Rate limit the unimplemented syscall event handler.

This introduces two new types of Emitters:
1. MultiEmitter, which will forward events to other registered Emitters, and
2. RateLimitedEmitter, which will forward events to a wrapped Emitter, subject
	to given rate limits.

The methods in the eventchannel package itself act like a multiEmitter, but is
not actually an Emitter. Now we have a DefaultEmitter, and the methods in
eventchannel simply forward calls to the DefaultEmitter.

The unimplemented syscall handler now uses a RateLimetedEmitter that wraps the
DefaultEmitter.

PiperOrigin-RevId: 260612770

2 files changed, 26 insertions, 13 deletions

diff --git a/pkg/sentry/kernel/kernel.go b/pkg/sentry/kernel/kernel.go
index 4c2d48e65..cf8bf3ecd 100644
--- a/pkg/sentry/kernel/kernel.go
+++ b/pkg/sentry/kernel/kernel.go
@@ -197,6 +197,11 @@ type Kernel struct {
 	// caches. Not all caches use it, only the caches that use host resources use
 	// the limiter. It may be nil if disabled.
 	DirentCacheLimiter *fs.DirentCacheLimiter
+
+	// unimplementedSyscallEmitter is used to emit unimplemented syscall
+	// events. This is initialized lazily on the first unimplemented
+	// syscall.
+	unimplementedSyscallEmitter eventchannel.Emitter `state:"nosave"`
 }
 
 // InitKernelArgs holds arguments to Init.
@@ -290,7 +295,6 @@ func (k *Kernel) Init(args InitKernelArgs) error {
 	k.monotonicClock = &timekeeperClock{tk: args.Timekeeper, c: sentrytime.Monotonic}
 	k.futexes = futex.NewManager()
 	k.netlinkPorts = port.New()
-
 	return nil
 }
 
@@ -1168,16 +1172,6 @@ func (k *Kernel) SupervisorContext() context.Context {
 	}
 }
 
-// EmitUnimplementedEvent emits an UnimplementedSyscall event via the event
-// channel.
-func (k *Kernel) EmitUnimplementedEvent(ctx context.Context) {
-	t := TaskFromContext(ctx)
-	eventchannel.Emit(&uspb.UnimplementedSyscall{
-		Tid:       int32(t.ThreadID()),
-		Registers: t.Arch().StateData().Proto(),
-	})
-}
-
 // SocketEntry represents a socket recorded in Kernel.sockets. It implements
 // refs.WeakRefUser for sockets stored in the socket table.
 //
@@ -1272,3 +1266,23 @@ func (ctx supervisorContext) Value(key interface{}) interface{} {
 		return nil
 	}
 }
+
+// Rate limits for the number of unimplemented syscall evants.
+const (
+	unimplementedSyscallsMaxRate = 100  // events per second
+	unimplementedSyscallBurst    = 1000 // events
+)
+
+// EmitUnimplementedEvent emits an UnimplementedSyscall event via the event
+// channel.
+func (k *Kernel) EmitUnimplementedEvent(ctx context.Context) {
+	if k.unimplementedSyscallEmitter == nil {
+		k.unimplementedSyscallEmitter = eventchannel.RateLimitedEmitterFrom(eventchannel.DefaultEmitter, unimplementedSyscallsMaxRate, unimplementedSyscallBurst)
+	}
+
+	t := TaskFromContext(ctx)
+	k.unimplementedSyscallEmitter.Emit(&uspb.UnimplementedSyscall{
+		Tid:       int32(t.ThreadID()),
+		Registers: t.Arch().StateData().Proto(),
+	})
+}
diff --git a/pkg/sentry/syscalls/linux/linux64.go b/pkg/sentry/syscalls/linux/linux64.go
index 51db2d8f7..ed996ba51 100644
--- a/pkg/sentry/syscalls/linux/linux64.go
+++ b/pkg/sentry/syscalls/linux/linux64.go
@@ -30,8 +30,7 @@ import (
 const _AUDIT_ARCH_X86_64 = 0xc000003e
 
 // AMD64 is a table of Linux amd64 syscall API with the corresponding syscall
-// numbers from Linux 4.4. The entries commented out are those syscalls we
-// don't currently support.
+// numbers from Linux 4.4.
 var AMD64 = &kernel.SyscallTable{
 	OS:   abi.Linux,
 	Arch: arch.AMD64,