wireguard: allowedips: add WGALLOWEDIP_F_REMOVE_ME flag

The current netlink API for WireGuard does not directly support removal
of allowed ips from a peer. A user can remove an allowed ip from a peer
in one of two ways:

1. By using the WGPEER_F_REPLACE_ALLOWEDIPS flag and providing a new
   list of allowed ips which omits the allowed ip that is to be removed.
2. By reassigning an allowed ip to a "dummy" peer then removing that
   peer with WGPEER_F_REMOVE_ME.

With the first approach, the driver completely rebuilds the allowed ip
list for a peer. If my current configuration is such that a peer has
allowed ips 192.168.0.2 and 192.168.0.3 and I want to remove 192.168.0.2
the actual transition looks like this.

[192.168.0.2, 192.168.0.3] <-- Initial state
[]                         <-- Step 1: Allowed ips removed for peer
[192.168.0.3]              <-- Step 2: Allowed ips added back for peer

This is true even if the allowed ip list is small and the update does
not need to be batched into multiple WG_CMD_SET_DEVICE requests, as the
removal and subsequent addition of ips is non-atomic within a single
request. Consequently, wg_allowedips_lookup_dst and
wg_allowedips_lookup_src may return NULL while reconfiguring a peer even
for packets bound for ips a user did not intend to remove leading to
unintended interruptions in connectivity. This presents in userspace as
failed calls to sendto and sendmsg for UDP sockets. In my case, I ran
netperf while repeatedly reconfiguring the allowed ips for a peer with
wg.

/usr/local/bin/netperf -H 10.102.73.72 -l 10m -t UDP_STREAM -- -R 1 -m 1024
send_data: data send error: No route to host (errno 113)
netperf: send_omni: send_data failed: No route to host

While this may not be of particular concern for environments where peers
and allowed ips are mostly static, systems like Cilium manage peers and
allowed ips in a dynamic environment where peers (i.e. Kubernetes nodes)
and allowed ips (i.e. pods running on those nodes) can frequently
change making WGPEER_F_REPLACE_ALLOWEDIPS problematic.

The second approach avoids any possible connectivity interruptions
but is hacky and less direct, requiring the creation of a temporary
peer just to dispose of an allowed ip.

Introduce a new flag called WGALLOWEDIP_F_REMOVE_ME which in the same
way that WGPEER_F_REMOVE_ME allows a user to remove a single peer from
a WireGuard device's configuration allows a user to remove an ip from a
peer's set of allowed ips. This enables incremental updates to a
device's configuration without any connectivity blips or messy
workarounds.

A corresponding patch for wg extends the existing `wg set` interface to
leverage this feature.

$ wg set wg0 peer <PUBKEY> allowed-ips +192.168.88.0/24,-192.168.0.1/32

When '+' or '-' is prepended to any ip in the list, wg clears
WGPEER_F_REPLACE_ALLOWEDIPS and sets the WGALLOWEDIP_F_REMOVE_ME flag on
any ip prefixed with '-'.

Signed-off-by: Jordan Rife <jordan@jrife.io>
[Jason: minor style nits, fixes to selftest, bump of wireguard-tools version]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20250521212707.1767879-5-Jason@zx2c4.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/uapi/linux/wireguard.h b/include/uapi/linux/wireguard.h
index ae88be14c947..8c26391196d5 100644
--- a/include/uapi/linux/wireguard.h
+++ b/include/uapi/linux/wireguard.h
@@ -101,6 +101,10 @@
  *                    WGALLOWEDIP_A_FAMILY: NLA_U16
  *                    WGALLOWEDIP_A_IPADDR: struct in_addr or struct in6_addr
  *                    WGALLOWEDIP_A_CIDR_MASK: NLA_U8
+ *                    WGALLOWEDIP_A_FLAGS: NLA_U32, WGALLOWEDIP_F_REMOVE_ME if
+ *                                         the specified IP should be removed;
+ *                                         otherwise, this IP will be added if
+ *                                         it is not already present.
  *                0: NLA_NESTED
  *                    ...
  *                0: NLA_NESTED
@@ -184,11 +188,16 @@ enum wgpeer_attribute {
 };
 #define WGPEER_A_MAX (__WGPEER_A_LAST - 1)
 
+enum wgallowedip_flag {
+	WGALLOWEDIP_F_REMOVE_ME = 1U << 0,
+	__WGALLOWEDIP_F_ALL = WGALLOWEDIP_F_REMOVE_ME
+};
 enum wgallowedip_attribute {
 	WGALLOWEDIP_A_UNSPEC,
 	WGALLOWEDIP_A_FAMILY,
 	WGALLOWEDIP_A_IPADDR,
 	WGALLOWEDIP_A_CIDR_MASK,
+	WGALLOWEDIP_A_FLAGS,
 	__WGALLOWEDIP_A_LAST
 };
 #define WGALLOWEDIP_A_MAX (__WGALLOWEDIP_A_LAST - 1)