies/policies/modules/MINUSCBCCIPHER.pmod, nothing shows or inform if the actual policy file (MINUSCBCCIPHER.pmod) was modified...
I think that should also be improved to avoid confusion.





Version-Release number of selected component (if applicable):
crypto-policies.noarch 20191128-2.git23e1bf1.el8




How reproducible:


We created this file:
sudo less /etc/crypto-policies/policies/modules/MINUSCBCCIPHER.pmod

cipher = -AES-128-CBC -AES-256-CBC -CAMELLIA-256-CBC -CAMELLIA-128-CBC

(again, based on the list of possible CBC related values described in /usr/share/crypto-policies/policies/DEFAULT.pol for "cipher =").
sudo update-crypto-policies --set DEFAULT:MINUSCBCCIPHER

We rebooted the server.
Based on the manpage, we saw "If absent, the value is derived from cipher.". We though it would be true and disable all CBC related cipher.

That did not work, the test proved ssh cbc cipher were not removed.
ssh -vv -oCiphers=aes128-cbc,aes256-cbc 127.0.0.1 Shows information, and I was able to connect.

When the CBC cipher are not there for ssh, it should show:

Unable to negotiate with 127.0.0.1 port 22: no matching cipher found. Their offer: aes256-gcm,chacha20-poly1305,aes256-ctr,aes128-gcm,aes128-ctr



We then continue and removed tls_cipher also:

tls_cipher = -AES-256-CBC -AES-128-CBC
cipher = -AES-128-CBC -AES-256-CBC -CAMELLIA-256-CBC -CAMELLIA-128-CBC

Same process, sudo update-crypto-policies --set DEFAULT:MINUSCBCCIPHER, reboot... ssh -vv -oCiphers=aes128-cbc,aes256-cbc 127.0.0.1 still worked.




We came to the conclusion:
We created a policy modification: /etc/crypto-policies/policies/modules/MINUSCBCCIPHER.pmod

To remove the CBC algorith from the server, modifying the DEFAULT profile, we have to write this in that MINUSCBCCIPHER.pmod file:

tls_cipher = -AES-256-CBC -AES-128-CBC
cipher = -AES-128-CBC -AES-256-CBC -CAMELLIA-256-CBC -CAMELLIA-128-CBC
ssh_cipher = -AES-128-CBC -AES-256-CBC


To remove the CBC algorithm from the server for SSHD only, modifying the DEFAULT profile, we have to write this in that MINUSCBCCIPHER.pmod file::
ssh_cipher = -AES-128-CBC -AES-256-CBC

(Note about ssh_cipher:
ssh_cipher = -AES-128-CBC -AES-256-CBC -CAMELLIA-256-CBC -CAMELLIA-128-CBC
 We don't specify these -CAMELLIA since they don't appear to be actually advertised or used by sshd: 
See:
 man sshd_config | grep -i cbc
                   3des-cbc
                   aes128-cbc
                   aes192-cbc
                   aes256-cbc )
3des-cbc is not advertised either from RHEL8.2 server, as seen with ssh -vv and looking at "Their offer": aes256-gcm,chacha20-poly1305,aes256-ctr,aes128-gcm,aes128-ctr)

In both cases, we have to set:
sudo update-crypto-policies --set DEFAULT:MINUSCBCCIPHER
And reboot the server, for the policy to take effect.


Thanks,


Steps to Reproduce:


Actual results:


Expected results:


Additional info: