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// Copyright (c) 2017, Google Inc.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
// break-hash parses an ELF binary containing the FIPS module and corrupts the
// first byte of the module. This should cause the integrity check to fail.
package main
import (
"bytes"
"crypto/hmac"
"crypto/sha512"
"debug/elf"
"encoding/hex"
"errors"
"fmt"
"io/ioutil"
"os"
)
func do(outPath, inPath string) error {
objectBytes, err := ioutil.ReadFile(inPath)
if err != nil {
return err
}
object, err := elf.NewFile(bytes.NewReader(objectBytes))
if err != nil {
return errors.New("failed to parse object: " + err.Error())
}
// Find the .text section.
var textSection *elf.Section
var textSectionIndex elf.SectionIndex
for i, section := range object.Sections {
if section.Name == ".text" {
textSectionIndex = elf.SectionIndex(i)
textSection = section
break
}
}
if textSection == nil {
return errors.New("failed to find .text section in object")
}
symbols, err := object.Symbols()
if err != nil {
fmt.Fprintf(os.Stderr, "%s\nTrying dynamic symbols\n", err)
symbols, err = object.DynamicSymbols()
}
if err != nil {
return errors.New("failed to parse symbols: " + err.Error())
}
// Find the start and end markers of the module.
var startSeen, endSeen bool
var start, end uint64
for _, symbol := range symbols {
if symbol.Section != textSectionIndex {
continue
}
switch symbol.Name {
case "BORINGSSL_bcm_text_start":
if startSeen {
return errors.New("duplicate start symbol found")
}
startSeen = true
start = symbol.Value
case "BORINGSSL_bcm_text_end":
if endSeen {
return errors.New("duplicate end symbol found")
}
endSeen = true
end = symbol.Value
default:
continue
}
}
if !startSeen || !endSeen {
return errors.New("could not find module in object")
}
moduleText := make([]byte, end-start)
if n, err := textSection.ReadAt(moduleText, int64(start-textSection.Addr)); err != nil {
return fmt.Errorf("failed to read from module start (at %d of %d) in .text: %s", start, textSection.Size, err)
} else if n != len(moduleText) {
return fmt.Errorf("short read from .text: wanted %d, got %d", len(moduleText), n)
}
// In order to match up the module start with the raw ELF contents,
// search for the first 256 bytes and assume that will be unique.
offset := bytes.Index(objectBytes, moduleText[:256])
if offset < 0 {
return errors.New("did not find module prefix in object file")
}
if bytes.Index(objectBytes[offset+1:], moduleText[:256]) >= 0 {
return errors.New("found two occurrences of prefix in object file")
}
// Corrupt the module in the ELF.
objectBytes[offset] ^= 1
// Calculate the before and after hash of the module.
var zeroKey [64]byte
mac := hmac.New(sha512.New, zeroKey[:])
mac.Write(moduleText)
hashWas := mac.Sum(nil)
moduleText[0] ^= 1
mac.Reset()
mac.Write(moduleText)
newHash := mac.Sum(nil)
fmt.Printf("Found start of module at offset 0x%x (VMA 0x%x):\n", start-textSection.Addr, start)
fmt.Println(hex.Dump(moduleText[:128]))
fmt.Printf("\nHash of module was: %x\n", hashWas)
fmt.Printf("Hash of corrupted module is: %x\n", newHash)
return ioutil.WriteFile(outPath, objectBytes, 0755)
}
func main() {
if len(os.Args) != 3 {
usage()
os.Exit(1)
}
if err := do(os.Args[2], os.Args[1]); err != nil {
fmt.Fprintf(os.Stderr, "%s\n", err)
os.Exit(1)
}
}
func usage() {
fmt.Fprintf(os.Stderr, "Usage: %s <input binary> <output path>\n", os.Args[0])
}
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