[PATCH] certs: Prevent spurious errors on repeated blacklisting

[Jarkko Sakkinen]

On Mon, Nov 07, 2022 at 08:40:09PM +0100, Mickaël Salaün wrote:
> 
> On 07/11/2022 17:35, Thomas Weißschuh wrote:
> > On 2022-11-07 17:20+0100, Mickaël Salaün wrote:
> > > 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 at weissschuh.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?
> > 
> > Indeed, sorry.
> > 
> > > > 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.
> > 
> > Will do.
> > 
> > My plan is to create a new function key_create() that does takes the core logic
> > of key_create_or_update() and fails with EEXIST if needed.
> > 
> > > > 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
> > 
> > I'll send a patch for that, too.
> 
> Good!
> 
> Jarkko, David, any though?

I'm happy to review a patch once it is available.

BR, Jarkko