Only use the NextHeader value of the first IPv6 fragment extension header.

As per RFC 8200 Section 4.5:
  The Next Header field of the last header of the Per-Fragment
  headers is obtained from the Next Header field of the first
  fragment's Fragment header.

Test:
  - pkg/tcpip/network/ipv6:ipv6_test
  - pkg/tcpip/network/ipv4:ipv4_test
  - pkg/tcpip/network/fragmentation:fragmentation_test

Updates #2197

PiperOrigin-RevId: 327671635

1 files changed, 17 insertions, 6 deletions

diff --git a/pkg/tcpip/network/fragmentation/reassembler.go b/pkg/tcpip/network/fragmentation/reassembler.go
index 50d30bbf0..f044867dc 100644
--- a/pkg/tcpip/network/fragmentation/reassembler.go
+++ b/pkg/tcpip/network/fragmentation/reassembler.go
@@ -34,6 +34,7 @@ type reassembler struct {
 	reassemblerEntry
 	id           FragmentID
 	size         int
+	proto        uint8
 	mu           sync.Mutex
 	holes        []hole
 	deleted      int
@@ -46,7 +47,6 @@ func newReassembler(id FragmentID) *reassembler {
 	r := &reassembler{
 		id:           id,
 		holes:        make([]hole, 0, 16),
-		deleted:      0,
 		heap:         make(fragHeap, 0, 8),
 		creationTime: time.Now(),
 	}
@@ -78,7 +78,7 @@ func (r *reassembler) updateHoles(first, last uint16, more bool) bool {
 	return used
 }
 
-func (r *reassembler) process(first, last uint16, more bool, vv buffer.VectorisedView) (buffer.VectorisedView, bool, int, error) {
+func (r *reassembler) process(first, last uint16, more bool, proto uint8, vv buffer.VectorisedView) (buffer.VectorisedView, uint8, bool, int, error) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	consumed := 0
@@ -86,7 +86,18 @@ func (r *reassembler) process(first, last uint16, more bool, vv buffer.Vectorise
 		// A concurrent goroutine might have already reassembled
 		// the packet and emptied the heap while this goroutine
 		// was waiting on the mutex. We don't have to do anything in this case.
-		return buffer.VectorisedView{}, false, consumed, nil
+		return buffer.VectorisedView{}, 0, false, consumed, nil
+	}
+	// For IPv6, it is possible to have different Protocol values between
+	// fragments of a packet (because, unlike IPv4, the Protocol is not used to
+	// identify a fragment). In this case, only the Protocol of the first
+	// fragment must be used as per RFC 8200 Section 4.5.
+	//
+	// TODO(gvisor.dev/issue/3648): The entire first IP header should be recorded
+	// here (instead of just the protocol) because most IP options should be
+	// derived from the first fragment.
+	if first == 0 {
+		r.proto = proto
 	}
 	if r.updateHoles(first, last, more) {
 		// We store the incoming packet only if it filled some holes.
@@ -96,13 +107,13 @@ func (r *reassembler) process(first, last uint16, more bool, vv buffer.Vectorise
 	}
 	// Check if all the holes have been deleted and we are ready to reassamble.
 	if r.deleted < len(r.holes) {
-		return buffer.VectorisedView{}, false, consumed, nil
+		return buffer.VectorisedView{}, 0, false, consumed, nil
 	}
 	res, err := r.heap.reassemble()
 	if err != nil {
-		return buffer.VectorisedView{}, false, consumed, fmt.Errorf("fragment reassembly failed: %v", err)
+		return buffer.VectorisedView{}, 0, false, consumed, fmt.Errorf("fragment reassembly failed: %w", err)
 	}
-	return res, true, consumed, nil
+	return res, r.proto, true, consumed, nil
 }
 
 func (r *reassembler) tooOld(timeout time.Duration) bool {