linux-kernel - Re: [PATCH] certs: Prevent spurious errors on repeated blacklisting

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Date:   Mon, 7 Nov 2022 17:20:53 +0100
From:   Mickaël Salaün <mic@...ikod.net>
To:     Thomas Weißschuh <linux@...ssschuh.net>
Cc:     David Howells <dhowells@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jarkko Sakkinen <jarkko@...nel.org>, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Mark Pearson <markpearson@...ovo.com>,
        Eric Snowberg <eric.snowberg@...cle.com>,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH] certs: Prevent spurious errors on repeated blacklisting


On 07/11/2022 16:55, Thomas Weißschuh wrote:
> On 2022-11-07 14:12+0100, Mickaël Salaün wrote:
>> This is a follow-up of
>> https://lore.kernel.org/r/c8c65713-5cda-43ad-8018-20f2e32e4432@t-8ch.de
>>
>> Added Jarkko, Mark Pearson, Eric Snowberg and more ML in Cc.
>>
>>
>> On 04/11/2022 02:47, Thomas Weißschuh wrote:
>>> When the blacklist keyring was changed to allow updates from the root
>>> user it gained an ->update() function that disallows all updates.
>>> When the a hash is blacklisted multiple times from the builtin or
>>> firmware-provided blacklist this spams prominent logs during boot:
>>>
>>> [    0.890814] blacklist: Problem blacklisting hash (-13)
>>>
>>> As all these repeated calls to mark_raw_hash_blacklisted() would create
>>> the same keyring entry again anyways these errors can be safely ignored.
>>
>> These errors can indeed be safely ignored, however they highlight issues
>> with some firmware vendors not checking nor optimizing their blocked hashes.
>> This raises security concerns, and it should be fixed by firmware vendors.
> 
> Thanks, I was not aware that these are worth fixing.
> 
>>> Fixes: 6364d106e041 ("certs: Allow root user to append signed hashes to the blacklist keyring")
>>> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
>>> ---
>>>    certs/blacklist.c | 4 +++-
>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/certs/blacklist.c b/certs/blacklist.c
>>> index 41f10601cc72..5f7f2882ced7 100644
>>> --- a/certs/blacklist.c
>>> +++ b/certs/blacklist.c
>>> @@ -191,7 +191,9 @@ static int mark_raw_hash_blacklisted(const char *hash)
>>>    				   BLACKLIST_KEY_PERM,
>>>    				   KEY_ALLOC_NOT_IN_QUOTA |
>>>    				   KEY_ALLOC_BUILT_IN);
>>> -	if (IS_ERR(key)) {
>>> +
>>> +	/* Blacklisting the same hash twice fails but would be idempotent */
>>> +	if (IS_ERR(key) && PTR_ERR(key) != -EACCES) {
>>
>> We should not hide EACCES errors. This logs issues, which is correct for
>> duplicate hashes, and can help firmware vendors to fix their database. I'd
>> really like to see a different log message instead: change the duplicate
>> entry error code from EACCES to EEXIST, and call pr_warn for this specific
>> case.
> 
> Returning EACCES would require some deeper changes to how the keyring is set up

I guess you meant EEXIST?

> or even changes to the keyring core itself to introduce a key_create() (without
> update) function.
> 
> Is this something you would take a look at, or should I try to do it?
> (I have no previous knowledge about the keyring subsystem)

Please take a look. I think it should not be too complex.

> 
> In any case it probably would also be good to log the problematic hashes
> themselves, so users can properly report the issue to their firmware vendors.

Agree

> 
>>>    		pr_err("Problem blacklisting hash (%ld)\n", PTR_ERR(key));
>>>    		return PTR_ERR(key);
>>>    	}
>>>
>>> base-commit: ee6050c8af96bba2f81e8b0793a1fc2f998fcd20