linux-kernel - Re: [PATCH v2 09/11] ima: Move validation of the keyrings conditional into ima_validate

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Message-ID: <20200706131845.GI4694@sequoia>
Date:   Mon, 6 Jul 2020 08:18:45 -0500
From:   Tyler Hicks <tyhicks@...ux.microsoft.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Prakhar Srivastava <prsriva02@...il.com>,
        linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v2 09/11] ima: Move validation of the keyrings
 conditional into ima_validate_rule()

On 2020-07-03 10:15:32, Mimi Zohar wrote:
> On Thu, 2020-07-02 at 17:16 -0500, Tyler Hicks wrote:
> > On 2020-06-30 19:07:29, Mimi Zohar wrote:
> > > On Fri, 2020-06-26 at 17:38 -0500, Tyler Hicks wrote:
> > > > Use ima_validate_rule() to ensure that the combination of a hook
> > > > function and the keyrings conditional is valid and that the keyrings
> > > > conditional is not specified without an explicit KEY_CHECK func
> > > > conditional. This is a code cleanup and has no user-facing change.
> > > > 
> > > > Signed-off-by: Tyler Hicks <tyhicks@...ux.microsoft.com>
> > > > ---
> > > > 
> > > > * v2
> > > >   - Allowed IMA_DIGSIG_REQUIRED, IMA_PERMIT_DIRECTIO,
> > > >     IMA_MODSIG_ALLOWED, and IMA_CHECK_BLACKLIST conditionals to be
> > > >     present in the rule entry flags for non-buffer hook functions.
> > > > 
> > > >  security/integrity/ima/ima_policy.c | 13 +++++++++++--
> > > >  1 file changed, 11 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> > > > index 8cdca2399d59..43d49ad958fb 100644
> > > > --- a/security/integrity/ima/ima_policy.c
> > > > +++ b/security/integrity/ima/ima_policy.c
> > > > @@ -1000,6 +1000,15 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
> > > >  		case KEXEC_KERNEL_CHECK:
> > > >  		case KEXEC_INITRAMFS_CHECK:
> > > >  		case POLICY_CHECK:
> > > > +			if (entry->flags & ~(IMA_FUNC | IMA_MASK | IMA_FSMAGIC |
> > > > +					     IMA_UID | IMA_FOWNER | IMA_FSUUID |
> > > > +					     IMA_INMASK | IMA_EUID | IMA_PCR |
> > > > +					     IMA_FSNAME | IMA_DIGSIG_REQUIRED |
> > > > +					     IMA_PERMIT_DIRECTIO |
> > > > +					     IMA_MODSIG_ALLOWED |
> > > > +					     IMA_CHECK_BLACKLIST))
> > > 
> > > Other than KEYRINGS, this patch should continue to behave the same.
> > >  However, this list gives the impressions that all of these flags are
> > > permitted on all of the above flags, which isn't true.
> > > 
> > > For example, both IMA_MODSIG_ALLOWED & IMA_CHECK_BLACKLIST are limited
> > > to appended signatures, meaning KERNEL_CHECK and KEXEC_KERNEL_CHECK.
> > 
> > Just to clarify, are both IMA_MODSIG_ALLOWED and IMA_CHECK_BLACKLIST
> > limited to KEXEC_KERNEL_CHECK, KEXEC_INITRAMFS_CHECK, and MODULE_CHECK?
> > That's what ima_hook_supports_modsig() suggests.
> 
> Theoretically that is true, but I have no idea how you would append a
> signature to the kexec boot command line.  The only users of appended
> signatures are currently kernel modules and the kexec'ed kernel image.

The discrepancy was with KEXEC_INITRAMFS_CHECK, not KEXEC_CMDLINE. I now
see that there's no support for initramfs signature verification in the
kexec code so I'll assume that ima_hook_supports_modsig() is wrong and
limit IMA_MODSIG_ALLOWED and IMA_CHECK_BLACKLIST to the
KEXEC_KERNEL_CHECK and MODULE_CHECK actions, as you originally
suggested.

Tyler

> 
> > 
> > >  Both should only be allowed on APPRAISE action rules.
> > 
> > For completeness, it looks like DONT_APPRAISE should not be allowed.
> 
> Good point.  
> 
> > 
> > > IMA_PCR should be limited to MEASURE action rules.
> > 
> > It looks like DONT_MEASURE should not be allowed.
> 
> The TPM PCR isn't a file attribute.
> 
> > 
> > > IMA_DIGSIG_REQUIRED should be limited to APPRAISE action rules.
> > 
> > It looks like DONT_APPRAISE should not be allowed.
> 
> Right, in all of these cases the DONT_XXXX isn't applicable.
> 
> Mimi