Read iptables via sockopts.

PiperOrigin-RevId: 264180125

3 files changed, 250 insertions, 9 deletions

diff --git a/pkg/sentry/socket/epsocket/stack.go b/pkg/sentry/socket/epsocket/stack.go
index 8f1572bf4..1b11f4b2d 100644
--- a/pkg/sentry/socket/epsocket/stack.go
+++ b/pkg/sentry/socket/epsocket/stack.go
@@ -198,8 +198,8 @@ func (s *Stack) IPTables() (iptables.IPTables, error) {
 
 // FillDefaultIPTables sets the stack's iptables to the default tables, which
 // allow and do not modify all traffic.
-func (s *Stack) FillDefaultIPTables() error {
-	return netfilter.FillDefaultIPTables(s.Stack)
+func (s *Stack) FillDefaultIPTables() {
+	netfilter.FillDefaultIPTables(s.Stack)
 }
 
 // Resume implements inet.Stack.Resume.
diff --git a/pkg/sentry/socket/netfilter/BUILD b/pkg/sentry/socket/netfilter/BUILD
index 3021f83e7..354a0d6ee 100644
--- a/pkg/sentry/socket/netfilter/BUILD
+++ b/pkg/sentry/socket/netfilter/BUILD
@@ -13,6 +13,7 @@ go_library(
     visibility = ["//pkg/sentry:internal"],
     deps = [
         "//pkg/abi/linux",
+        "//pkg/binary",
         "//pkg/sentry/kernel",
         "//pkg/sentry/usermem",
         "//pkg/syserr",
diff --git a/pkg/sentry/socket/netfilter/netfilter.go b/pkg/sentry/socket/netfilter/netfilter.go
index efdb42903..9f87c32f1 100644
--- a/pkg/sentry/socket/netfilter/netfilter.go
+++ b/pkg/sentry/socket/netfilter/netfilter.go
@@ -17,7 +17,10 @@
 package netfilter
 
 import (
+	"fmt"
+
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/binary"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/usermem"
 	"gvisor.dev/gvisor/pkg/syserr"
@@ -26,21 +29,258 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
+// errorTargetName is used to mark targets as error targets. Error targets
+// shouldn't be reached - an error has occurred if we fall through to one.
+const errorTargetName = "ERROR"
+
+// metadata is opaque to netstack. It holds data that we need to translate
+// between Linux's and netstack's iptables representations.
+type metadata struct {
+	HookEntry  [linux.NF_INET_NUMHOOKS]uint32
+	Underflow  [linux.NF_INET_NUMHOOKS]uint32
+	NumEntries uint32
+	Size       uint32
+}
+
 // GetInfo returns information about iptables.
 func GetInfo(t *kernel.Task, ep tcpip.Endpoint, outPtr usermem.Addr) (linux.IPTGetinfo, *syserr.Error) {
-	// TODO(b/129292233): Implement.
-	return linux.IPTGetinfo{}, syserr.ErrInvalidArgument
+	// Read in the struct and table name.
+	var info linux.IPTGetinfo
+	if _, err := t.CopyIn(outPtr, &info); err != nil {
+		return linux.IPTGetinfo{}, syserr.FromError(err)
+	}
+
+	// Find the appropriate table.
+	table, err := findTable(ep, info.TableName())
+	if err != nil {
+		return linux.IPTGetinfo{}, err
+	}
+
+	// Get the hooks that apply to this table.
+	info.ValidHooks = table.ValidHooks()
+
+	// Grab the metadata struct, which is used to store information (e.g.
+	// the number of entries) that applies to the user's encoding of
+	// iptables, but not netstack's.
+	metadata := table.Metadata().(metadata)
+
+	// Set values from metadata.
+	info.HookEntry = metadata.HookEntry
+	info.Underflow = metadata.Underflow
+	info.NumEntries = metadata.NumEntries
+	info.Size = metadata.Size
+
+	return info, nil
 }
 
 // GetEntries returns netstack's iptables rules encoded for the iptables tool.
 func GetEntries(t *kernel.Task, ep tcpip.Endpoint, outPtr usermem.Addr, outLen int) (linux.KernelIPTGetEntries, *syserr.Error) {
-	// TODO(b/129292233): Implement.
-	return linux.KernelIPTGetEntries{}, syserr.ErrInvalidArgument
+	// Read in the struct and table name.
+	var userEntries linux.IPTGetEntries
+	if _, err := t.CopyIn(outPtr, &userEntries); err != nil {
+		return linux.KernelIPTGetEntries{}, syserr.FromError(err)
+	}
+
+	// Find the appropriate table.
+	table, err := findTable(ep, userEntries.TableName())
+	if err != nil {
+		return linux.KernelIPTGetEntries{}, err
+	}
+
+	// Convert netstack's iptables rules to something that the iptables
+	// tool can understand.
+	entries, _, err := convertNetstackToBinary(userEntries.TableName(), table)
+	if err != nil {
+		return linux.KernelIPTGetEntries{}, err
+	}
+	if binary.Size(entries) > uintptr(outLen) {
+		return linux.KernelIPTGetEntries{}, syserr.ErrInvalidArgument
+	}
+
+	return entries, nil
+}
+
+func findTable(ep tcpip.Endpoint, tableName string) (iptables.Table, *syserr.Error) {
+	ipt, err := ep.IPTables()
+	if err != nil {
+		return iptables.Table{}, syserr.FromError(err)
+	}
+	table, ok := ipt.Tables[tableName]
+	if !ok {
+		return iptables.Table{}, syserr.ErrInvalidArgument
+	}
+	return table, nil
 }
 
 // FillDefaultIPTables sets stack's IPTables to the default tables and
 // populates them with metadata.
-func FillDefaultIPTables(stack *stack.Stack) error {
-	stack.SetIPTables(iptables.DefaultTables())
-	return nil
+func FillDefaultIPTables(stack *stack.Stack) {
+	ipt := iptables.DefaultTables()
+
+	// In order to fill in the metadata, we have to translate ipt from its
+	// netstack format to Linux's giant-binary-blob format.
+	for name, table := range ipt.Tables {
+		_, metadata, err := convertNetstackToBinary(name, table)
+		if err != nil {
+			panic(fmt.Errorf("Unable to set default IP tables: %v", err))
+		}
+		table.SetMetadata(metadata)
+		ipt.Tables[name] = table
+	}
+
+	stack.SetIPTables(ipt)
+}
+
+// convertNetstackToBinary converts the iptables as stored in netstack to the
+// format expected by the iptables tool. Linux stores each table as a binary
+// blob that can only be traversed by parsing a bit, reading some offsets,
+// jumping to those offsets, parsing again, etc.
+func convertNetstackToBinary(name string, table iptables.Table) (linux.KernelIPTGetEntries, metadata, *syserr.Error) {
+	// Return values.
+	var entries linux.KernelIPTGetEntries
+	var meta metadata
+
+	// The table name has to fit in the struct.
+	if linux.XT_TABLE_MAXNAMELEN < len(name) {
+		return linux.KernelIPTGetEntries{}, metadata{}, syserr.ErrInvalidArgument
+	}
+	copy(entries.Name[:], name)
+
+	// Deal with the built in chains first (INPUT, OUTPUT, etc.). Each of
+	// these chains ends with an unconditional policy entry.
+	for hook := iptables.Prerouting; hook < iptables.NumHooks; hook++ {
+		chain, ok := table.BuiltinChains[hook]
+		if !ok {
+			// This table doesn't support this hook.
+			continue
+		}
+
+		// Sanity check.
+		if len(chain.Rules) < 1 {
+			return linux.KernelIPTGetEntries{}, metadata{}, syserr.ErrInvalidArgument
+		}
+
+		for ruleIdx, rule := range chain.Rules {
+			// If this is the first rule of a builtin chain, set
+			// the metadata hook entry point.
+			if ruleIdx == 0 {
+				meta.HookEntry[hook] = entries.Size
+			}
+
+			// Each rule corresponds to an entry.
+			entry := linux.KernelIPTEntry{
+				IPTEntry: linux.IPTEntry{
+					NextOffset:   linux.SizeOfIPTEntry,
+					TargetOffset: linux.SizeOfIPTEntry,
+				},
+			}
+
+			for _, matcher := range rule.Matchers {
+				// Serialize the matcher and add it to the
+				// entry.
+				serialized := marshalMatcher(matcher)
+				entry.Elems = append(entry.Elems, serialized...)
+				entry.NextOffset += uint16(len(serialized))
+				entry.TargetOffset += uint16(len(serialized))
+			}
+
+			// Serialize and append the target.
+			serialized := marshalTarget(rule.Target)
+			entry.Elems = append(entry.Elems, serialized...)
+			entry.NextOffset += uint16(len(serialized))
+
+			// The underflow rule is the last rule in the chain,
+			// and is an unconditional rule (i.e. it matches any
+			// packet). This is enforced when saving iptables.
+			if ruleIdx == len(chain.Rules)-1 {
+				meta.Underflow[hook] = entries.Size
+			}
+
+			entries.Size += uint32(entry.NextOffset)
+			entries.Entrytable = append(entries.Entrytable, entry)
+			meta.NumEntries++
+		}
+
+	}
+
+	// TODO(gvisor.dev/issue/170): Deal with the user chains here. Each of
+	// these starts with an error node holding the chain's name and ends
+	// with an unconditional return.
+
+	// Lastly, each table ends with an unconditional error target rule as
+	// its final entry.
+	errorEntry := linux.KernelIPTEntry{
+		IPTEntry: linux.IPTEntry{
+			NextOffset:   linux.SizeOfIPTEntry,
+			TargetOffset: linux.SizeOfIPTEntry,
+		},
+	}
+	var errorTarget linux.XTErrorTarget
+	errorTarget.Target.TargetSize = linux.SizeOfXTErrorTarget
+	copy(errorTarget.ErrorName[:], errorTargetName)
+	copy(errorTarget.Target.Name[:], errorTargetName)
+
+	// Serialize and add it to the list of entries.
+	errorTargetBuf := make([]byte, 0, linux.SizeOfXTErrorTarget)
+	serializedErrorTarget := binary.Marshal(errorTargetBuf, usermem.ByteOrder, errorTarget)
+	errorEntry.Elems = append(errorEntry.Elems, serializedErrorTarget...)
+	errorEntry.NextOffset += uint16(len(serializedErrorTarget))
+
+	entries.Size += uint32(errorEntry.NextOffset)
+	entries.Entrytable = append(entries.Entrytable, errorEntry)
+	meta.NumEntries++
+	meta.Size = entries.Size
+
+	return entries, meta, nil
+}
+
+func marshalMatcher(matcher iptables.Matcher) []byte {
+	switch matcher.(type) {
+	default:
+		// TODO(gvisor.dev/issue/170): We don't support any matchers yet, so
+		// any call to marshalMatcher will panic.
+		panic(fmt.Errorf("unknown matcher of type %T", matcher))
+	}
+}
+
+func marshalTarget(target iptables.Target) []byte {
+	switch target.(type) {
+	case iptables.UnconditionalAcceptTarget:
+		return marshalUnconditionalAcceptTarget()
+	default:
+		panic(fmt.Errorf("unknown target of type %T", target))
+	}
+}
+
+func marshalUnconditionalAcceptTarget() []byte {
+	// The target's name will be the empty string.
+	target := linux.XTStandardTarget{
+		Target: linux.XTEntryTarget{
+			TargetSize: linux.SizeOfXTStandardTarget,
+		},
+		Verdict: translateStandardVerdict(iptables.Accept),
+	}
+
+	ret := make([]byte, 0, linux.SizeOfXTStandardTarget)
+	return binary.Marshal(ret, usermem.ByteOrder, target)
+}
+
+// translateStandardVerdict translates verdicts the same way as the iptables
+// tool.
+func translateStandardVerdict(verdict iptables.Verdict) int32 {
+	switch verdict {
+	case iptables.Accept:
+		return -linux.NF_ACCEPT - 1
+	case iptables.Drop:
+		return -linux.NF_DROP - 1
+	case iptables.Queue:
+		return -linux.NF_QUEUE - 1
+	case iptables.Return:
+		return linux.NF_RETURN
+	case iptables.Jump:
+		// TODO(gvisor.dev/issue/170): Support Jump.
+		panic("Jump isn't supported yet")
+	default:
+		panic(fmt.Sprintf("unknown standard verdict: %d", verdict))
+	}
 }