Re: [PATCH v2 11/12] ima: Introduce template field evmsig and write to field sig as fallback

[Mimi Zohar]

On Thu, 2020-09-17 at 15:05 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> > Sent: Thursday, September 17, 2020 4:25 PM
> > Hi Roberto,
> > 
> > On Fri, 2020-09-04 at 11:26 +0200, Roberto Sassu wrote:
> > > With the patch to accept EVM portable signatures when the
> > > appraise_type=imasig requirement is specified in the policy, appraisal can
> > > be successfully done even if the file does not have an IMA signature.
> > >
> > > However, remote attestation would not see that a different signature
> > type
> > > was used, as only IMA signatures can be included in the measurement list.
> > > This patch solves the issue by introducing the new template field 'evmsig'
> > > to show EVM portable signatures and by including its value in the existing
> > > field 'sig' if the IMA signature is not found.
> > >
> > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > Suggested-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> > 
> > Thank you!   Just a minor comment below.
> > 
> > <snip>
> > 
> > > diff --git a/security/integrity/ima/ima_template_lib.c
> > b/security/integrity/ima/ima_template_lib.c
> > > index c022ee9e2a4e..2c596c2a89cc 100644
> > > --- a/security/integrity/ima/ima_template_lib.c
> > > +++ b/security/integrity/ima/ima_template_lib.c
> > >
> > > @@ -438,7 +439,7 @@ int ima_eventsig_init(struct ima_event_data
> > *event_data,
> > >  	struct evm_ima_xattr_data *xattr_value = event_data->xattr_value;
> > >
> > >  	if ((!xattr_value) || (xattr_value->type !=
> > EVM_IMA_XATTR_DIGSIG))
> > > -		return 0;
> > > +		return ima_eventevmsig_init(event_data, field_data);
> > >
> > >  	return ima_write_template_field_data(xattr_value, event_data-
> > >xattr_len,
> > >  					     DATA_FMT_HEX, field_data);
> > > @@ -484,3 +485,39 @@ int ima_eventmodsig_init(struct ima_event_data
> > *event_data,
> > >  	return ima_write_template_field_data(data, data_len,
> > DATA_FMT_HEX,
> > >  					     field_data);
> > >  }
> > > +
> > > +/*
> > > + *  ima_eventevmsig_init - include the EVM portable signature as part of
> > the
> > > + *  template data
> > > + */
> > > +int ima_eventevmsig_init(struct ima_event_data *event_data,
> > > +			 struct ima_field_data *field_data)
> > > +{
> > > +	struct evm_ima_xattr_data *xattr_data = NULL;
> > > +	int rc = 0;
> > > +
> > > +	if (!event_data->file)
> > > +		return 0;
> > > +
> > > +	if (!(file_inode(event_data->file)->i_opflags & IOP_XATTR))
> > > +		return 0;
> > > +
> > > +	rc = vfs_getxattr_alloc(file_dentry(event_data->file),
> > XATTR_NAME_EVM,
> > > +				(char **)&xattr_data, 0, GFP_NOFS);
> > > +	if (rc <= 0) {
> > > +		if (!rc || rc == -ENODATA)
> > > +			return 0;
> > > +
> > > +		return rc;
> > 
> > We're including the EVM signature on a best effort basis to help with
> > attestation.  Do we really care why it failed?   Are we going to act on
> > it?
> 
> Hi Mimi
> 
> other template field functions have a similar behavior. They return
> an error if an operation necessary to retrieve the data cannot be
> performed. Should I always return 0?

The EVM signature case is more similar to the IMA signature case, than
to other fields.  In the signature cases, if the signature exists, it
is included.   My suggestion is based on the difference in how the
vfs_getxattr_alloc() results are handled.

thanks,

Mimi