cifs: try to handle the MUST SecurityFlags sanely

The cifs.ko SecurityFlags interface wins my award for worst-designed
interface ever, but we're sort of stuck with it since it's documented
and people do use it (even if it doesn't work correctly).

Case in point -- you can specify multiple sets of "MUST" flags. It makes
absolutely no sense, but you can do it.

What should the effect be in such a case? No one knows or seems to have
considered this so far, so let's define it now. If you try to specify
multiple MUST flags, clear any other MAY or MUST bits except for the
ones that involve signing.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/cifs_debug.c

@@ -597,6 +597,32 @@ static int cifs_security_flags_proc_open(struct inode *inode, struct file *file)
 	return single_open(file, cifs_security_flags_proc_show, NULL);
 }
 
+/*
+ * Ensure that if someone sets a MUST flag, that we disable all other MAY
+ * flags except for the ones corresponding to the given MUST flag. If there are
+ * multiple MUST flags, then try to prefer more secure ones.
+ */
+static void
+cifs_security_flags_handle_must_flags(unsigned int *flags)
+{
+	unsigned int signflags = *flags & CIFSSEC_MUST_SIGN;
+
+	if ((*flags & CIFSSEC_MUST_KRB5) == CIFSSEC_MUST_KRB5)
+		*flags = CIFSSEC_MUST_KRB5;
+	else if ((*flags & CIFSSEC_MUST_NTLMSSP) == CIFSSEC_MUST_NTLMSSP)
+		*flags = CIFSSEC_MUST_NTLMSSP;
+	else if ((*flags & CIFSSEC_MUST_NTLMV2) == CIFSSEC_MUST_NTLMV2)
+		*flags = CIFSSEC_MUST_NTLMV2;
+	else if ((*flags & CIFSSEC_MUST_NTLM) == CIFSSEC_MUST_NTLM)
+		*flags = CIFSSEC_MUST_NTLM;
+	else if ((*flags & CIFSSEC_MUST_LANMAN) == CIFSSEC_MUST_LANMAN)
+		*flags = CIFSSEC_MUST_LANMAN;
+	else if ((*flags & CIFSSEC_MUST_PLNTXT) == CIFSSEC_MUST_PLNTXT)
+		*flags = CIFSSEC_MUST_PLNTXT;
+
+	*flags |= signflags;
+}
+
 static ssize_t cifs_security_flags_proc_write(struct file *file,
 		const char __user *buffer, size_t count, loff_t *ppos)
 {
@@ -650,6 +676,8 @@ static ssize_t cifs_security_flags_proc_write(struct file *file,
 		return -EINVAL;
 	}
 
+	cifs_security_flags_handle_must_flags(&flags);
+
 	/* flags look ok - update the global security flags for cifs module */
 	global_secflags = flags;
 	if (global_secflags & CIFSSEC_MUST_SIGN) {