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Add bounds checking to all variable length allocs.

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Several of the bitcoin data structures contain variable length entries,
many of which have well-defined maximum limits.  However, there are still
a few cases, such as variable length strings and number of transactions
which don't have clearly defined maximum limits.  Instead they are only
limited by the maximum size of a message.

In order to efficiently decode messages, space is pre-allocated for the
slices which hold these variable length pieces as to avoid needing to
dynamically grow the backing arrays.  Due to this however, it was
previously possible to claim extremely high slice lengths which exceed
available memory (or maximum allowed slice lengths).

This commit imposes limits to all of these cases based on calculating
the maximum possible number of elements that could fit into a message
and using those as sane upper limits.

The variable length string case was found (and tests added to hit it) by
drahn@ which prompted an audit to find all cases.
This commit is contained in:
Dave Collins 2013-10-24 23:13:51 -05:00
parent 70aa92bf0d
commit 5cc32bbfc7
3 changed files with 121 additions and 13 deletions
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21
common.go
View file

@ -8,6 +8,7 @@ import (
"crypto/rand"
"crypto/sha256"
"encoding/binary"
"fmt"
"io"
"math"
)
@ -121,13 +122,27 @@ func writeVarInt(w io.Writer, pver uint32, val uint64) error {
// readVarString reads a variable length string from r and returns it as a Go
// string. A varString is encoded as a varInt containing the length of the
// string, and the bytes that represent the string itself.
// string, and the bytes that represent the string itself. An error is returned
// if the length is greater than the maximum block payload size, since it would
// not be possible to put a varString of that size into a block anyways and it
// also helps protect against memory exhuastion attacks and forced panics
// through malformed messages.
func readVarString(r io.Reader, pver uint32) (string, error) {
slen, err := readVarInt(r, pver)
count, err := readVarInt(r, pver)
if err != nil {
return "", err
}
buf := make([]byte, slen)
// Prevent variable length strings that are larger than the maximum
// message size. It would be possible to cause memory exhaustion and
// panics without a sane upper bound on this count.
if count > maxMessagePayload {
str := fmt.Sprintf("variable length string is too long "+
"[count %d, max %d]", count, maxMessagePayload)
return "", messageError("readVarString", str)
}
buf := make([]byte, count)
err = readElement(r, buf)
if err != nil {
return "", err

30
msgblock.go
View file

@ -6,6 +6,7 @@ package btcwire
import (
"bytes"
"fmt"
"io"
)
@ -22,6 +23,10 @@ const MaxBlocksPerMsg = 500
// MaxBlockPayload is the maximum bytes a block message can be in bytes.
const MaxBlockPayload = 1000000 // Not actually 1MB which would be 1024 * 1024
// maxTxPerBlock is the maximum number of transactions that could
// possibly fit into a block.
const maxTxPerBlock = (MaxBlockPayload / minTxPayload) + 1
// TxLoc holds locator data for the offset and length of where a transaction is
// located within a MsgBlock data buffer.
type TxLoc struct {
@ -72,8 +77,18 @@ func (msg *MsgBlock) BtcDecode(r io.Reader, pver uint32) error {
return err
}
msg.Transactions = make([]*MsgTx, 0, msg.Header.TxnCount)
for i := uint64(0); i < msg.Header.TxnCount; i++ {
// Prevent more transactions than could possibly fit into a block.
// It would be possible to cause memory exhaustion and panics without
// a sane upper bound on this count.
txCount := msg.Header.TxnCount
if txCount > maxTxPerBlock {
str := fmt.Sprintf("too many transactions to fit into a block "+
"[count %d, max %d]", txCount, maxTxPerBlock)
return messageError("MsgBlock.BtcDecode", str)
}
msg.Transactions = make([]*MsgTx, 0, txCount)
for i := uint64(0); i < txCount; i++ {
tx := MsgTx{}
err := tx.BtcDecode(r, pver)
if err != nil {
@ -115,9 +130,18 @@ func (msg *MsgBlock) DeserializeTxLoc(r *bytes.Buffer) ([]TxLoc, error) {
return nil, err
}
// Prevent more transactions than could possibly fit into a block.
// It would be possible to cause memory exhaustion and panics without
// a sane upper bound on this count.
txCount := msg.Header.TxnCount
if txCount > maxTxPerBlock {
str := fmt.Sprintf("too many transactions to fit into a block "+
"[count %d, max %d]", txCount, maxTxPerBlock)
return nil, messageError("MsgBlock.DeserializeTxLoc", str)
}
// Deserialize each transaction while keeping track of its location
// within the byte stream.
txCount := msg.Header.TxnCount
msg.Transactions = make([]*MsgTx, 0, txCount)
txLocs := make([]TxLoc, txCount)
for i := uint64(0); i < txCount; i++ {

83
msgtx.go
View file

@ -6,9 +6,17 @@ package btcwire
import (
"bytes"
"fmt"
"io"
)
// TxVersion is the current latest supported transaction version.
const TxVersion = 1
// MaxTxInSequenceNum is the maximum sequence number the sequence field
// of a transaction input can be.
const MaxTxInSequenceNum uint32 = 0xffffffff
// defaultTxInOutAlloc is the default size used for the backing array for
// transaction inputs and outputs. The array will dynamically grow as needed,
// but this figure is intended to provide enough space for the number of
@ -16,12 +24,33 @@ import (
// backing array multiple times.
const defaultTxInOutAlloc = 15
// TxVersion is the current latest supported transaction version.
const TxVersion = 1
const (
// minTxInPayload is the minimum payload size for a transaction input.
// PreviousOutpoint.Hash + PreviousOutpoint.Index 4 bytes + Varint for
// SignatureScript length 1 byte + Sequence 4 bytes.
minTxInPayload = 9 + HashSize
// MaxTxInSequenceNum is the maximum sequence number the sequence field
// of a transaction input can be.
const MaxTxInSequenceNum uint32 = 0xffffffff
// maxTxInPerMessage is the maximum number of transactions inputs that
// a transaction which fits into a message could possibly have.
maxTxInPerMessage = (maxMessagePayload / minTxInPayload) + 1
// minTxOutPayload is the minimum payload size for a transaction output.
// Value 8 bytes + Varint for PkScript length 1 byte.
minTxOutPayload = 9
// maxTxOutPerMessage is the maximum number of transactions outputs that
// a transaction which fits into a message could possibly have.
maxTxOutPerMessage = (maxMessagePayload / minTxOutPayload) + 1
// minTxPayload is the minimum payload size for a transaction. Note
// that any realistically usable transaction must have at least one
// input or output, but that is a rule enforced at a higher layer, so
// it is intentionally not included here.
// Version 4 bytes + Varint number of transaction inputs 1 byte + Varint
// number of transaction outputs 1 byte + LockTime 4 bytes + min input
// payload + min output payload.
minTxPayload = 10
)
// OutPoint defines a bitcoin data type that is used to track previous
// transaction outputs.
@ -191,6 +220,16 @@ func (msg *MsgTx) BtcDecode(r io.Reader, pver uint32) error {
return err
}
// Prevent more input transactions than could possibly fit into a
// message. It would be possible to cause memory exhaustion and panics
// without a sane upper bound on this count.
if count > uint64(maxTxInPerMessage) {
str := fmt.Sprintf("too many input transactions to fit into "+
"max message size [count %d, max %d]", count,
maxTxInPerMessage)
return messageError("MsgTx.BtcDecode", str)
}
msg.TxIn = make([]*TxIn, 0, count)
for i := uint64(0); i < count; i++ {
ti := TxIn{}
@ -206,6 +245,16 @@ func (msg *MsgTx) BtcDecode(r io.Reader, pver uint32) error {
return err
}
// Prevent more output transactions than could possibly fit into a
// message. It would be possible to cause memory exhaustion and panics
// without a sane upper bound on this count.
if count > uint64(maxTxOutPerMessage) {
str := fmt.Sprintf("too many output transactions to fit into "+
"max message size [count %d, max %d]", count,
maxTxOutPerMessage)
return messageError("MsgTx.BtcDecode", str)
}
msg.TxOut = make([]*TxOut, 0, count)
for i := uint64(0); i < count; i++ {
to := TxOut{}
@ -362,6 +411,16 @@ func readTxIn(r io.Reader, pver uint32, version uint32, ti *TxIn) error {
return err
}
// Prevent signature script larger than the max message size. It would
// be possible to cause memory exhaustion and panics without a sane
// upper bound on this count.
if count > uint64(maxMessagePayload) {
str := fmt.Sprintf("transaction input signature script is "+
"larger than max message size [count %d, max %d]",
count, maxMessagePayload)
return messageError("MsgTx.BtcDecode", str)
}
b := make([]byte, count)
err = readElement(r, b)
if err != nil {
@ -413,12 +472,22 @@ func readTxOut(r io.Reader, pver uint32, version uint32, to *TxOut) error {
return err
}
slen, err := readVarInt(r, pver)
count, err := readVarInt(r, pver)
if err != nil {
return err
}
b := make([]byte, slen)
// Prevent public key script larger than the max message size. It would
// be possible to cause memory exhaustion and panics without a sane
// upper bound on this count.
if count > uint64(maxMessagePayload) {
str := fmt.Sprintf("transaction output public key script is "+
"larger than max message size [count %d, max %d]",
count, maxMessagePayload)
return messageError("MsgTx.BtcDecode", str)
}
b := make([]byte, count)
err = readElement(r, b)
if err != nil {
return err