ima: re-evaluate files on privileged mounted filesystems

This patch addresses the fuse privileged mounted filesystems in a "secure"
environment, with a correctly enforced security policy, which is willing
to assume the inherent risk of specific fuse filesystems that are well
defined and properly implemented.

As there is no way for the kernel to detect file changes, the kernel
ignores the cached file integrity results and re-measures, re-appraises,
and re-audits the file.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Dongsu Park <dongsu@kinvolk.io>
Cc: Alban Crequy <alban@kinvolk.io>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

security/integrity/ima/ima_main.c

@@ -25,6 +25,7 @@
 #include <linux/xattr.h>
 #include <linux/ima.h>
 #include <linux/iversion.h>
+#include <linux/fs.h>
 
 #include "ima.h"
 
@@ -230,9 +231,17 @@ static int process_measurement(struct file *file, const struct cred *cred,
 				 IMA_APPRAISE_SUBMASK | IMA_APPRAISED_SUBMASK |
 				 IMA_ACTION_FLAGS);
 
-	if (test_and_clear_bit(IMA_CHANGE_XATTR, &iint->atomic_flags))
-		/* reset all flags if ima_inode_setxattr was called */
+	/*
+	 * Re-evaulate the file if either the xattr has changed or the
+	 * kernel has no way of detecting file change on the filesystem.
+	 * (Limited to privileged mounted filesystems.)
+	 */
+	if (test_and_clear_bit(IMA_CHANGE_XATTR, &iint->atomic_flags) ||
+	    ((inode->i_sb->s_iflags & SB_I_IMA_UNVERIFIABLE_SIGNATURE) &&
+	     !(inode->i_sb->s_iflags & SB_I_UNTRUSTED_MOUNTER))) {
 		iint->flags &= ~IMA_DONE_MASK;
+		iint->measured_pcrs = 0;
+	}
 
 	/* Determine if already appraised/measured based on bitmask
 	 * (IMA_MEASURE, IMA_MEASURED, IMA_XXXX_APPRAISE, IMA_XXXX_APPRAISED,