fix(doh,doq): reject oversized upstream DNS responses

DoH, DoH3, and DoQ response paths previously used io.ReadAll on
attacker-controlled upstream responses before enforcing any protocol-level
size limit. A malicious or compromised upstream could return an oversized
body or stream and force ctrld to buffer unbounded data before eventually
failing DNS parsing.

Cap DoH/DoH3 response bodies at dns.MaxMsgSize and cap DoQ streams at the
2-byte length prefix plus dns.MaxMsgSize. Also limit non-200 DoH error
bodies so error formatting cannot consume large upstream responses.

doq.go

@@ -17,6 +17,12 @@ import (
 	"github.com/quic-go/quic-go"
 )
 
+// doqMaxResponseSize caps the bytes read from a DoQ stream: a 2-byte
+// length prefix plus a DNS message bounded by dns.MaxMsgSize. Anything
+// larger cannot be a valid response and is rejected before buffering more
+// data from the upstream.
+const doqMaxResponseSize = 2 + dns.MaxMsgSize
+
 type doqResolver struct {
 	uc *UpstreamConfig
 }
@@ -191,7 +197,13 @@ func (p *doqConnPool) doResolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, er
 		return nil, err
 	}
 
-	buf, err := io.ReadAll(stream)
+	// A DoQ response is a 2-byte length prefix followed by a DNS message.
+	// The DNS message is bounded by the protocol at dns.MaxMsgSize, so a
+	// well-formed response is at most doqMaxResponseSize bytes. Read one
+	// byte past that cap to distinguish "at limit" from "over limit" and
+	// reject oversized responses before they can drive memory growth from
+	// a malicious or compromised upstream.
+	buf, err := io.ReadAll(io.LimitReader(stream, doqMaxResponseSize+1))
 	if err != nil {
 		p.putConn(conn, false)
 		return nil, err
@@ -203,6 +215,11 @@ func (p *doqConnPool) doResolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, er
 		return nil, io.EOF
 	}
 
+	if len(buf) > doqMaxResponseSize {
+		p.putConn(conn, false)
+		return nil, fmt.Errorf("DoQ response exceeds %d-byte maximum", doqMaxResponseSize)
+	}
+
 	// RFC 9250: each DoQ DNS message is encoded as a 2-octet length field
 	// followed by the DNS message. Reject responses that are shorter than
 	// the prefix or whose prefix declares more bytes than were received,