On Tue, 2013-02-12 at 13:52 -0500, Vivek Goyal wrote:
> On Tue, Feb 12, 2013 at 12:14:07PM -0500, Mimi Zohar wrote:
>
> [..]
> > > > > --- a/security/integrity/ima/ima_appraise.c
> > > > > +++ b/security/integrity/ima/ima_appraise.c
> > > > > @@ -124,19 +124,26 @@ int ima_appraise_measurement(int func, struct
> > > > > integrity_iint_cache *iint,
> > > > > enum integrity_status status = INTEGRITY_UNKNOWN;
> > > > > const char *op = "appraise_data";
> > > > > char *cause = "unknown";
> > > > > - int rc;
> > > > > + int rc, audit_info = 0;
> > > > >
> > > > > if (!ima_appraise)
> > > > > return 0;
> > > > > - if (!inode->i_op->getxattr)
> > > > > + if (!inode->i_op->getxattr) {
> > > > > + /* getxattr not supported. file couldn't have been
> > > > > signed */
> > > > > + if (iint->flags & IMA_DIGSIG_OPTIONAL)
> > > > > + return INTEGRITY_PASS;
> > > > > return INTEGRITY_UNKNOWN;
> > > > > + }
> > > > >
> > > >
> > > > Please don't change the result of the appraisal like this. A single
> > > > change can be made towards the bottom of process_measurement().
> > >
> > > I don't want to pass integrity in all cases of INTEGRITY_UNKNOWN. So
> > > I can probably maintain a bool variable, say pass_appraisal, and set
> > > that here and at the end of function, parse that variable and change
> > > the status accordingly.
> >
> > process_measurement() is the only caller of ima_appraise_measurement().
> > Leave the results of ima_appraise_measurement() alone. There's already
> > code at the end of process_measurement() which decides what to return.
> > Just modify it based on the appraisal results.
>
> Ok, I can do that. There is a small concern though. That is what to do
> when rc = INTEGRITY_UKNOWN and IMA_DIGSIG_OPTIONAL flag is set.
>
> ima_appraise_measurement() returns INTEGRITY_UKNOWN when file system
> does not support xattrs or if security xattr is not enabled. In this
> case it is desirable to allow access if IMA_DIGSIG_OPTIONAL flag is
> set.
Right, 'INTEGRITY_UNKNOWN' means that we can't reason, for whatever
reason, about the integrity of the file.
> But INTEGRITY_UNKNOWN is also also returned when integrity_digsig_verify()
> fails and returns -EOPNOTSUPP.
In this case, it is Kconfig based.
> I feel that in this case it is not very appropriate to pass appraisal and
> let executable run. If digital signatures are present but we can't verify
> those (Say some algorithm is not supported in kernel). In that case I
> think it makes sense to fail the signature.
>
> rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
> xattr_value->digest, rc - 1,
> iint->ima_xattr.digest,
> IMA_DIGEST_SIZE);
> if (rc == -EOPNOTSUPP) {
> status = INTEGRITY_UNKNOWN;
>
>
> So how to handle this case.
>
> I am wondering why do we reutrn INTEGRITY_UNKNOWN above and not
> INTEGRITY_FAIL.
We still can't reason about the integrity of the file. For all we know,
it could be a validly signed file, just verification wasn't enabled.
> Will it make sense to fail signature in case of -EOPNOTSUPP.
> rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
> xattr_value->digest, rc - 1,
> iint->ima_xattr.digest,
> IMA_DIGEST_SIZE);
> if (rc)
> status = INTEGRITY_FAIL;
> else
> status = INTEGRITY_PASS;
>
>
Please don't.
Mimi
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