src

witness.go (12786B)

---

 // Copyright 2021 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package vulncheck
 
 import (
 	"container/list"
 	"fmt"
 	"go/ast"
 	"go/token"
 	"sort"
 	"strconv"
 	"strings"
 	"sync"
 	"unicode"
 
 	"golang.org/x/tools/go/packages"
 )
 
 // CallStack is a call stack starting with a client
 // function or method and ending with a call to a
 // vulnerable symbol.
 type CallStack []StackEntry
 
 // StackEntry is an element of a call stack.
 type StackEntry struct {
 	// Function whose frame is on the stack.
 	Function *FuncNode
 
 	// Call is the call site inducing the next stack frame.
 	// nil when the frame represents the last frame in the stack.
 	Call *CallSite
 }
 
 // sourceCallstacks returns representative call stacks for each
 // vulnerability in res. The returned call stacks are heuristically
 // ordered by how seemingly easy is to understand them: shorter
 // call stacks with less dynamic call sites appear earlier in the
 // returned slices.
 //
 // sourceCallstacks performs a breadth-first search of res.CallGraph
 // starting at the vulnerable symbol and going up until reaching an entry
 // function or method in res.CallGraph.Entries. During this search,
 // each function is visited at most once to avoid potential
 // exponential explosion. Hence, not all call stacks are analyzed.
 func sourceCallstacks(res *Result) map[*Vuln]CallStack {
 	var (
 		wg sync.WaitGroup
 		mu sync.Mutex
 	)
 	stackPerVuln := make(map[*Vuln]CallStack)
 	for _, vuln := range res.Vulns {
 		vuln := vuln
 		wg.Add(1)
 		go func() {
 			cs := sourceCallstack(vuln, res)
 			mu.Lock()
 			stackPerVuln[vuln] = cs
 			mu.Unlock()
 			wg.Done()
 		}()
 	}
 	wg.Wait()
 
 	updateInitPositions(stackPerVuln)
 	return stackPerVuln
 }
 
 // sourceCallstack finds a representative call stack for vuln.
 // This is a shortest unique call stack with the least
 // number of dynamic call sites.
 func sourceCallstack(vuln *Vuln, res *Result) CallStack {
 	vulnSink := vuln.CallSink
 	if vulnSink == nil {
 		return nil
 	}
 
 	entries := make(map[*FuncNode]bool)
 	for _, e := range res.EntryFunctions {
 		entries[e] = true
 	}
 
 	seen := make(map[*FuncNode]bool)
 
 	// Do a BFS from the vuln sink to the entry points
 	// and find the representative call stack. This is
 	// the shortest call stack that goes through the
 	// least number of dynamic call sites. We first
 	// collect all candidate call stacks of the shortest
 	// length and then pick the best one accordingly.
 	var candidates []CallStack
 	candDepth := 0
 	queue := list.New()
 	queue.PushBack(&callChain{f: vulnSink})
 
 	// We want to avoid call stacks that go through
 	// other vulnerable symbols of the same package
 	// for the same vulnerability. In other words,
 	// we want unique call stacks.
 	skipSymbols := make(map[*FuncNode]bool)
 	for _, v := range res.Vulns {
 		if v.CallSink != nil && v != vuln &&
 			v.OSV == vuln.OSV && v.Package == vuln.Package {
 			skipSymbols[v.CallSink] = true
 		}
 	}
 
 	for queue.Len() > 0 {
 		front := queue.Front()
 		c := front.Value.(*callChain)
 		queue.Remove(front)
 
 		f := c.f
 		if seen[f] {
 			continue
 		}
 		seen[f] = true
 
 		// Pick a single call site for each function in determinstic order.
 		// A single call site is sufficient as we visit a function only once.
 		for _, cs := range callsites(f.CallSites, seen) {
 			nStack := &callChain{f: cs.Parent, call: cs, child: c}
 			if !skipSymbols[cs.Parent] {
 				queue.PushBack(nStack)
 			}
 
 			if entries[cs.Parent] {
 				ns := nStack.CallStack()
 				if len(candidates) == 0 || len(ns) == candDepth {
 					// The case where we either have not identified
 					// any call stacks or just found one of the same
 					// length as the previous ones.
 					candidates = append(candidates, ns)
 					candDepth = len(ns)
 				} else {
 					// We just found a candidate call stack whose
 					// length is greater than what we previously
 					// found. We can thus safely disregard this
 					// call stack and stop searching since we won't
 					// be able to find any better candidates.
 					queue.Init() // clear the list, effectively exiting the outer loop
 				}
 			}
 		}
 	}
 
 	// Sort candidate call stacks by their number of dynamic call
 	// sites and return the first one.
 	sort.SliceStable(candidates, func(i int, j int) bool {
 		s1, s2 := candidates[i], candidates[j]
 		if w1, w2 := weight(s1), weight(s2); w1 != w2 {
 			return w1 < w2
 		}
 
 		// At this point, the stableness/determinism of
 		// sorting is guaranteed by the determinism of
 		// the underlying call graph and the call stack
 		// search algorithm.
 		return true
 	})
 	if len(candidates) == 0 {
 		return nil
 	}
 	return candidates[0]
 }
 
 // callsites picks a call site from sites for each non-visited function.
 // For each such function, the smallest (posLess) call site is chosen. The
 // returned slice is sorted by caller functions (funcLess). Assumes callee
 // of each call site is the same.
 func callsites(sites []*CallSite, visited map[*FuncNode]bool) []*CallSite {
 	minCs := make(map[*FuncNode]*CallSite)
 	for _, cs := range sites {
 		if visited[cs.Parent] {
 			continue
 		}
 		if csLess(cs, minCs[cs.Parent]) {
 			minCs[cs.Parent] = cs
 		}
 	}
 
 	var fs []*FuncNode
 	for _, cs := range minCs {
 		fs = append(fs, cs.Parent)
 	}
 	sort.SliceStable(fs, func(i, j int) bool { return funcLess(fs[i], fs[j]) })
 
 	var css []*CallSite
 	for _, f := range fs {
 		css = append(css, minCs[f])
 	}
 	return css
 }
 
 // callChain models a chain of function calls.
 type callChain struct {
 	call  *CallSite // nil for entry points
 	f     *FuncNode
 	child *callChain
 }
 
 // CallStack converts callChain to CallStack type.
 func (c *callChain) CallStack() CallStack {
 	if c == nil {
 		return nil
 	}
 	return append(CallStack{StackEntry{Function: c.f, Call: c.call}}, c.child.CallStack()...)
 }
 
 // weight computes an approximate measure of how easy is to understand the call
 // stack when presented to the client as a witness. The smaller the value, the more
 // understandable the stack is. Currently defined as the number of unresolved
 // call sites in the stack.
 func weight(stack CallStack) int {
 	w := 0
 	for _, e := range stack {
 		if e.Call != nil && !e.Call.Resolved {
 			w += 1
 		}
 	}
 	return w
 }
 
 // csLess compares two call sites by their locations and, if needed,
 // their string representation.
 func csLess(cs1, cs2 *CallSite) bool {
 	if cs2 == nil {
 		return true
 	}
 
 	// fast code path
 	if p1, p2 := cs1.Pos, cs2.Pos; p1 != nil && p2 != nil {
 		if posLess(*p1, *p2) {
 			return true
 		}
 		if posLess(*p2, *p1) {
 			return false
 		}
 		// for sanity, should not occur in practice
 		return fmt.Sprintf("%v.%v", cs1.RecvType, cs2.Name) < fmt.Sprintf("%v.%v", cs2.RecvType, cs2.Name)
 	}
 
 	// code path rarely exercised
 	if cs2.Pos == nil {
 		return true
 	}
 	if cs1.Pos == nil {
 		return false
 	}
 	// should very rarely occur in practice
 	return fmt.Sprintf("%v.%v", cs1.RecvType, cs2.Name) < fmt.Sprintf("%v.%v", cs2.RecvType, cs2.Name)
 }
 
 // posLess compares two positions by their line and column number,
 // and filename if needed.
 func posLess(p1, p2 token.Position) bool {
 	if p1.Line < p2.Line {
 		return true
 	}
 	if p2.Line < p1.Line {
 		return false
 	}
 
 	if p1.Column < p2.Column {
 		return true
 	}
 	if p2.Column < p1.Column {
 		return false
 	}
 
 	return strings.Compare(p1.Filename, p2.Filename) == -1
 }
 
 // funcLess compares two function nodes by locations of
 // corresponding functions and, if needed, their string representation.
 func funcLess(f1, f2 *FuncNode) bool {
 	if p1, p2 := f1.Pos, f2.Pos; p1 != nil && p2 != nil {
 		if posLess(*p1, *p2) {
 			return true
 		}
 		if posLess(*p2, *p1) {
 			return false
 		}
 		// for sanity, should not occur in practice
 		return f1.String() < f2.String()
 	}
 
 	if f2.Pos == nil {
 		return true
 	}
 	if f1.Pos == nil {
 		return false
 	}
 	// should happen only for inits
 	return f1.String() < f2.String()
 }
 
 // updateInitPositions populates non-existing positions of init functions
 // and their respective calls in callStacks (see #51575).
 func updateInitPositions(callStacks map[*Vuln]CallStack) {
 	for _, cs := range callStacks {
 		for i := range cs {
 			updateInitPosition(&cs[i])
 			if i != len(cs)-1 {
 				updateInitCallPosition(&cs[i], cs[i+1])
 			}
 		}
 	}
 }
 
 // updateInitCallPosition updates the position of a call to init in a stack frame, if
 // one already does not exist:
 //
 //	P1.init -> P2.init: position of call to P2.init is the position of "import P2"
 //	statement in P1
 //
 //	P.init -> P.init#d: P.init is an implicit init. We say it calls the explicit
 //	P.init#d at the place of "package P" statement.
 func updateInitCallPosition(curr *StackEntry, next StackEntry) {
 	call := curr.Call
 	if !isInit(next.Function) || (call.Pos != nil && call.Pos.IsValid()) {
 		// Skip non-init functions and inits whose call site position is available.
 		return
 	}
 
 	var pos token.Position
 	if curr.Function.Name == "init" && curr.Function.Package == next.Function.Package {
 		// We have implicit P.init calling P.init#d. Set the call position to
 		// be at "package P" statement position.
 		pos = packageStatementPos(curr.Function.Package)
 	} else {
 		// Choose the beginning of the import statement as the position.
 		pos = importStatementPos(curr.Function.Package, next.Function.Package.PkgPath)
 	}
 
 	call.Pos = &pos
 }
 
 func importStatementPos(pkg *packages.Package, importPath string) token.Position {
 	var importSpec *ast.ImportSpec
 spec:
 	for _, f := range pkg.Syntax {
 		for _, impSpec := range f.Imports {
 			// Import spec paths have quotation marks.
 			impSpecPath, err := strconv.Unquote(impSpec.Path.Value)
 			if err != nil {
 				panic(fmt.Sprintf("import specification: package path has no quotation marks: %v", err))
 			}
 			if impSpecPath == importPath {
 				importSpec = impSpec
 				break spec
 			}
 		}
 	}
 
 	if importSpec == nil {
 		// for sanity, in case of a wild call graph imprecision
 		return token.Position{}
 	}
 
 	// Choose the beginning of the import statement as the position.
 	return pkg.Fset.Position(importSpec.Pos())
 }
 
 func packageStatementPos(pkg *packages.Package) token.Position {
 	if len(pkg.Syntax) == 0 {
 		return token.Position{}
 	}
 	// Choose beginning of the package statement as the position. Pick
 	// the first file since it is as good as any.
 	return pkg.Fset.Position(pkg.Syntax[0].Package)
 }
 
 // updateInitPosition updates the position of P.init function in a stack frame if one
 // is not available. The new position is the position of the "package P" statement.
 func updateInitPosition(se *StackEntry) {
 	fun := se.Function
 	if !isInit(fun) || (fun.Pos != nil && fun.Pos.IsValid()) {
 		// Skip non-init functions and inits whose position is available.
 		return
 	}
 
 	pos := packageStatementPos(fun.Package)
 	fun.Pos = &pos
 }
 
 func isInit(f *FuncNode) bool {
 	// A source init function, or anonymous functions used in inits, will
 	// be named "init#x" by vulncheck (more precisely, ssa), where x is a
 	// positive integer. Implicit inits are named simply "init".
 	return f.Name == "init" || strings.HasPrefix(f.Name, "init#")
 }
 
 // binaryCallstacks computes representative call stacks for binary results.
 func binaryCallstacks(vr *Result) map[*Vuln]CallStack {
 	callstacks := map[*Vuln]CallStack{}
 	for _, vv := range uniqueVulns(vr.Vulns) {
 		f := &FuncNode{Package: vv.Package, Name: vv.Symbol}
 		parts := strings.Split(vv.Symbol, ".")
 		if len(parts) != 1 {
 			f.RecvType = parts[0]
 			f.Name = parts[1]
 		}
 		callstacks[vv] = CallStack{StackEntry{Function: f}}
 	}
 	return callstacks
 }
 
 // uniqueVulns does for binary mode what sourceCallstacks does for source mode.
 // It tries not to report redundant symbols. Since there are no call stacks in
 // binary mode, the following approximate approach is used. Do not report unexported
 // symbols for a <vulnID, pkg, module> triple if there are some exported symbols.
 // Otherwise, report all unexported symbols to avoid not reporting anything.
 func uniqueVulns(vulns []*Vuln) []*Vuln {
 	type key struct {
 		id  string
 		pkg string
 		mod string
 	}
 	hasExported := make(map[key]bool)
 	for _, v := range vulns {
 		if isExported(v.Symbol) {
 			k := key{id: v.OSV.ID, pkg: v.Package.PkgPath, mod: v.Package.Module.Path}
 			hasExported[k] = true
 		}
 	}
 
 	var uniques []*Vuln
 	for _, v := range vulns {
 		k := key{id: v.OSV.ID, pkg: v.Package.PkgPath, mod: v.Package.Module.Path}
 		if isExported(v.Symbol) || !hasExported[k] {
 			uniques = append(uniques, v)
 		}
 	}
 	return uniques
 }
 
 // isExported checks if the symbol is exported. Assumes that the
 // symbol is of the form "identifier", "identifier1.identifier2",
 // or "identifier.".
 func isExported(symbol string) bool {
 	parts := strings.Split(symbol, ".")
 	last := parts[len(parts)-1]
 	if last == "" { // case for "identifier."
 		return false
 	}
 	return unicode.IsUpper(rune(last[0]))
 }