driver: fix missed-wakeup race in ring buffer Alertable signalingHEADmaster

Add MemoryBarrier() between store-load pairs in the Dekker-style
synchronization used by the Receive ring's Alertable/Tail protocol.

On x86-64, WriteRelease/ReadAcquire only prevent compiler reordering and
provide acquire/release semantics, but do not emit MFENCE — the only
instruction that prevents store-load reordering across cores. Without a
full barrier, both the userspace producer and the kernel consumer can
simultaneously read stale values:

  Userspace: STORE(Tail)  ...  LOAD(Alertable) -> sees FALSE (stale)
  Driver:    STORE(Alertable=TRUE) ... LOAD(Tail) -> sees old tail

The driver then enters KeWaitForMultipleObjects with no pending
SetEvent, sleeping until a TCP retransmission (typically 4-5s later)
re-triggers the send path and wins the race.

The fix adds MemoryBarrier() (MFENCE on x86) on both sides:
- api/session.c WintunSendPacket: between WriteULongRelease(Tail) and
  ReadAcquire(Alertable)
- driver/twintun.c TunProcessReceiveData: between
  WriteRelease(Alertable, TRUE) and ReadULongAcquire(Tail)

This guarantees that at least one side always observes the other's
store, preventing the missed wakeup while preserving the Alertable
optimization that avoids unnecessary SetEvent syscalls.

Reported-by: Alexey Lapuka <alexey@twingate.com>
Reference: https://lists.zx2c4.com/pipermail/wireguard/2026-February/009523.html
Signed-off-by: Simon Rozman <simon.rozman@amebis.si>

2 files changed, 2 insertions, 0 deletions

diff --git a/api/session.c b/api/session.c
index ab96c64..13d5bca 100644
--- a/api/session.c
+++ b/api/session.c
@@ -302,6 +302,7 @@ WintunSendPacket(TUN_SESSION *Session, const BYTE *Packet)
     if (Session->Descriptor.Receive.Ring->Tail != Session->Receive.TailRelease)
     {
         WriteULongRelease(&Session->Descriptor.Receive.Ring->Tail, Session->Receive.TailRelease);
+        MemoryBarrier();
         if (ReadAcquire(&Session->Descriptor.Receive.Ring->Alertable))
             SetEvent(Session->Descriptor.Receive.TailMoved);
     }
diff --git a/driver/wintun.c b/driver/wintun.c
index 82e346b..72ba5d3 100644
--- a/driver/wintun.c
+++ b/driver/wintun.c
@@ -481,6 +481,7 @@ TunProcessReceiveData(_Inout_ TUN_CTX *Ctx)
             if (RingHead == RingTail)
             {
                 WriteRelease(&Ring->Alertable, TRUE);
+                MemoryBarrier();
                 RingTail = ReadULongAcquire(&Ring->Tail);
                 if (RingHead == RingTail)
                 {