Re: [PATCH v5 22/23] integrity: Move integrity functions to the LSM infrastructure

[Roberto Sassu]

On Fri, 2023-11-17 at 16:22 -0500, Paul Moore wrote:
> On Thu, Nov 16, 2023 at 5:08 AM Roberto Sassu
> <roberto.sassu@xxxxxxxxxxxxxxx> wrote:
> > On Wed, 2023-11-15 at 23:33 -0500, Paul Moore wrote:
> > > On Nov  7, 2023 Roberto Sassu <roberto.sassu@xxxxxxxxxxxxxxx> wrote:
> 
> ...
> 
> > > > +/*
> > > > + * Perform the initialization of the 'integrity', 'ima' and 'evm' LSMs to
> > > > + * ensure that the management of integrity metadata is working at the time
> > > > + * IMA and EVM hooks are registered to the LSM infrastructure, and to keep
> > > > + * the original ordering of IMA and EVM functions as when they were hardcoded.
> > > > + */
> > > >  static int __init integrity_lsm_init(void)
> > > >  {
> > > > +   const struct lsm_id *lsmid;
> > > > +
> > > >     iint_cache =
> > > >         kmem_cache_create("iint_cache", sizeof(struct integrity_iint_cache),
> > > >                           0, SLAB_PANIC, iint_init_once);
> > > > +   /*
> > > > +    * Obtain either the IMA or EVM LSM ID to register integrity-specific
> > > > +    * hooks under that LSM, since there is no LSM ID assigned to the
> > > > +    * 'integrity' LSM.
> > > > +    */
> > > > +   lsmid = ima_get_lsm_id();
> > > > +   if (!lsmid)
> > > > +           lsmid = evm_get_lsm_id();
> > > > +   /* No point in continuing, since both IMA and EVM are disabled. */
> > > > +   if (!lsmid)
> > > > +           return 0;
> > > > +
> > > > +   security_add_hooks(integrity_hooks, ARRAY_SIZE(integrity_hooks), lsmid);
> > > 
> > > Ooof.  I understand, or at least I think I understand, why the above
> > > hack is needed, but I really don't like the idea of @integrity_hooks
> > > jumping between IMA and EVM depending on how the kernel is configured.
> > > 
> > > Just to make sure I'm understanding things correctly, the "integrity"
> > > LSM exists to ensure the proper hook ordering between IMA/EVM, shared
> > > metadata management for IMA/EVM, and a little bit of a hack to solve
> > > some kernel module loading issues with signatures.  Is that correct?
> > > 
> > > I see that patch 23/23 makes some nice improvements to the metadata
> > > management, moving them into LSM security blobs, but it appears that
> > > they are still shared, and thus the requirement is still there for
> > > an "integrity" LSM to manage the shared blobs.
> > 
> > Yes, all is correct.
> 
> Thanks for the clarification, more on this below.
> 
> > > I'd like to hear everyone's honest opinion on this next question: do
> > > we have any hope of separating IMA and EVM so they are independent
> > > (ignore the ordering issues for a moment), or are we always going to
> > > need to have the "integrity" LSM to manage shared resources, hooks,
> > > etc.?
> > 
> > I think it should not be technically difficult to do it. But, it would
> > be very important to understand all the implications of doing those
> > changes.
> > 
> > Sorry, for now I don't see an immediate need to do that, other than
> > solving this LSM naming issue. I tried to find the best solution I
> > could.
> 
> I first want to say that I think you've done a great job thus far, and
> I'm very grateful for the work you've done.  We can always use more
> help in the kernel security space and I'm very happy to see your
> contributions - thank you :)

Thank you!

> I'm concerned about the integrity LSM because it isn't really a LSM,
> it is simply an implementation artifact from a time when only one LSM
> was enabled.  Now that we have basic support for stacking LSMs, as we
> promote integrity/IMA/EVM I think this is the perfect time to move
> away from the "integrity" portion and integrate the necessary
> functionality into the IMA and EVM LSMs.  This is even more important
> now that we are looking at making the LSMs more visible to userspace
> via syscalls; how would you explain to a developer or user the need
> for an "integrity" LSM along with the IMA and EVM LSMs?
> 
> Let's look at the three things the the integrity code provides in this patchset:
> 
> * IMA/EVM hook ordering
> 
> For better or worse, we have requirements on LSM ordering today that
> are enforced only by convention, the BPF LSM being the perfect
> example.  As long as we document this in Kconfig I think we are okay.
> 
> * Shared metadata
> 
> Looking at the integrity_iint_cache struct at the end of your patchset
> I see the following:
> 
>   struct integrity_iint_cache {
>     struct mutex mutex;
>     struct inode *inode;
>     u64 version;
>     unsigned long flags;
>     unsigned long measured_pcrs;
>     unsigned long atomic_flags;
>     enum integrity_status ima_file_status:4;
>     enum integrity_status ima_mmap_status:4;
>     enum integrity_status ima_bprm_status:4;
>     enum integrity_status ima_read_status:4;
>     enum integrity_status ima_creds_status:4;
>     enum integrity_status evm_status:4;
>     struct ima_digest_data *ima_hash;
>   };
> 
> Now that we are stashing the metadata in the inode, we should be able
> to remove the @inode field back pointer.  It seems like we could
> duplicate @mutex and @version without problem.
> 
> I only see the @measured_pcrs, @atomic_flags used in the IMA code.
> 
> I only see the @ima_XXX_status fields using in the IMA code, and the
> @evm_status used in the EVM code.
> 
> I only see the @ima_hash field used by the IMA code.
> 
> I do see both IMA and EVM using the @flags field, but only one case
> (IMA_NEW_FILE) where one LSM (EVM) looks for another flags (IMA).  I'm
> not sure how difficult that would be to untangle, but I imagine we
> could do something here; if we had to, we could make EVM be dependent
> on IMA in Kconfig and add a function call to check on the inode
> status.  Although I hope we could find a better solution.
> 
> * Kernel module loading hook (integrity_kernel_module_request(...))
> 
> My guess is that this is really an IMA hook, but I can't say for
> certain.  If it is needed for EVM we could always duplicate it across
> the IMA and EVM LSMs, it is trivially small and one extra strcmp() at
> kernel module load time doesn't seem awful to me.

Ok... so, for now I'm trying to separate them just to see if it is
possible. Will send just the integrity-related patches shortly.

Thanks

Roberto