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

[Roberto Sassu <roberto.sassu@huawei.com>]

> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> 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@huawei.com>
> > Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
> 
> 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?

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> Mimi
> 
> > +	}
> > +
> > +	if (xattr_data->type != EVM_XATTR_PORTABLE_DIGSIG) {
> > +		kfree(xattr_data);
> > +		return 0;
> > +	}
> > +
> > +	rc = ima_write_template_field_data((char *)xattr_data, rc,
> DATA_FMT_HEX,
> > +					   field_data);
> > +	kfree(xattr_data);
> > +	return rc;
> > +}
>