On Mon, Nov 20, 2023 at 3:16 AM Roberto Sassu
<roberto.sassu@xxxxxxxxxxxxxxx> wrote:
> 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?
Perhaps, but if it allows us to remove another layer of dynamic memory
I would argue that it may be worth the cost. It's also worth
considering the size of integrity_iint_cache, while it isn't small, it
isn't exactly huge either.
> 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 (?).
That would be excellent, getting rid of a layer of locking would be significant.
> 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.
I understand, and I agree that an iterative approach is a good idea, I
just want to make sure we keep things tidy from a user perspective,
i.e. not exposing the "integrity" LSM when it isn't required.
--
paul-moore.com
