netlink: Relax attr validation for fixed length types

Commit 28033ae4e0f5 ("net: netlink: Update attr validation to require
exact length for some types") requires attributes using types NLA_U* and
NLA_S* to have an exact length. This change is exposing bugs in various
userspace commands that are sending attributes with an invalid length
(e.g., attribute has type NLA_U8 and userspace sends NLA_U32). While
the commands are clearly broken and need to be fixed, users are arguing
that the sudden change in enforcement is breaking older commands on
newer kernels for use cases that otherwise "worked".

Relax the validation to print a warning mesage similar to what is done
for messages containing extra bytes after parsing.

Fixes: 28033ae4e0f5 ("net: netlink: Update attr validation to require exact length for some types")
Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/nlattr.c

@@ -15,7 +15,11 @@
 #include <linux/types.h>
 #include <net/netlink.h>
 
-/* for these data types attribute length must be exactly given size */
+/* For these data types, attribute length should be exactly the given
+ * size. However, to maintain compatibility with broken commands, if the
+ * attribute length does not match the expected size a warning is emitted
+ * to the user that the command is sending invalid data and needs to be fixed.
+ */
 static const u8 nla_attr_len[NLA_TYPE_MAX+1] = {
 	[NLA_U8]	= sizeof(u8),
 	[NLA_U16]	= sizeof(u16),
@@ -28,8 +32,16 @@ static const u8 nla_attr_len[NLA_TYPE_MAX+1] = {
 };
 
 static const u8 nla_attr_minlen[NLA_TYPE_MAX+1] = {
+	[NLA_U8]	= sizeof(u8),
+	[NLA_U16]	= sizeof(u16),
+	[NLA_U32]	= sizeof(u32),
+	[NLA_U64]	= sizeof(u64),
 	[NLA_MSECS]	= sizeof(u64),
 	[NLA_NESTED]	= NLA_HDRLEN,
+	[NLA_S8]	= sizeof(s8),
+	[NLA_S16]	= sizeof(s16),
+	[NLA_S32]	= sizeof(s32),
+	[NLA_S64]	= sizeof(s64),
 };
 
 static int validate_nla_bitfield32(const struct nlattr *nla,
@@ -69,11 +81,9 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 
 	BUG_ON(pt->type > NLA_TYPE_MAX);
 
-	/* for data types NLA_U* and NLA_S* require exact length */
-	if (nla_attr_len[pt->type]) {
-		if (attrlen != nla_attr_len[pt->type])
-			return -ERANGE;
-		return 0;
+	if (nla_attr_len[pt->type] && attrlen != nla_attr_len[pt->type]) {
+		pr_warn_ratelimited("netlink: '%s': attribute type %d has an invalid length.\n",
+				    current->comm, type);
 	}
 
 	switch (pt->type) {