net/tstun: fix gvisor inbound GSO packet injection (#13283)

buffs[0] was not sized to hold pkt with GSO, resulting in a panic.

Updates tailscale/corp#22511

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/tstun/wrap.go

@@ -1258,8 +1258,8 @@ func (t *Wrapper) SetJailedFilter(filt *filter.Filter) {
 //
 // pkt will be copied into buffs before writing to the underlying tun.Device.
 // Therefore, callers must allocate and pass a buffs slice that is sized
-// appropriately for holding pkt.Size() + PacketStartOffset as either a single
-// element (buffs[0]), or split across multiple elements if the originating
+// appropriately for holding pkt.Size() + PacketStartOffset as a single
+// element (buffs[0]) and split across multiple elements if the originating
 // stack supports GSO. sizes must be sized with similar consideration,
 // len(buffs) should be equal to len(sizes). If any len(buffs[<index>]) was
 // mutated by InjectInboundPacketBuffer it will be reset to cap(buffs[<index>])

wgengine/netstack/netstack.go

@@ -826,14 +826,19 @@ func (ns *Impl) DialContextUDP(ctx context.Context, ipp netip.AddrPort) (*gonet.
 // across subsequent inbound packet injection calls.
 func (ns *Impl) getInjectInboundBuffsSizes() (buffs [][]byte, sizes []int) {
 	batchSize := 1
-	if ns.linkEP.SupportedGSO() == stack.HostGSOSupported {
+	gsoEnabled := ns.linkEP.SupportedGSO() == stack.HostGSOSupported
+	if gsoEnabled {
 		batchSize = conn.IdealBatchSize
 	}
 	buffs = make([][]byte, batchSize)
 	sizes = make([]int, batchSize)
 	for i := 0; i < batchSize; i++ {
+		if i == 0 && gsoEnabled {
+			buffs[i] = make([]byte, tstun.PacketStartOffset+ns.linkEP.GSOMaxSize())
+		} else {
 			buffs[i] = make([]byte, tstun.PacketStartOffset+tstun.DefaultTUNMTU())
 		}
+	}
 	return buffs, sizes
 }