netlink: add mask validation

We don't have good validation policy for existing unsigned int attrs
which serve as flags (for new ones we could use NLA_BITFIELD32).
With increased use of policy dumping having the validation be
expressed as part of the policy is important. Add validation
policy in form of a mask of supported/valid bits.

Support u64 in the uAPI to be future-proof, but really for now
the embedded mask member can only hold 32 bits, so anything with
bit 32+ set will always fail validation.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -200,6 +200,7 @@ enum nla_policy_validation {
 	NLA_VALIDATE_RANGE_WARN_TOO_LONG,
 	NLA_VALIDATE_MIN,
 	NLA_VALIDATE_MAX,
+	NLA_VALIDATE_MASK,
 	NLA_VALIDATE_RANGE_PTR,
 	NLA_VALIDATE_FUNCTION,
 };
@@ -317,6 +318,7 @@ struct nla_policy {
 	u16		len;
 	union {
 		const u32 bitfield32_valid;
+		const u32 mask;
 		const char *reject_message;
 		const struct nla_policy *nested_policy;
 		struct netlink_range_validation *range;
@@ -368,6 +370,8 @@ struct nla_policy {
 	(tp == NLA_S8 || tp == NLA_S16 || tp == NLA_S32 || tp == NLA_S64)
 
 #define __NLA_ENSURE(condition) BUILD_BUG_ON_ZERO(!(condition))
+#define NLA_ENSURE_UINT_TYPE(tp)			\
+	(__NLA_ENSURE(__NLA_IS_UINT_TYPE(tp)) + tp)
 #define NLA_ENSURE_UINT_OR_BINARY_TYPE(tp)		\
 	(__NLA_ENSURE(__NLA_IS_UINT_TYPE(tp) ||	\
 		      tp == NLA_MSECS ||		\
@@ -416,6 +420,12 @@ struct nla_policy {
 	.max = _max,					\
 }
 
+#define NLA_POLICY_MASK(tp, _mask) {			\
+	.type = NLA_ENSURE_UINT_TYPE(tp),		\
+	.validation_type = NLA_VALIDATE_MASK,		\
+	.mask = _mask,					\
+}
+
 #define NLA_POLICY_VALIDATE_FN(tp, fn, ...) {		\
 	.type = NLA_ENSURE_NO_VALIDATION_PTR(tp),	\
 	.validation_type = NLA_VALIDATE_FUNCTION,	\

include/uapi/linux/netlink.h

@@ -331,6 +331,7 @@ enum netlink_attribute_type {
  *	the index, if limited inside the nesting (U32)
  * @NL_POLICY_TYPE_ATTR_BITFIELD32_MASK: valid mask for the
  *	bitfield32 type (U32)
+ * @NL_POLICY_TYPE_ATTR_MASK: mask of valid bits for unsigned integers (U64)
  * @NL_POLICY_TYPE_ATTR_PAD: pad attribute for 64-bit alignment
  */
 enum netlink_policy_type_attr {
@@ -346,6 +347,7 @@ enum netlink_policy_type_attr {
 	NL_POLICY_TYPE_ATTR_POLICY_MAXTYPE,
 	NL_POLICY_TYPE_ATTR_BITFIELD32_MASK,
 	NL_POLICY_TYPE_ATTR_PAD,
+	NL_POLICY_TYPE_ATTR_MASK,
 
 	/* keep last */
 	__NL_POLICY_TYPE_ATTR_MAX,

lib/nlattr.c

@@ -323,6 +323,37 @@ static int nla_validate_int_range(const struct nla_policy *pt,
 	}
 }
 
+static int nla_validate_mask(const struct nla_policy *pt,
+			     const struct nlattr *nla,
+			     struct netlink_ext_ack *extack)
+{
+	u64 value;
+
+	switch (pt->type) {
+	case NLA_U8:
+		value = nla_get_u8(nla);
+		break;
+	case NLA_U16:
+		value = nla_get_u16(nla);
+		break;
+	case NLA_U32:
+		value = nla_get_u32(nla);
+		break;
+	case NLA_U64:
+		value = nla_get_u64(nla);
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	if (value & ~(u64)pt->mask) {
+		NL_SET_ERR_MSG_ATTR(extack, nla, "reserved bit set");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 static int validate_nla(const struct nlattr *nla, int maxtype,
 			const struct nla_policy *policy, unsigned int validate,
 			struct netlink_ext_ack *extack, unsigned int depth)
@@ -503,6 +534,11 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 		if (err)
 			return err;
 		break;
+	case NLA_VALIDATE_MASK:
+		err = nla_validate_mask(pt, nla, extack);
+		if (err)
+			return err;
+		break;
 	case NLA_VALIDATE_FUNCTION:
 		if (pt->validate) {
 			err = pt->validate(nla, extack);

net/netlink/policy.c

@@ -263,6 +263,14 @@ int netlink_policy_dump_write(struct sk_buff *skb,
 		else
 			type = NL_ATTR_TYPE_U64;
 
+		if (pt->validation_type == NLA_VALIDATE_MASK) {
+			if (nla_put_u64_64bit(skb, NL_POLICY_TYPE_ATTR_MASK,
+					      pt->mask,
+					      NL_POLICY_TYPE_ATTR_PAD))
+				goto nla_put_failure;
+			break;
+		}
+
 		nla_get_range_unsigned(pt, &range);
 
 		if (nla_put_u64_64bit(skb, NL_POLICY_TYPE_ATTR_MIN_VALUE_U,