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

[Roberto Sassu]

On Wed, 2023-12-06 at 11:11 -0500, Mimi Zohar wrote:
> On Wed, 2023-12-06 at 14:10 +0100, Roberto Sassu wrote:
> > On Mon, 2023-12-04 at 14:26 +0100, Roberto Sassu wrote:
> 
> ...
> > > If the result of this patch set should be that IMA and EVM become
> > > proper LSMs without the shared integrity layer, instead of collapsing
> > > all changes in this patch set, I think we should first verify if IMA
> > > and EVM can be really independent. Once we guarantee that, we can
> > > proceed making the proper LSMs.
> > > 
> > > These are the changes I have in mind:
> > > 
> > > 1) Fix evm_verifyxattr(), and make it work without integrity_iint_cache
> > > 2) Remove the integrity_iint_cache parameter from evm_verifyxattr(),
> > >    since the other callers are not going to use it
> > 
> > Ehm, I checked better.
> > 
> > integrity_inode_get() is public too (although it is not exported). So,
> > a caller (not IMA) could do:
> > 
> > iint = integrity_inode_get(inode);
> > status = evm_verifyxattr(..., iint);
> > 
> > However, it should not call integrity_inode_free(), which is also in
> > include/linux/integrity.h, since this is going to be called by
> > security_inode_free() (currently).

Oh, I needed to add a check for the iint here:


void integrity_inode_free(struct inode *inode)
{
	struct integrity_iint_cache *iint;

	if (!IS_IMA(inode))
		return;

	iint = integrity_iint_find(inode);
	if (!iint)                          <=== this
		return;

	integrity_inode_set_iint(inode, NULL);

	iint_free(iint);
}

> Calling integrity_inode_free() directly would release the iint early.  
> As a result, IMA would then need to re-allocate it on next access. 
> Other than impacting IMA's performance, is this a problem?

Uhm, I think the iint could be freed while IMA is using it, for example
in process_measurement().

Roberto

> > > 3) Create an internal function with the original parameters to be used
> > >    by IMA
> > > 4) Introduce evm_post_path_mknod(), which similarly to
> > >    ima_post_path_mknod(), sets IMA_NEW_FILE for new files
> > 
> > I just realized that also this is changing the current behavior.
> > 
> > IMA would clear IMA_NEW_FILE in ima_check_last_writer(), while EVM
> > wouldn't (unless we implement the file_release hook in EVM too).
> 
> True
> 
> Mimi
> 
> > > 5) Add hardcoded call to evm_post_path_mknod() after
> > >    ima_post_path_mknod() in security.c
> > > 
> > > If we think that this is good enough, we proceed with the move of IMA
> > > and EVM functions to the LSM infrastructure (patches v7 19-21).
> > > 
> > > The next patches are going to be similar to patches v6 22-23, but
> > > unlike those, their goal would be simply to split metadata, not to make
> > > IMA and EVM independent, which at this point has been addressed
> > > separately in the prerequisite patches.
> > > 
> > > The final patch is to remove the 'integrity' LSM and the integrity
> > > metadata management code, which now is not used anymore.
> > > 
> > > Would that work?
> > 
> > We are not making much progress, I'm going to follow any recommendation
> > that would move this forward.
>