Subject: Re: [PATCH v10 12/12] ima: Store the measurement again when appraising a modsig

From: Mimi Zohar <[email-masked]>
Date: Tue, 28 May 2019 10:09:20 -0400

Hi Thiago,

On Thu, 2019-04-18 at 00:51 -0300, Thiago Jung Bauermann wrote:
> If the IMA template contains the "modsig" or "d-modsig" field, then the
> modsig should be added to the measurement list when the file is appraised.
> 
> And that is what normally happens, but if a measurement rule caused a file
> containing a modsig to be measured before a different rule causes it to be
> appraised, the resulting measurement entry will not contain the modsig
> because it is only fetched during appraisal. When the appraisal rule
> triggers, it won't store a new measurement containing the modsig because
> the file was already measured.
> 
> We need to detect that situation and store an additional measurement with
> the modsig. This is done by adding an IMA_MEASURE action flag if we read a
> modsig and the IMA template contains a modsig field.

With the new per policy rule "template" support being added, this
patch needs to be modified so that the per policy "template" format is
checked.  ima_template_has_modsig() should be called with the
template_desc being used.

thanks,

Mimi

> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 8e6475854351..f91ed4189f98 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -282,9 +282,17 @@ static int process_measurement(struct file *file, const struct cred *cred,
>  		/* read 'security.ima' */
>  		xattr_len = ima_read_xattr(file_dentry(file), &xattr_value);
>  
> -		/* Read the appended modsig if allowed by the policy. */
> -		if (iint->flags & IMA_MODSIG_ALLOWED)
> -			ima_read_modsig(func, buf, size, &modsig);
> +		/*
> +		 * Read the appended modsig, if allowed by the policy, and allow
> +		 * an additional measurement list entry, if needed, based on the
> +		 * template format.
> +		 */
> +		if (iint->flags & IMA_MODSIG_ALLOWED) {
> +			rc = ima_read_modsig(func, buf, size, &modsig);
> +
> +			if (!rc && ima_template_has_modsig())
> +				action |= IMA_MEASURE;
> +		}
>