From 1cf89f5339b549236f38ce5fbc40f7bf993d9626 Mon Sep 17 00:00:00 2001
From: Jordan Whited <jordan@tailscale.com>
Date: Wed, 8 Nov 2023 14:06:20 -0800
Subject: [PATCH] tun: fix Device.Read() buf length assumption on Windows

The length of a packet read from the underlying TUN device may exceed
the length of a supplied buffer when MTU exceeds device.MaxMessageSize.

Reviewed-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 tun/tun_windows.go | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/tun/tun_windows.go b/tun/tun_windows.go
index 34f2980..2af8e3e 100644
--- a/tun/tun_windows.go
+++ b/tun/tun_windows.go
@@ -160,11 +160,10 @@ retry:
 		packet, err := tun.session.ReceivePacket()
 		switch err {
 		case nil:
-			packetSize := len(packet)
-			copy(bufs[0][offset:], packet)
-			sizes[0] = packetSize
+			n := copy(bufs[0][offset:], packet)
+			sizes[0] = n
 			tun.session.ReleaseReceivePacket(packet)
-			tun.rate.update(uint64(packetSize))
+			tun.rate.update(uint64(n))
 			return 1, nil
 		case windows.ERROR_NO_MORE_ITEMS:
 			if !shouldSpin || uint64(nanotime()-start) >= spinloopDuration {