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;
}
--
paul-moore.com
