Don't read forwarding from netstack in sentry

https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt:
  /proc/sys/net/ipv4/* Variables:

  ip_forward - BOOLEAN
    0 - disabled (default)
    not 0 - enabled

    Forward Packets between interfaces.

    This variable is special, its change resets all configuration
    parameters to their default state (RFC1122 for hosts, RFC1812
    for routers)

/proc/sys/net/ipv4/ip_forward only does work when its value is changed
and always returns the last written value. The last written value may
not reflect the current state of the netstack (e.g. when `ip_forward`
was written a value of "1" then disable forwarding on an interface)
so there is no need for sentry to probe netstack to get the current
forwarding state of interfaces.

```
~$ cat /proc/sys/net/ipv4/ip_forward
0
~$ sudo bash -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
~$ cat /proc/sys/net/ipv4/ip_forward
1
~$ sudo sysctl -a | grep ipv4 | grep forward
net.ipv4.conf.all.forwarding = 1
net.ipv4.conf.default.forwarding = 1
net.ipv4.conf.eno1.forwarding = 1
net.ipv4.conf.lo.forwarding = 1
net.ipv4.conf.wlp1s0.forwarding = 1
net.ipv4.ip_forward = 1
net.ipv4.ip_forward_update_priority = 1
net.ipv4.ip_forward_use_pmtu = 0
~$ sudo sysctl -w net.ipv4.conf.wlp1s0.forwarding=0
net.ipv4.conf.wlp1s0.forwarding = 0
~$ sudo sysctl -a | grep ipv4 | grep forward
net.ipv4.conf.all.forwarding = 1
net.ipv4.conf.default.forwarding = 1
net.ipv4.conf.eno1.forwarding = 1
net.ipv4.conf.lo.forwarding = 1
net.ipv4.conf.wlp1s0.forwarding = 0
net.ipv4.ip_forward = 1
net.ipv4.ip_forward_update_priority = 1
net.ipv4.ip_forward_use_pmtu = 0
~$ cat /proc/sys/net/ipv4/ip_forward
1
~$ sudo bash -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
~$ sudo sysctl -a | grep ipv4 | grep forward
net.ipv4.conf.all.forwarding = 1
net.ipv4.conf.default.forwarding = 1
net.ipv4.conf.eno1.forwarding = 1
net.ipv4.conf.lo.forwarding = 1
net.ipv4.conf.wlp1s0.forwarding = 0
net.ipv4.ip_forward = 1
net.ipv4.ip_forward_update_priority = 1
net.ipv4.ip_forward_use_pmtu = 0
~$ sudo bash -c "echo 0 > /proc/sys/net/ipv4/ip_forward"
~$ sudo sysctl -a | grep ipv4 | grep forward
sysctl: unable to open directory "/proc/sys/fs/binfmt_misc/"
net.ipv4.conf.all.forwarding = 0
net.ipv4.conf.default.forwarding = 0
net.ipv4.conf.eno1.forwarding = 0
net.ipv4.conf.lo.forwarding = 0
net.ipv4.conf.wlp1s0.forwarding = 0
net.ipv4.ip_forward = 0
net.ipv4.ip_forward_update_priority = 1
net.ipv4.ip_forward_use_pmtu = 0
~$ cat /proc/sys/net/ipv4/ip_forward
0
```

In the above example we can see that writing "1" to
/proc/sys/net/ipv4/ip_forward configures the stack to be a router (all
interfaces are configured to enable forwarding). However, if we manually
update an interace (`wlp1s0`) to not forward packets,
/proc/sys/net/ipv4/ip_forward continues to return the last written value
of "1", even though not all interfaces will forward packets.

Also note that writing the same value twice has no effect; work is
performed iff the value changes.

This change also removes the 'unset' state from sentry's ip forwarding
data structures as an 'unset' ip forwarding value is the same as leaving
forwarding disabled as the stack is always brought up with forwarding
initially disabled; disabling forwarding on a newly created stack is a
no-op.

PiperOrigin-RevId: 373853106

2 files changed, 6 insertions, 16 deletions

diff --git a/pkg/sentry/fs/proc/sys_net.go b/pkg/sentry/fs/proc/sys_net.go
index 1d09afdd7..4893af56b 100644
--- a/pkg/sentry/fs/proc/sys_net.go
+++ b/pkg/sentry/fs/proc/sys_net.go
@@ -403,7 +403,7 @@ type ipForwarding struct {
 	// enabled stores the IPv4 forwarding state on save.
 	// We must save/restore this here, since a netstack instance
 	// is created on restore.
-	enabled *bool
+	enabled bool
 }
 
 func newIPForwardingInode(ctx context.Context, msrc *fs.MountSource, s inet.Stack) *fs.Inode {
@@ -461,13 +461,8 @@ func (f *ipForwardingFile) Read(ctx context.Context, _ *fs.File, dst usermem.IOS
 		return 0, io.EOF
 	}
 
-	if f.ipf.enabled == nil {
-		enabled := f.stack.Forwarding(ipv4.ProtocolNumber)
-		f.ipf.enabled = &enabled
-	}
-
 	val := "0\n"
-	if *f.ipf.enabled {
+	if f.ipf.enabled {
 		// Technically, this is not quite compatible with Linux. Linux
 		// stores these as an integer, so if you write "2" into
 		// ip_forward, you should get 2 back.
@@ -494,11 +489,8 @@ func (f *ipForwardingFile) Write(ctx context.Context, _ *fs.File, src usermem.IO
 	if err != nil {
 		return n, err
 	}
-	if f.ipf.enabled == nil {
-		f.ipf.enabled = new(bool)
-	}
-	*f.ipf.enabled = v != 0
-	return n, f.stack.SetForwarding(ipv4.ProtocolNumber, *f.ipf.enabled)
+	f.ipf.enabled = v != 0
+	return n, f.stack.SetForwarding(ipv4.ProtocolNumber, f.ipf.enabled)
 }
 
 // portRangeInode implements fs.InodeOperations. It provides and allows
diff --git a/pkg/sentry/fs/proc/sys_net_state.go b/pkg/sentry/fs/proc/sys_net_state.go
index 4cb4741af..51d2be647 100644
--- a/pkg/sentry/fs/proc/sys_net_state.go
+++ b/pkg/sentry/fs/proc/sys_net_state.go
@@ -47,9 +47,7 @@ func (s *tcpSack) afterLoad() {
 
 // afterLoad is invoked by stateify.
 func (ipf *ipForwarding) afterLoad() {
-	if ipf.enabled != nil {
-		if err := ipf.stack.SetForwarding(ipv4.ProtocolNumber, *ipf.enabled); err != nil {
-			panic(fmt.Sprintf("failed to set IPv4 forwarding [%v]: %v", *ipf.enabled, err))
-		}
+	if err := ipf.stack.SetForwarding(ipv4.ProtocolNumber, ipf.enabled); err != nil {
+		panic(fmt.Sprintf("ipf.stack.SetForwarding(%d, %t): %s", ipv4.ProtocolNumber, ipf.enabled, err))
 	}
 }