RE: [PATCH] ima: Calculate digest in ima_inode_hash() if not available

[Roberto Sassu]

> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> Sent: Sunday, February 13, 2022 2:06 PM
> Hi Roberto,
> 
> On Fri, 2022-02-11 at 11:48 +0100, Roberto Sassu wrote:
> > __ima_inode_hash() checks if a digest has been already calculated by
> > looking for the integrity_iint_cache structure associated to the passed
> > inode.
> >
> > Users of ima_file_hash() and ima_inode_hash() (e.g. eBPF) might be
> > interested in obtaining the information without having to setup an IMA
> > policy so that the digest is always available at the time they call one of
> > those functions.
> >
> > Open a new file descriptor in __ima_inode_hash(), so that this function
> > could invoke ima_collect_measurement() to calculate the digest if it is not
> > available. Still return -EOPNOTSUPP if the calculation failed.
> >
> > Instead of opening a new file descriptor, the one from ima_file_hash()
> > could have been used. However, since ima_inode_hash() was created to
> obtain
> > the digest when the file descriptor is not available, it could benefit from
> > this change too. Also, the opened file descriptor might be not suitable for
> > use (file descriptor opened not for reading).
> >
> > This change does not cause memory usage increase, due to using a temporary
> > integrity_iint_cache structure for the digest calculation, and due to
> > freeing the ima_digest_data structure inside integrity_iint_cache before
> > exiting from __ima_inode_hash().
> >
> > Finally, update the test by removing ima_setup.sh (it is not necessary
> > anymore to set an IMA policy) and by directly executing /bin/true.
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> 
> Although this patch doesn't directly modify either ima_file_hash() or
> ima_inode_hash(),  this change affects both functions.  ima_file_hash()
> was introduced to be used with eBPF.  Based on Florent's post, changing
> the ima_file_hash() behavor seems fine.  Since I have no idea whether
> anyone is still using ima_inode_hash(), perhaps it would be safer to
> limit this behavior change to just ima_file_hash().

Hi Mimi

ok.

I found that just checking that iint->ima_hash is not NULL is not enough
(ima_inode_hash() might still return the old digest after a file write).
Should I replace that check with !(iint->flags & IMA_COLLECTED)?
Or should I do only for ima_file_hash() and recalculate the digest
if necessary?

> Please update the ima_file_hash() doc.  While touching this area, I'd
> appreciate your fixing the first doc line in both ima_file_hash() and
> ima_inode_hash() cases, which wraps spanning two lines.

Did you mean to make the description shorter or to have everything
in one line? According to the kernel documentation (kernel-doc.rst),
having the brief description in multiple lines should be fine.

> Please split the IMA from the eBPF changes.

Ok.

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Zhong Ronghua

> --
> thanks,
> 
> Mimi