RE: [RFC][PATCH 1/3] evm: Move hooks outside LSM infrastructure

[Roberto Sassu]

> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> Sent: Friday, May 8, 2020 7:08 PM
> On Fri, 2020-05-08 at 10:20 +0000, Roberto Sassu wrote:
> > > From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> > > On Thu, 2020-05-07 at 16:47 +0000, Roberto Sassu wrote:
> 
> <snip>
> 
> > > > > the file metadata to the file data. ÂThe IMA and EVM policies really
> > > > > need to be in sync.
> > > >
> > > > It would be nice, but at the moment EVM considers also files that are
> > > > not selected by the IMA policy. An example of why this is a problem is
> > > > the audit service that fails to start when it tries to adjust the
> permissions
> > > > of the log files. Those files don't have security.evm because they are
> > > > not appraised by IMA, but EVM denies the operation.
> > >
> > > No, this is a timing issue as to whether or not the builtin policy or
> > > a custom policy has been loaded. ÂA custom policy could exclude the
> > > log files based on LSM labels, but they are included in the builtin
> > > policy.
> >
> > Yes, I was referring to a custom policy. In this case, EVM will not adapt
> > to the custom policy but still verifies all files. If access control is done
> > exclusively by IMA at the time evm_verifyxattr() is called, we wouldn't
> > need to add security.evm to all files.
> 
> Roberto, EVM is only triggered by IMA, unless you've modified the
> kernel to do otherwise.

EVM would deny xattr/attr operations even if IMA is disabled in the
kernel configuration. For example, evm_setxattr() returns the value
from evm_protect_xattr(). IMA is not involved there.

> I'm not interested in a complicated solution, just one that addresses
> the new EVM immutable and portable signature. ÂIt might require EVM
> HMAC, IMA differentiating between a new file and an existing file, or
> it might require writing the new EVM signature last, after all the
> other xattrs or metadata are updated. ÂPlease nothing that changes
> existing expectations.

Ok. Introducing the new status INTEGRITY_FAIL_IMMUTABLE, as I
mentioned in '[PATCH] ima: Allow imasig requirement to be satisfied by
EVM portable signatures' seems to have an additional benefit. We
could introduce an additional exception in evm_protect_xattr(), other
than INTEGRITY_NOXATTRS, as we know that xattr/attr update won't
cause HMAC update.

However, it won't work unless the IMA policy says that the file should
be appraised when the mknod() system call is executed. Otherwise,
integrity_iint_cache is not created for the file and the IMA_NEW_FILE
flag is not set.

Granting an exception for INTEGRITY_FAIL_IMMUTABLE solves the case
where security.evm is the first xattr set. If a protected xattr is the first to
be added, then we also have to handle the INTEGRITY_NOLABEL error.
It should be fine to add an exception for this error if the HMAC key is not
loaded.

This still does not solve all problems. INTEGRITY_NOLABEL cannot be
ignored if the HMAC key is loaded, which means that all files need to be
protected by EVM to avoid issues like the one I described (auditd).

Roberto

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