Smack: Verify read access on file open - v3

Smack believes that many of the operatons that can
be performed on an open file descriptor are read operations.
The fstat and lseek system calls are examples.
An implication of this is that files shouldn't be open
if the task doesn't have read access even if it has
write access and the file is being opened write only.

Targeted for git://git.gitorious.org/smack-next/kernel.git

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -1462,19 +1462,32 @@ static int smack_file_receive(struct file *file)
 /**
  * smack_file_open - Smack dentry open processing
  * @file: the object
- * @cred: unused
+ * @cred: task credential
  *
  * Set the security blob in the file structure.
+ * Allow the open only if the task has read access. There are
+ * many read operations (e.g. fstat) that you can do with an
+ * fd even if you have the file open write-only.
  *
  * Returns 0
  */
 static int smack_file_open(struct file *file, const struct cred *cred)
 {
+	struct task_smack *tsp = cred->security;
 	struct inode_smack *isp = file_inode(file)->i_security;
+	struct smk_audit_info ad;
+	int rc;
 
-	file->f_security = isp->smk_inode;
+	if (smack_privileged(CAP_MAC_OVERRIDE))
+		return 0;
 
-	return 0;
+	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_PATH);
+	smk_ad_setfield_u_fs_path(&ad, file->f_path);
+	rc = smk_access(tsp->smk_task, isp->smk_inode, MAY_READ, &ad);
+	if (rc == 0)
+		file->f_security = isp->smk_inode;
+
+	return rc;
 }
 
 /*