Message-ID: <20230913.Ceifae7ievei@digikod.net>
Date:   Wed, 13 Sep 2023 12:21:58 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Eric Snowberg <eric.snowberg@...cle.com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        David Howells <dhowells@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "mic@...ux.microsoft.com" <mic@...ux.microsoft.com>,
        Kanth Ghatraju <kanth.ghatraju@...cle.com>,
        Konrad Wilk <konrad.wilk@...cle.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>
Subject: Re: [PATCH] certs: Restrict blacklist updates to the secondary
 trusted keyring

On Wed, Sep 13, 2023 at 02:40:17AM +0000, Eric Snowberg wrote:
> 
> 
> > On Sep 12, 2023, at 4:47 PM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
> > 
> > On Tue, 2023-09-12 at 17:11 +0000, Eric Snowberg wrote:
> >> 
> >>> On Sep 12, 2023, at 5:54 AM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
> >>> 
> >>> On Tue, 2023-09-12 at 02:00 +0000, Eric Snowberg wrote:
> >>>> 
> >>>>> On Sep 11, 2023, at 5:08 PM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
> >>>>> 
> >>>>> On Mon, 2023-09-11 at 22:17 +0000, Eric Snowberg wrote:
> >>>>>> 
> >>>>>>> On Sep 11, 2023, at 10:51 AM, Mickaël Salaün <mic@...ikod.net> wrote:
> >>>>>>> 
> >>>>>>> On Mon, Sep 11, 2023 at 09:29:07AM -0400, Mimi Zohar wrote:
> >>>>>>>> Hi Eric,
> >>>>>>>> 
> >>>>>>>> On Fri, 2023-09-08 at 17:34 -0400, Eric Snowberg wrote:
> >>>>>>>>> Currently root can dynamically update the blacklist keyring if the hash
> >>>>>>>>> being added is signed and vouched for by the builtin trusted keyring.
> >>>>>>>>> Currently keys in the secondary trusted keyring can not be used.
> >>>>>>>>> 
> >>>>>>>>> Keys within the secondary trusted keyring carry the same capabilities as
> >>>>>>>>> the builtin trusted keyring.  Relax the current restriction for updating
> >>>>>>>>> the .blacklist keyring and allow the secondary to also be referenced as
> >>>>>>>>> a trust source.  Since the machine keyring is linked to the secondary
> >>>>>>>>> trusted keyring, any key within it may also be used.
> >>>>>>>>> 
> >>>>>>>>> An example use case for this is IMA appraisal.  Now that IMA both
> >>>>>>>>> references the blacklist keyring and allows the machine owner to add
> >>>>>>>>> custom IMA CA certs via the machine keyring, this adds the additional
> >>>>>>>>> capability for the machine owner to also do revocations on a running
> >>>>>>>>> system.
> >>>>>>>>> 
> >>>>>>>>> IMA appraisal usage example to add a revocation for /usr/foo:
> >>>>>>>>> 
> >>>>>>>>> sha256sum /bin/foo | awk '{printf "bin:" $1}' > hash.txt
> >>>>>>>>> 
> >>>>>>>>> openssl smime -sign -in hash.txt -inkey machine-private-key.pem \
> >>>>>>>>>    -signer machine-certificate.pem -noattr -binary -outform DER \
> >>>>>>>>>    -out hash.p7s
> >>>>>>>>> 
> >>>>>>>>> keyctl padd blacklist "$(< hash.txt)" %:.blacklist < hash.p7s
> >>>>>>>>> 
> >>>>>>>>> Signed-off-by: Eric Snowberg <eric.snowberg@...cle.com>
> >>>>>>>> 
> >>>>>>>> The secondary keyring may include both CA and code signing keys.  With
> >>>>>>>> this change any key loaded onto the secondary keyring may blacklist a
> >>>>>>>> hash.  Wouldn't it make more sense to limit blacklisting
> >>>>>>>> certificates/hashes to at least CA keys? 
> >>>>>>> 
> >>>>>>> Some operational constraints may limit what a CA can sign.
> >>>>>> 
> >>>>>> Agreed.  
> >>>>>> 
> >>>>>> Is there precedents for requiring this S/MIME to be signed by a CA? 
> >>>>>> 
> >>>>>>> This change is critical and should be tied to a dedicated kernel config
> >>>>>>> (disabled by default), otherwise existing systems using this feature
> >>>>>>> will have their threat model automatically changed without notice.
> >>>>>> 
> >>>>>> Today we have INTEGRITY_CA_MACHINE_KEYRING_MAX.  This can 
> >>>>>> be enabled to enforce CA restrictions on the machine keyring.  Mimi, would 
> >>>>>> this be a suitable solution for what you are after?
> >>>>> 
> >>>>> There needs to be some correlation between the file hashes being added
> >>>>> to the blacklist and the certificate that signed them.  Without that
> >>>>> correlation, any key on the secondary trusted keyring could add any
> >>>>> file hashes it wants to the blacklist.
> >>>> 
> >>>> Today any key in the secondary trusted keyring can be used to validate a 
> >>>> signed kernel module.  At a later time, if a new hash is added to the blacklist 
> >>>> keyring to revoke loading a signed kernel module,  the ability to do the 
> >>>> revocation with this additional change would be more restrictive than loading 
> >>>> the original module.
> >>> 
> >>> A public key on the secondary keyring is used to verify code that it
> >>> signed, but does not impact any other code. Allowing any public key on
> >>> the secondary keyring to blacklist any file hash is giving it more
> >>> privileges than it originally had.
> >>> 
> >>> This requirement isn't different than how Certificate Revocation List
> >>> (CRL) work.  Not any CA can revoke a certificate.
> >> 
> >> In UEFI Secure Boot we have the Forbidden Signature Database (DBX).  
> >> Root can update the DBX on a host.  The requirement placed on updating 
> >> it is the new DBX entry must be signed by any key contained within the 
> >> KEK.  Following a reboot, all DBX entries load into the .blacklist keyring.  
> >> There is not a requirement similar to how CRL’s work here, any KEK key 
> >> can be used.
> >> 
> >> With architectures booted through a shim there is the MOKX.  Similar to 
> >> DBX, MOKX have the same capabilities, however they do not need to be 
> >> signed by any key, the machine owner must show they have physical 
> >> presence (and potentially a MOK password) for inclusion.  Again there 
> >> is not a requirement similar to how CRL’s work here either.  The machine 
> >> owner can decide what is included.
> >> 
> >> Today when a kernel is built, any number of keys may be included within 
> >> the builtin trusted keyring.  The keys included in the kernel may not have 
> >> a single usage field set or the CA bit set.  There are no requirements on 
> >> how these keys get used later on.  Any key in the builtin trusted keyring 
> >> can be used to sign a revocation that can be added to the blacklist keyring.  
> >> Additionally, any key in the MOK can be used to sign this kernel and it will 
> >> boot.  Before booting the kernel, MOK keys have more privileges than 
> >> after the kernel is booted in some instances.
> >> 
> >> Today MOK keys can be loaded into the machine keyring.  These keys get 
> >> linked to the secondary trusted keyring.  Currently key usage enforcement
> >> is being applied to these keys behind some Kconfig options.  By default 
> >> anything in the secondary has the same capabilities as the builtin trusted 
> >> keyring.  What is challenging here with this request is the inconsistency 
> >> between how everything else currently works. 
> >> 
> >> Root can not arbitrarily add things to the secondary trusted keyring.  These 
> >> keys must be signed by something in either the machine or the builtin.  In 
> >> this thread [1], Jarkko is saying CA based infrastructure should be a policy 
> >> decision not to be enforced by the kernel. Wouldn’t this apply here as well?
> >> 
> >> 1. https://lore.kernel.org/lkml/CVGUFUEQVCHS.37OA20PNG9EVB@suppilovahvero/
> > 
> > Mickaël said, "This change is critical and should be tied to a
> > dedicated kernel config
> > (disabled by default), otherwise existing systems using this feature
> > will have their threat model automatically changed without notice."
> 
> I was thinking he meant it is critical not to change the current behavior
> by limiting blacklisting to only CA keys.  Not that it was critical to add
> CA enforcement.  Maybe Mickaël can comment?

I meant that applying this patch as-is may change the threat model used
by some users. Currently, only signed hashes vouched by the builtin
trusted keyring are valid. If we extend this mechanism to the secondary
trusted keyring without notice, this means that more certificates could
vouch blacklisted hashes, which may include some certificates with an
initial different usage.

See commit 4da8f8c8a1e0 ("dm verity: Add support for signature
verification with 2nd keyring") that adds
CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING:
https://lore.kernel.org/all/20201023170512.201124-1-mic@digikod.net/

> 
> > As a possible alternative I suggested limiting which file hashes the
> > certs on the secondary (or machine) keyring could blacklist.
> 
> I’m not sure I completely understand your suggestion here.
> Do you mean, verify the CA bit is set for secondary keys, but
> ignore the bit for builtin?  And then only use those keys to add
> hashes to the blacklist keyring?  If I have that right, what would 
> be the justification for the change based on how things currently
> get included in the blacklist keyring?  Thanks.

I'd like to be able to specify which kind of certificate can vouch for
blacklisting hashes, and for other usages, but AFAIK this is not the
path Linux has followed (e.g. unlike Windows). We only have the keyring
to identify an usage, which is unfortunate. On the other side, this
approach lets users manage their certificates without constraint, which
is quite (too?) flexible. A complementary approach would be to create an
LSM (or a dedicated interface) to tie certificate properties to a set of
kernel usages, while still letting users configure these constraints.

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