> On Mar 15, 2021, at 12:01 PM, Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > On 15/03/2021 17:59, Eric Snowberg wrote: >> >>> On Mar 12, 2021, at 10:12 AM, Mickaël Salaün <mic@xxxxxxxxxxx> wrote: >>> >>> From: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> >>> >>> Add a kernel option SYSTEM_BLACKLIST_AUTH_UPDATE to enable the root user >>> to dynamically add new keys to the blacklist keyring. This enables to >>> invalidate new certificates, either from being loaded in a keyring, or >>> from being trusted in a PKCS#7 certificate chain. This also enables to >>> add new file hashes to be denied by the integrity infrastructure. >>> >>> Being able to untrust a certificate which could have normaly been >>> trusted is a sensitive operation. This is why adding new hashes to the >>> blacklist keyring is only allowed when these hashes are signed and >>> vouched by the builtin trusted keyring. A blacklist hash is stored as a >>> key description. The PKCS#7 signature of this description must be >>> provided as the key payload. >>> >>> Marking a certificate as untrusted should be enforced while the system >>> is running. It is then forbiden to remove such blacklist keys. >>> >>> Update blacklist keyring, blacklist key and revoked certificate access rights: >>> * allows the root user to search for a specific blacklisted hash, which >>> make sense because the descriptions are already viewable; >>> * forbids key update (blacklist and asymmetric ones); >>> * restricts kernel rights on the blacklist keyring to align with the >>> root user rights. >>> >>> See help in tools/certs/print-cert-tbs-hash.sh . >>> >>> Cc: David Howells <dhowells@xxxxxxxxxx> >>> Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx> >>> Cc: Eric Snowberg <eric.snowberg@xxxxxxxxxx> >>> Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx> >>> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> >>> Link: https://lore.kernel.org/r/20210312171232.2681989-6-mic@xxxxxxxxxxx >> >> I tried testing this, it doesn’t work as I would expect. >> Here is my test setup: >> >> Kernel built with two keys compiled into the builtin_trusted_keys keyring >> >> Generated a tbs cert from one of the keys and signed it with the other key >> >> As root, added the tbs cert hash to the blacklist keyring >> >> Verified the tbs hash is in the blacklist keyring >> >> Enabled lockdown to enforce kernel module signature checking >> >> Signed a kernel module with the key I just blacklisted >> >> Load the kernel module >> >> I’m seeing the kernel module load, I would expect this to fail, since the >> key is now blacklisted. Or is this change just supposed to prevent new keys >> from being added in the future? > > This is the expected behavior and the way the blacklist keyring is > currently used, as explained in the commit message: > "This enables to invalidate new certificates, either from being loaded > in a keyring, or from being trusted in a PKCS#7 certificate chain." > > If you want a (trusted root) key to be untrusted, you need to remove it > from the keyring, which is not allowed for the builtin trusted keyring. Is there a non technical reason why this can not be changed to also apply to builtin trusted keys? If a user had the same tbs cert hash in their dbx and soon mokx, the hash would show up in the .blacklist keyring and invalidate any key in the builtin_trusted_keys keyring. After adding the same hash with this series, it shows up in the .blacklist_keyring but the value is ignored by operations using the builtin_trusted_keys keyring. It just seems incomplete to me, or did I miss an earlier discussion on this topic?
