Re: [PATCH v5 23/23] integrity: Switch from rbtree to LSM-managed blob for integrity_iint_cache

[Roberto Sassu]

On Fri, 2023-11-17 at 15:57 -0500, Paul Moore wrote:
> On Nov  7, 2023 Roberto Sassu <roberto.sassu@xxxxxxxxxxxxxxx> wrote:
> > 
> > Before the security field of kernel objects could be shared among LSMs with
> > the LSM stacking feature, IMA and EVM had to rely on an alternative storage
> > of inode metadata. The association between inode metadata and inode is
> > maintained through an rbtree.
> > 
> > Because of this alternative storage mechanism, there was no need to use
> > disjoint inode metadata, so IMA and EVM today still share them.
> > 
> > With the reservation mechanism offered by the LSM infrastructure, the
> > rbtree is no longer necessary, as each LSM could reserve a space in the
> > security blob for each inode. However, since IMA and EVM share the
> > inode metadata, they cannot directly reserve the space for them.
> > 
> > Instead, request from the 'integrity' LSM a space in the security blob for
> > the pointer of inode metadata (integrity_iint_cache structure). The other
> > reason for keeping the 'integrity' LSM is to preserve the original ordering
> > of IMA and EVM functions as when they were hardcoded.
> > 
> > Prefer reserving space for a pointer to allocating the integrity_iint_cache
> > structure directly, as IMA would require it only for a subset of inodes.
> > Always allocating it would cause a waste of memory.
> > 
> > Introduce two primitives for getting and setting the pointer of
> > integrity_iint_cache in the security blob, respectively
> > integrity_inode_get_iint() and integrity_inode_set_iint(). This would make
> > the code more understandable, as they directly replace rbtree operations.
> > 
> > Locking is not needed, as access to inode metadata is not shared, it is per
> > inode.
> > 
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > Reviewed-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> > Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> > ---
> >  security/integrity/iint.c      | 71 +++++-----------------------------
> >  security/integrity/integrity.h | 20 +++++++++-
> >  2 files changed, 29 insertions(+), 62 deletions(-)
> > 
> > diff --git a/security/integrity/iint.c b/security/integrity/iint.c
> > index 882fde2a2607..a5edd3c70784 100644
> > --- a/security/integrity/iint.c
> > +++ b/security/integrity/iint.c
> > @@ -231,6 +175,10 @@ static int __init integrity_lsm_init(void)
> >  	return 0;
> >  }
> >  
> > +struct lsm_blob_sizes integrity_blob_sizes __ro_after_init = {
> > +	.lbs_inode = sizeof(struct integrity_iint_cache *),
> > +};
> 
> I'll admit that I'm likely missing an important detail, but is there
> a reason why you couldn't stash the integrity_iint_cache struct
> directly in the inode's security blob instead of the pointer?  For
> example:
> 
>   struct lsm_blob_sizes ... = {
>     .lbs_inode = sizeof(struct integrity_iint_cache),
>   };
> 
>   struct integrity_iint_cache *integrity_inode_get(inode)
>   {
>     if (unlikely(!inode->isecurity))
>       return NULL;
>     return inode->i_security + integrity_blob_sizes.lbs_inode;
>   }

It would increase memory occupation. Sometimes the IMA policy
encompasses a small subset of the inodes. Allocating the full
integrity_iint_cache would be a waste of memory, I guess?

On the other hand... (did not think fully about that) if we embed the
full structure in the security blob, we already have a mutex available
to use, and we don't need to take the inode lock (?).

I'm fully convinced that we can improve the implementation
significantly. I just was really hoping to go step by step and not
accumulating improvements as dependency for moving IMA and EVM to the
LSM infrastructure.

Thanks

Roberto