ima: prevent a file already mmap'ed write to be mmap'ed execute

The kernel calls deny_write_access() to prevent a file already opened
for write from being executed and also prevents files being executed
from being opened for write.  For some reason this does not extend to
files being mmap'ed execute.

From an IMA perspective, measuring/appraising the integrity of a file
being mmap'ed shared execute, without first making sure the file cannot
be modified, makes no sense.  This patch prevents files, in policy,
already mmap'ed shared write, from being mmap'ed execute.
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_main.c

@@ -72,6 +72,27 @@ static int __init hash_setup(char *str)
 }
 __setup("ima_hash=", hash_setup);
 
+/* Prevent mmap'ing a file execute that is already mmap'ed write */
+static int mmap_violation_check(enum ima_hooks func, struct file *file,
+				char **pathbuf, const char **pathname,
+				char *filename)
+{
+	struct inode *inode;
+	int rc = 0;
+
+	if ((func == MMAP_CHECK) && mapping_writably_mapped(file->f_mapping)) {
+		rc = -ETXTBSY;
+		inode = file_inode(file);
+
+		if (!*pathbuf)	/* ima_rdwr_violation possibly pre-fetched */
+			*pathname = ima_d_path(&file->f_path, pathbuf,
+					       filename);
+		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, *pathname,
+				    "mmap_file", "mmapped_writers", rc, 0);
+	}
+	return rc;
+}
+
 /*
  * ima_rdwr_violation_check
  *
@@ -270,8 +291,12 @@ static int process_measurement(struct file *file, const struct cred *cred,
 
 	/* Nothing to do, just return existing appraised status */
 	if (!action) {
-		if (must_appraise)
-			rc = ima_get_cache_status(iint, func);
+		if (must_appraise) {
+			rc = mmap_violation_check(func, file, &pathbuf,
+						  &pathname, filename);
+			if (!rc)
+				rc = ima_get_cache_status(iint, func);
+		}
 		goto out_locked;
 	}
 
@@ -298,6 +323,9 @@ static int process_measurement(struct file *file, const struct cred *cred,
 		rc = ima_appraise_measurement(func, iint, file, pathname,
 					      xattr_value, xattr_len);
 		inode_unlock(inode);
+		if (!rc)
+			rc = mmap_violation_check(func, file, &pathbuf,
+						  &pathname, filename);
 	}
 	if (action & IMA_AUDIT)
 		ima_audit_measurement(iint, pathname);