tun: use correct IP header comparisons in tcpGRO() and tcpPacketsCanCoalesce()

tcpGRO() was using an incorrect IPv4 more fragments bit mask.

tcpPacketsCanCoalesce() was not distinguishing tcp6 from tcp4, and TTL
values were not compared. TTL values should be equal at the IP layer,
otherwise the packets should not coalesce. This tracks with the kernel.

Reviewed-by: Denton Gentry <dgentry@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 25 insertions, 10 deletions

diff --git a/tun/tcp_offload_linux.go b/tun/tcp_offload_linux.go
index 4912efd..39a7180 100644
--- a/tun/tcp_offload_linux.go
+++ b/tun/tcp_offload_linux.go
@@ -189,14 +189,29 @@ func tcpPacketsCanCoalesce(pkt []byte, iphLen, tcphLen uint8, seq uint32, pshSet
 			return coalesceUnavailable
 		}
 	}
-	if pkt[1] != pktTarget[1] {
-		// cannot coalesce with unequal ToS values
-		return coalesceUnavailable
-	}
-	if pkt[6]>>5 != pktTarget[6]>>5 {
-		// cannot coalesce with unequal DF or reserved bits. MF is checked
-		// further up the stack.
-		return coalesceUnavailable
+	if pkt[0]>>4 == 6 {
+		if pkt[0] != pktTarget[0] || pkt[1]>>4 != pktTarget[1]>>4 {
+			// cannot coalesce with unequal Traffic class values
+			return coalesceUnavailable
+		}
+		if pkt[7] != pktTarget[7] {
+			// cannot coalesce with unequal Hop limit values
+			return coalesceUnavailable
+		}
+	} else {
+		if pkt[1] != pktTarget[1] {
+			// cannot coalesce with unequal ToS values
+			return coalesceUnavailable
+		}
+		if pkt[6]>>5 != pktTarget[6]>>5 {
+			// cannot coalesce with unequal DF or reserved bits. MF is checked
+			// further up the stack.
+			return coalesceUnavailable
+		}
+		if pkt[8] != pktTarget[8] {
+			// cannot coalesce with unequal TTL values
+			return coalesceUnavailable
+		}
 	}
 	// seq adjacency
 	lhsLen := item.gsoSize
@@ -366,7 +381,7 @@ func coalesceTCPPackets(mode canCoalesce, pkt []byte, pktBuffsIndex int, gsoSize
 }
 
 const (
-	ipv4FlagMoreFragments = 0x80
+	ipv4FlagMoreFragments uint8 = 0x20
 )
 
 const (
@@ -409,7 +424,7 @@ func tcpGRO(bufs [][]byte, offset int, pktI int, table *tcpGROTable, isV6 bool)
 		return false
 	}
 	if !isV6 {
-		if pkt[6]&ipv4FlagMoreFragments != 0 || (pkt[6]<<3 != 0 || pkt[7] != 0) {
+		if pkt[6]&ipv4FlagMoreFragments != 0 || pkt[6]<<3 != 0 || pkt[7] != 0 {
 			// no GRO support for fragmented segments for now
 			return false
 		}