imapserver: prevent unbounded memory allocations when handling a command

some commands, like search, can specify any number of literals, of arbitrary
size.  we already limited individual literals to 100kb. but you could specify
many of them, causing unbounded memory consumption. this change adds a limit of
1000 literals in a command, and a limit of 1mb of total combined memory for
literals. once the limits are exceeded, a TOOBIG error code is returned.

unbounded memory use could only be triggered on authenticated connections.

this addresses the same issue as CVE-2024-34055 for cyrus-imap, by damian
poddebniak.

imapclient/client.go

@@ -238,19 +238,27 @@ func (c *Conn) Write(buf []byte) (n int, rerr error) {
 
 // WriteSyncLiteral first writes the synchronous literal size, then read the
 // continuation "+" and finally writes the data.
-func (c *Conn) WriteSyncLiteral(s string) (rerr error) {
+func (c *Conn) WriteSyncLiteral(s string) (untagged []Untagged, rerr error) {
 	defer c.recover(&rerr)
 
 	_, err := fmt.Fprintf(c.conn, "{%d}\r\n", len(s))
 	c.xcheckf(err, "write sync literal size")
-	line, err := c.Readline()
-	c.xcheckf(err, "read line")
-	if !strings.HasPrefix(line, "+") {
-		c.xerrorf("no continuation received for sync literal")
-	}
+
+	plus, err := c.r.Peek(1)
+	c.xcheckf(err, "read continuation")
+	if plus[0] == '+' {
+		_, err = c.Readline()
+		c.xcheckf(err, "read continuation line")
+
 		_, err = c.conn.Write([]byte(s))
 		c.xcheckf(err, "write literal data")
-	return nil
+		return nil, nil
+	}
+	untagged, result, err := c.Response()
+	if err == nil && result.Status == OK {
+		c.xerrorf("no continuation, but invalid ok response (%q)", result.More)
+	}
+	return untagged, fmt.Errorf("no continuation (%s)", result.Status)
 }
 
 // Transactf writes format and args as an IMAP command, using Commandf with an

imapserver/parse.go

@@ -52,6 +52,8 @@ type parser struct {
 	upper       string
 	o           int      // Current offset in parsing.
 	contexts    []string // What we're parsing, for error messages.
+	literals    int      // Literals in command, for limit.
+	literalSize int64    // Total size of literals in command, for limit.
 	conn        *conn
 }
 
@@ -70,7 +72,7 @@ func toUpper(s string) string {
 }
 
 func newParser(s string, conn *conn) *parser {
-	return &parser{s, toUpper(s), 0, nil, conn}
+	return &parser{s, toUpper(s), 0, nil, 0, 0, conn}
 }
 
 func (p *parser) xerrorf(format string, args ...any) {
@@ -302,7 +304,7 @@ func (p *parser) xstring() (r string) {
 		}
 		p.xerrorf("missing closing dquote in string")
 	}
-	size, sync := p.xliteralSize(100*1024, false)
+	size, sync := p.xliteralSize(false, true)
 	s := p.conn.xreadliteral(size, sync)
 	line := p.conn.readline(false)
 	p.orig, p.upper, p.o = line, toUpper(line), 0
@@ -741,23 +743,47 @@ func (p *parser) xdateTime() time.Time {
 }
 
 // ../rfc/9051:6655 ../rfc/7888:330 ../rfc/3501:4801
-func (p *parser) xliteralSize(maxSize int64, lit8 bool) (size int64, sync bool) {
+func (p *parser) xliteralSize(lit8 bool, checkSize bool) (size int64, sync bool) {
 	// todo: enforce that we get non-binary when ~ isn't present?
 	if lit8 {
 		p.take("~")
 	}
 	p.xtake("{")
 	size = p.xnumber64()
-	if maxSize > 0 && size > maxSize {
-		// ../rfc/7888:249
-		line := fmt.Sprintf("* BYE [ALERT] Max literal size %d is larger than allowed %d in this context", size, maxSize)
-		err := errors.New("literal too big")
-		panic(syntaxError{line, "TOOBIG", err.Error(), err})
-	}
 
 	sync = !p.take("+")
 	p.xtake("}")
 	p.xempty()
+
+	if checkSize {
+		// ../rfc/7888:249
+		var errmsg string
+		const (
+			litSizeMax      = 100 * 1024
+			totalLitSizeMax = 10 * litSizeMax
+			litMax          = 1000
+		)
+		p.literalSize += size
+		p.literals++
+		if size > litSizeMax {
+			errmsg = fmt.Sprintf("max literal size %d is larger than allowed %d", size, litSizeMax)
+		} else if p.literalSize > totalLitSizeMax {
+			errmsg = fmt.Sprintf("max total literal size for command %d is larger than allowed %d", p.literalSize, totalLitSizeMax)
+		} else if p.literals > litMax {
+			errmsg = fmt.Sprintf("max literals for command %d is larger than allowed %d", p.literals, litMax)
+		}
+		if errmsg != "" {
+			// ../rfc/9051:357 ../rfc/3501:347
+			err := errors.New("literal too big: " + errmsg)
+			if sync {
+				errmsg = ""
+			} else {
+				errmsg = "* BYE [ALERT] " + errmsg
+			}
+			panic(syntaxError{errmsg, "TOOBIG", err.Error(), err})
+		}
+	}
+
 	return size, sync
 }
 

imapserver/search_test.go

@@ -1,6 +1,7 @@
 package imapserver
 
 import (
+	"fmt"
 	"strconv"
 	"strings"
 	"testing"
@@ -338,6 +339,53 @@ func TestSearch(t *testing.T) {
 	tc.client.Enable("IMAP4rev2")
 	tc.transactf("ok", `search undraft`)
 	tc.xesearch(esearchall("1:2"))
+
+	// Long commands should be rejected, not allocating too much memory.
+	lit := make([]byte, 100*1024+1)
+	for i := range lit {
+		lit[i] = 'x'
+	}
+	writeTextLit := func(n int, expok bool) {
+		_, err := fmt.Fprintf(tc.client, " TEXT ")
+		tcheck(t, err, "write text")
+
+		_, err = fmt.Fprintf(tc.client, "{%d}\r\n", n)
+		tcheck(t, err, "write literal size")
+		line, err := tc.client.Readline()
+		tcheck(t, err, "read line")
+		if expok && !strings.HasPrefix(line, "+") {
+			tcheck(t, fmt.Errorf("no continuation after writing size: %s", line), "sending literal")
+		} else if !expok && !strings.HasPrefix(line, "x0 BAD [TOOBIG]") {
+			tcheck(t, fmt.Errorf("got line %s", line), "expected TOOBIG error")
+		}
+		if !expok {
+			return
+		}
+		_, err = tc.client.Write(lit[:n])
+		tcheck(t, err, "write literal data")
+	}
+
+	// More than 100k for a literal.
+	_, err := fmt.Fprintf(tc.client, "x0 uid search")
+	tcheck(t, err, "write start of uit search")
+	writeTextLit(100*1024+1, false)
+
+	// More than 1mb total for literals.
+	_, err = fmt.Fprintf(tc.client, "x0 uid search")
+	tcheck(t, err, "write start of uit search")
+	for i := 0; i < 10; i++ {
+		writeTextLit(100*1024, true)
+	}
+	writeTextLit(1, false)
+
+	// More than 1000 literals.
+	_, err = fmt.Fprintf(tc.client, "x0 uid search")
+	tcheck(t, err, "write start of uit search")
+	for i := 0; i < 1000; i++ {
+		writeTextLit(1, true)
+	}
+	writeTextLit(1, false)
+
 }
 
 // esearchall makes an UntaggedEsearch response with All set, for comparisons.

imapserver/server.go

@@ -2729,7 +2729,7 @@ func (c *conn) cmdAppend(tag, cmd string, p *parser) {
 	// todo: this is only relevant if we also support the CATENATE extension?
 	// ../rfc/6855:204
 	utf8 := p.take("UTF8 (")
-	size, sync := p.xliteralSize(0, utf8)
+	size, sync := p.xliteralSize(utf8, false)
 
 	name = xcheckmailboxname(name, true)
 	c.xdbread(func(tx *bstore.Tx) {