genetlink: reject use of nlmsg_flags for new commands

Commit 9c5d03d36251 ("genetlink: start to validate reserved header bytes")
introduced extra validation for genetlink headers. We had to gate it
to only apply to new commands, to maintain bug-wards compatibility.
Use this opportunity (before the new checks make it to Linus's tree)
to add more conditions.

Validate that Generic Netlink families do not use nlmsg_flags outside
of the well-understood set.

Link: https://lore.kernel.org/all/20220928073709.1b93b74a@kernel.org/
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20220929142809.1167546-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
index 7c136de..39b7c00 100644 (file)
--- a/net/netlink/genetlink.c
+++ b/net/netlink/genetlink.c
@@ -739,6 +739,36 @@ out:
        return err;
 }
 
+static int genl_header_check(const struct genl_family *family,
+                            struct nlmsghdr *nlh, struct genlmsghdr *hdr,
+                            struct netlink_ext_ack *extack)
+{
+       u16 flags;
+
+       /* Only for commands added after we started validating */
+       if (hdr->cmd < family->resv_start_op)
+               return 0;
+
+       if (hdr->reserved) {
+               NL_SET_ERR_MSG(extack, "genlmsghdr.reserved field is not 0");
+               return -EINVAL;
+       }
+
+       /* Old netlink flags have pretty loose semantics, allow only the flags
+        * consumed by the core where we can enforce the meaning.
+        */
+       flags = nlh->nlmsg_flags;
+       if ((flags & NLM_F_DUMP) == NLM_F_DUMP) /* DUMP is 2 bits */
+               flags &= ~NLM_F_DUMP;
+       if (flags & ~(NLM_F_REQUEST | NLM_F_ACK | NLM_F_ECHO)) {
+               NL_SET_ERR_MSG(extack,
+                              "ambiguous or reserved bits set in nlmsg_flags");
+               return -EINVAL;
+       }
+
+       return 0;
+}
+
 static int genl_family_rcv_msg(const struct genl_family *family,
                               struct sk_buff *skb,
                               struct nlmsghdr *nlh,
@@ -757,7 +787,7 @@ static int genl_family_rcv_msg(const struct genl_family *family,
        if (nlh->nlmsg_len < nlmsg_msg_size(hdrlen))
                return -EINVAL;
 
-       if (hdr->cmd >= family->resv_start_op && hdr->reserved)
+       if (genl_header_check(family, nlh, hdr, extack))
                return -EINVAL;
 
        if (genl_get_cmd(hdr->cmd, family, &op))