[release-branch.go1.16] regexp/syntax: reject very deeply nested regexps in Parse

The regexp code assumes it can recurse over the structure of
a regexp safely. Go's growable stacks make that reasonable
for all plausible regexps, but implausible ones can reach the
“infinite recursion?” stack limit.

This CL limits the depth of any parsed regexp to 1000.
That is, the depth of the parse tree is required to be ≤ 1000.
Regexps that require deeper parse trees will return ErrInternalError.
A future CL will change the error to ErrInvalidDepth,
but using ErrInternalError for now avoids introducing new API
in point releases when this is backported.

Fixes #51112.
Fixes #51117.

Change-Id: I97d2cd82195946eb43a4ea8561f5b95f91fb14c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/384616
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/384855

2 files changed, 77 insertions, 2 deletions

diff --git a/src/regexp/syntax/parse.go b/src/regexp/syntax/parse.go
index 7b4030935a..d7cf2afa5e 100644
--- a/src/regexp/syntax/parse.go
+++ b/src/regexp/syntax/parse.go
@@ -76,13 +76,29 @@ const (
 	opVerticalBar
 )
 
+// maxHeight is the maximum height of a regexp parse tree.
+// It is somewhat arbitrarily chosen, but the idea is to be large enough
+// that no one will actually hit in real use but at the same time small enough
+// that recursion on the Regexp tree will not hit the 1GB Go stack limit.
+// The maximum amount of stack for a single recursive frame is probably
+// closer to 1kB, so this could potentially be raised, but it seems unlikely
+// that people have regexps nested even this deeply.
+// We ran a test on Google's C++ code base and turned up only
+// a single use case with depth > 100; it had depth 128.
+// Using depth 1000 should be plenty of margin.
+// As an optimization, we don't even bother calculating heights
+// until we've allocated at least maxHeight Regexp structures.
+const maxHeight = 1000
+
 type parser struct {
 	flags       Flags     // parse mode flags
 	stack       []*Regexp // stack of parsed expressions
 	free        *Regexp
 	numCap      int // number of capturing groups seen
 	wholeRegexp string
-	tmpClass    []rune // temporary char class work space
+	tmpClass    []rune          // temporary char class work space
+	numRegexp   int             // number of regexps allocated
+	height      map[*Regexp]int // regexp height for height limit check
 }
 
 func (p *parser) newRegexp(op Op) *Regexp {
@@ -92,16 +108,52 @@ func (p *parser) newRegexp(op Op) *Regexp {
 		*re = Regexp{}
 	} else {
 		re = new(Regexp)
+		p.numRegexp++
 	}
 	re.Op = op
 	return re
 }
 
 func (p *parser) reuse(re *Regexp) {
+	if p.height != nil {
+		delete(p.height, re)
+	}
 	re.Sub0[0] = p.free
 	p.free = re
 }
 
+func (p *parser) checkHeight(re *Regexp) {
+	if p.numRegexp < maxHeight {
+		return
+	}
+	if p.height == nil {
+		p.height = make(map[*Regexp]int)
+		for _, re := range p.stack {
+			p.checkHeight(re)
+		}
+	}
+	if p.calcHeight(re, true) > maxHeight {
+		panic(ErrInternalError)
+	}
+}
+
+func (p *parser) calcHeight(re *Regexp, force bool) int {
+	if !force {
+		if h, ok := p.height[re]; ok {
+			return h
+		}
+	}
+	h := 1
+	for _, sub := range re.Sub {
+		hsub := p.calcHeight(sub, false)
+		if h < 1+hsub {
+			h = 1 + hsub
+		}
+	}
+	p.height[re] = h
+	return h
+}
+
 // Parse stack manipulation.
 
 // push pushes the regexp re onto the parse stack and returns the regexp.
@@ -137,6 +189,7 @@ func (p *parser) push(re *Regexp) *Regexp {
 	}
 
 	p.stack = append(p.stack, re)
+	p.checkHeight(re)
 	return re
 }
 
@@ -246,6 +299,7 @@ func (p *parser) repeat(op Op, min, max int, before, after, lastRepeat string) (
 	re.Sub = re.Sub0[:1]
 	re.Sub[0] = sub
 	p.stack[n-1] = re
+	p.checkHeight(re)
 
 	if op == OpRepeat && (min >= 2 || max >= 2) && !repeatIsValid(re, 1000) {
 		return "", &Error{ErrInvalidRepeatSize, before[:len(before)-len(after)]}
@@ -693,6 +747,21 @@ func literalRegexp(s string, flags Flags) *Regexp {
 // Flags, and returns a regular expression parse tree. The syntax is
 // described in the top-level comment.
 func Parse(s string, flags Flags) (*Regexp, error) {
+	return parse(s, flags)
+}
+
+func parse(s string, flags Flags) (_ *Regexp, err error) {
+	defer func() {
+		switch r := recover(); r {
+		default:
+			panic(r)
+		case nil:
+			// ok
+		case ErrInternalError:
+			err = &Error{Code: ErrInternalError, Expr: s}
+		}
+	}()
+
 	if flags&Literal != 0 {
 		// Trivial parser for literal string.
 		if err := checkUTF8(s); err != nil {
@@ -704,7 +773,6 @@ func Parse(s string, flags Flags) (*Regexp, error) {
 	// Otherwise, must do real work.
 	var (
 		p          parser
-		err        error
 		c          rune
 		op         Op
 		lastRepeat string
diff --git a/src/regexp/syntax/parse_test.go b/src/regexp/syntax/parse_test.go
index 5581ba1ca5..1ef6d8a3fe 100644
--- a/src/regexp/syntax/parse_test.go
+++ b/src/regexp/syntax/parse_test.go
@@ -207,6 +207,11 @@ var parseTests = []parseTest{
 	// Valid repetitions.
 	{`((((((((((x{2}){2}){2}){2}){2}){2}){2}){2}){2}))`, ``},
 	{`((((((((((x{1}){2}){2}){2}){2}){2}){2}){2}){2}){2})`, ``},
+
+	// Valid nesting.
+	{strings.Repeat("(", 999) + strings.Repeat(")", 999), ``},
+	{strings.Repeat("(?:", 999) + strings.Repeat(")*", 999), ``},
+	{"(" + strings.Repeat("|", 12345) + ")", ``}, // not nested at all
 }
 
 const testFlags = MatchNL | PerlX | UnicodeGroups
@@ -482,6 +487,8 @@ var invalidRegexps = []string{
 	`a{100000}`,
 	`a{100000,}`,
 	"((((((((((x{2}){2}){2}){2}){2}){2}){2}){2}){2}){2})",
+	strings.Repeat("(", 1000) + strings.Repeat(")", 1000),
+	strings.Repeat("(?:", 1000) + strings.Repeat(")*", 1000),
 	`\Q\E*`,
 }