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

[Roberto Sassu]

> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> Sent: Monday, May 11, 2020 11:37 PM
> On Mon, 2020-05-11 at 14:13 +0000, Roberto Sassu wrote:
> > > 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.
> 
> CommitÂae1ba1676b88 ("EVM: Allow userland to permit modification of
> EVM-protected metadata") introducedÂEVM_ALLOW_METADATA_WRITES
> to allow
> writing the EVM portable and immutable file signatures.

According to Documentation/ABI/testing/evm:

Note that once a key has been loaded, it will no longer be
possible to enable metadata modification.

We load the EVM key from /etc/keys/x509_evm.der, during kernel
initialization, which excludes the possibility of using that flag. Deferring
the loading of the key is not possible if that key is used for appraisal and
enforcement is enabled from the beginning.

Also, once the flag is cleared portable signatures cannot be installed
until reboot. This seems a big limitation especially when software
updates are installed on a running system.

> > > 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:q
> 
> > > 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.
> 
> Refer to Documentation/ABI/testing/evm describes on how to permit
> writing the security.evm signatures.

It doesn't seem there is a solution for adding portable signatures at
run-time.

> > 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).
> 
> The application still needs to defer writing the EVM portable and
> immutable file signatures until after all the other xattrs are written
> otherwise it won't validate.

EVM won't allow that because at the second setxattr() it will find that
security.evm is missing.

Roberto

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