CRI stream server: Fix goroutine leak in Exec

In the CRI streaming server, a goroutine (`handleResizeEvents`) is launched
to handle terminal resize events if a TTY is asked for with an exec; this
is the sender of terminal resize events. Another goroutine is launched
shortly after successful process startup to actually do something with
these events, however the issue arises if the exec process fails to start
for any reason that would have `process.Start` return non-nil. The receiver
goroutine never gets launched so the sender is stuck blocked on a channel send
infinitely.

This could be used in a malicious manner by repeatedly launching execs
with a command that doesn't exist in the image, as a single goroutine
will get leaked on every invocation which will slowly grow containerd's
memory usage.

Signed-off-by: Danny Canter <danny@dcantah.dev>

pkg/cri/streaming/remotecommand/httpstream.go

@@ -33,6 +33,7 @@ limitations under the License.
 package remotecommand
 
 import (
+	gocontext "context"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -132,7 +133,7 @@ func createStreams(req *http.Request, w http.ResponseWriter, opts *Options, supp
 
 	if ctx.resizeStream != nil {
 		ctx.resizeChan = make(chan remotecommand.TerminalSize)
-		go handleResizeEvents(ctx.resizeStream, ctx.resizeChan)
+		go handleResizeEvents(req.Context(), ctx.resizeStream, ctx.resizeChan)
 	}
 
 	return ctx, true
@@ -425,7 +426,7 @@ WaitForStreams:
 // supportsTerminalResizing returns false because v1ProtocolHandler doesn't support it.
 func (*v1ProtocolHandler) supportsTerminalResizing() bool { return false }
 
-func handleResizeEvents(stream io.Reader, channel chan<- remotecommand.TerminalSize) {
+func handleResizeEvents(ctx gocontext.Context, stream io.Reader, channel chan<- remotecommand.TerminalSize) {
 	defer runtime.HandleCrash()
 	defer close(channel)
 
@@ -435,7 +436,15 @@ func handleResizeEvents(stream io.Reader, channel chan<- remotecommand.TerminalS
 		if err := decoder.Decode(&size); err != nil {
 			break
 		}
-		channel <- size
+
+		select {
+		case channel <- size:
+		case <-ctx.Done():
+			// To avoid leaking this routine, exit if the http request finishes. This path
+			// would generally be hit if starting the process fails and nothing is started to
+			// ingest these resize events.
+			return
+		}
 	}
 }