rik/nanopb
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity
Files
29479a7cca1c7f52f8da57fe3ba76459fbb8a63c
nanopb/pb_decode.c
Petteri Aimonen 29479a7cca Protect against size_t overflows in pb_dec_bytes/pb_dec_string. 
Possible consequences of bug:
1) Denial of service by causing a crash
   Possible when all of the following apply:
      - Untrusted data is passed to pb_decode()
      - The top-level message contains a static string field as the first field.
   Causes a single write of '0' byte to 1 byte before the message struct.

2) Remote code execution
   Possible when all of the following apply:
      - 64-bit platform
      - The message or a submessage contains a static string field.
      - Decoding directly from a custom pb_istream_t
      - bytes_left on the stream is set to larger than 4 GB
   Causes a write of up to 4 GB of data past the string field.

--

Detailed analysis follows

In the following consideration, I define "platform bitness" as equal to
number of bits in size_t datatype. Therefore most 8-bit platforms are
regarded as 16-bit for the purposes of this discussion.

1. The overflow in pb_dec_string

The overflow happens in this computation:

uint32_t size;
size_t alloc_size;
alloc_size = size + 1;

There are two ways in which the overflow can occur: In the uint32_t
addition, or in the cast to size_t. This depends on the platform
bitness.

On 32- and 64-bit platforms, the size has to be UINT32_MAX for the
overflow to occur. In that case alloc_size will be 0.

On 16-bit platforms, overflow will happen whenever size is more than
UINT16_MAX, and resulting alloc_size is attacker controlled.

For static fields, the alloc_size value is just checked against the
field data size. For pointer fields, the alloc_size value is passed to
malloc(). End result in both cases is the same, the storage is 0 or
just a few bytes in length.

On 16-bit platforms, another overflow occurs in the call to pb_read(),
when passing the original size. An attacker will want the passed value
to be larger than the alloc_size, therefore the only reasonable choice
is to have size = UINT16_MAX and alloc_size = 0. Any larger multiple
will truncate to the same values.

At this point we have read atleast the tag and the string length of the
message, i.e. atleast 3 bytes. The maximum initial value for stream
bytes_left is SIZE_MAX, thus at this point at most SIZE_MAX-3 bytes are
remaining.

On 32-bit and 16-bit platforms this means that the size passed to
pb_read() is always larger than the number of remaining bytes. This
causes pb_read() to fail immediately, before reading any bytes.

On 64-bit platforms, it is possible for the bytes_left value to be set
to a value larger than UINT32_MAX, which is the wraparound point in
size calculation. In this case pb_read() will succeed and write up to 4
GB of attacker controlled data over the RAM that comes after the string
field.

On all platforms, there is an unconditional write of a terminating null
byte. Because the size of size_t typically reflects the size of the
processor address space, a write at UINT16_MAX or UINT32_MAX bytes
after the string field actually wraps back to before the string field.
Consequently, on 32-bit and 16-bit platforms, the bug causes a single
write of '0' byte at one byte before the string field.

If the string field is in the middle of a message, this will just
corrupt other data in the message struct. Because the message contents
is attacker controlled anyway, this is a non-issue. However, if the
string field is the first field in the top-level message, it can
corrupt other data on the stack/heap before it. Typically a single '0'
write at a location not controlled by attacker is enough only for a
denial-of-service attack.

When using pointer fields and malloc(), the attacker controlled
alloc_size will cause a 0-size allocation to happen. By the same logic
as before, on 32-bit and 16-bit platforms this causes a '0' byte write
only. On 64-bit platforms, however, it will again allow up to 4 GB of
malicious data to be written over memory, if the stream length allows
the read.

2. The overflow in pb_dec_bytes

This overflow happens in the PB_BYTES_ARRAY_T_ALLOCSIZE macro:

The computation is done in size_t data type this time. This means that
an overflow is possible only when n is larger than SIZE_MAX -
offsetof(..). The offsetof value in this case is equal to
sizeof(pb_size_t) bytes.

Because the incoming size value is limited to 32 bits, no overflow can
happen here on 64-bit platforms.

The size will be passed to pb_read(). Like before, on 32-bit and 16-bit
platforms the read will always fail before writing anything.

This leaves only the write of bdest->size as exploitable. On statically
allocated fields, the size field will always be allocated, regardless
of alloc_size. In this case, no buffer overflow is possible here, but
user code could possibly use the attacker controlled size value and
read past a buffer.

If the field is allocated through malloc(), this will allow a write of
sizeof(pb_size_t) attacker controlled bytes to past a 0-byte long
buffer. In typical malloc implementations, this will either fit in
unused alignment padding area, or cause a heap corruption and a crash.
Under very exceptional situation it could allow attacker to influence
the behaviour of malloc(), possibly jumping into an attacker-controlled
location and thus leading to remote code execution.
2014-09-08 18:35:50 +03:00

735 lines
20 KiB
C
Raw Blame History

/* pb_decode.c -- decode a protobuf using minimal resources
*
* 2011 Petteri Aimonen <jpa@kapsi.fi>
*/
/* The warn_unused_result attribute appeared first in gcc-3.4.0 */
#if !defined(__GNUC__) || ( __GNUC__ < 3) || (__GNUC__ == 3 && __GNUC_MINOR__ < 4)
#define checkreturn
#else
/* Verify that we remember to check all return values for proper error propagation */
#define checkreturn __attribute__((warn_unused_result))
#endif
#define NANOPB_INTERNALS
#include "pb.h"
#include "pb_decode.h"
#include <string.h>
typedef bool (*pb_decoder_t)(pb_istream_t *stream, const pb_field_t *field, void *dest) checkreturn;
/* --- Function pointers to field decoders ---
* Order in the array must match pb_action_t LTYPE numbering.
*/
static const pb_decoder_t PB_DECODERS[PB_LTYPES_COUNT] = {
&pb_dec_varint,
&pb_dec_svarint,
&pb_dec_fixed32,
&pb_dec_fixed64,
&pb_dec_bytes,
&pb_dec_string,
&pb_dec_submessage
};
/**************
* pb_istream *
**************/
static bool checkreturn buf_read(pb_istream_t *stream, uint8_t *buf, size_t count)
{
uint8_t *source = (uint8_t*)stream->state;
stream->state = source + count;
if (buf != NULL)
{
while (count--)
*buf++ = *source++;
}
return true;
}
bool checkreturn pb_read(pb_istream_t *stream, uint8_t *buf, size_t count)
{
#ifndef PB_BUFFER_ONLY
if (buf == NULL && stream->callback != buf_read)
{
/* Skip input bytes */
uint8_t tmp[16];
while (count > 16)
{
if (!pb_read(stream, tmp, 16))
return false;
count -= 16;
}
return pb_read(stream, tmp, count);
}
#endif
if (stream->bytes_left < count)
PB_RETURN_ERROR(stream, "end-of-stream");
#ifndef PB_BUFFER_ONLY
if (!stream->callback(stream, buf, count))
PB_RETURN_ERROR(stream, "io error");
#else
if (!buf_read(stream, buf, count))
return false;
#endif
stream->bytes_left -= count;
return true;
}
pb_istream_t pb_istream_from_buffer(uint8_t *buf, size_t bufsize)
{
pb_istream_t stream;
#ifdef PB_BUFFER_ONLY
stream.callback = NULL;
#else
stream.callback = &buf_read;
#endif
stream.state = buf;
stream.bytes_left = bufsize;
#ifndef PB_NO_ERRMSG
stream.errmsg = NULL;
#endif
return stream;
}
/********************
* Helper functions *
********************/
static bool checkreturn pb_decode_varint32(pb_istream_t *stream, uint32_t *dest)
{
uint8_t byte;
uint32_t result;
if (!pb_read(stream, &byte, 1))
return false;
if (!(byte & 0x80))
{
/* Quick case, 1 byte value */
result = byte;
}
else
{
/* Multibyte case */
uint8_t bitpos = 7;
result = byte & 0x7F;
do
{
if (bitpos >= 32)
PB_RETURN_ERROR(stream, "varint overflow");
if (!pb_read(stream, &byte, 1))
return false;
result |= (uint32_t)(byte & 0x7F) << bitpos;
bitpos = (uint8_t)(bitpos + 7);
} while (byte & 0x80);
}
*dest = result;
return true;
}
bool checkreturn pb_decode_varint(pb_istream_t *stream, uint64_t *dest)
{
uint8_t byte;
uint8_t bitpos = 0;
uint64_t result = 0;
do
{
if (bitpos >= 64)
PB_RETURN_ERROR(stream, "varint overflow");
if (!pb_read(stream, &byte, 1))
return false;
result |= (uint64_t)(byte & 0x7F) << bitpos;
bitpos = (uint8_t)(bitpos + 7);
} while (byte & 0x80);
*dest = result;
return true;
}
bool checkreturn pb_skip_varint(pb_istream_t *stream)
{
uint8_t byte;
do
{
if (!pb_read(stream, &byte, 1))
return false;
} while (byte & 0x80);
return true;
}
bool checkreturn pb_skip_string(pb_istream_t *stream)
{
uint32_t length;
if (!pb_decode_varint32(stream, &length))
return false;
return pb_read(stream, NULL, length);
}
bool checkreturn pb_decode_tag(pb_istream_t *stream, pb_wire_type_t *wire_type, uint32_t *tag, bool *eof)
{
uint32_t temp;
*eof = false;
*wire_type = (pb_wire_type_t) 0;
*tag = 0;
if (!pb_decode_varint32(stream, &temp))
{
if (stream->bytes_left == 0)
*eof = true;
return false;
}
if (temp == 0)
{
*eof = true; /* Special feature: allow 0-terminated messages. */
return false;
}
*tag = temp >> 3;
*wire_type = (pb_wire_type_t)(temp & 7);
return true;
}
bool checkreturn pb_skip_field(pb_istream_t *stream, pb_wire_type_t wire_type)
{
switch (wire_type)
{
case PB_WT_VARINT: return pb_skip_varint(stream);
case PB_WT_64BIT: return pb_read(stream, NULL, 8);
case PB_WT_STRING: return pb_skip_string(stream);
case PB_WT_32BIT: return pb_read(stream, NULL, 4);
default: PB_RETURN_ERROR(stream, "invalid wire_type");
}
}
/* Read a raw value to buffer, for the purpose of passing it to callback as
* a substream. Size is maximum size on call, and actual size on return.
*/
static bool checkreturn read_raw_value(pb_istream_t *stream, pb_wire_type_t wire_type, uint8_t *buf, size_t *size)
{
size_t max_size = *size;
switch (wire_type)
{
case PB_WT_VARINT:
*size = 0;
do
{
(*size)++;
if (*size > max_size) return false;
if (!pb_read(stream, buf, 1)) return false;
} while (*buf++ & 0x80);
return true;
case PB_WT_64BIT:
*size = 8;
return pb_read(stream, buf, 8);
case PB_WT_32BIT:
*size = 4;
return pb_read(stream, buf, 4);
default: PB_RETURN_ERROR(stream, "invalid wire_type");
}
}
/* Decode string length from stream and return a substream with limited length.
* Remember to close the substream using pb_close_string_substream().
*/
bool checkreturn pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream)
{
uint32_t size;
if (!pb_decode_varint32(stream, &size))
return false;
*substream = *stream;
if (substream->bytes_left < size)
PB_RETURN_ERROR(stream, "parent stream too short");
substream->bytes_left = size;
stream->bytes_left -= size;
return true;
}
void pb_close_string_substream(pb_istream_t *stream, pb_istream_t *substream)
{
stream->state = substream->state;
#ifndef PB_NO_ERRMSG
stream->errmsg = substream->errmsg;
#endif
}
/* Iterator for pb_field_t list */
typedef struct {
const pb_field_t *start; /* Start of the pb_field_t array */
const pb_field_t *current; /* Current position of the iterator */
unsigned field_index; /* Zero-based index of the field. */
unsigned required_field_index; /* Zero-based index that counts only the required fields */
void *dest_struct; /* Pointer to the destination structure to decode to */
void *pData; /* Pointer where to store current field value */
void *pSize; /* Pointer where to store the size of current array field */
} pb_field_iterator_t;
static void pb_field_init(pb_field_iterator_t *iter, const pb_field_t *fields, void *dest_struct)
{
iter->start = iter->current = fields;
iter->field_index = 0;
iter->required_field_index = 0;
iter->pData = (char*)dest_struct + iter->current->data_offset;
iter->pSize = (char*)iter->pData + iter->current->size_offset;
iter->dest_struct = dest_struct;
}
static bool pb_field_next(pb_field_iterator_t *iter)
{
bool notwrapped = true;
size_t prev_size = iter->current->data_size;
if (PB_HTYPE(iter->current->type) == PB_HTYPE_ARRAY)
prev_size *= iter->current->array_size;
if (PB_HTYPE(iter->current->type) == PB_HTYPE_REQUIRED)
iter->required_field_index++;
iter->current++;
iter->field_index++;
if (iter->current->tag == 0)
{
iter->current = iter->start;
iter->field_index = 0;
iter->required_field_index = 0;
iter->pData = iter->dest_struct;
prev_size = 0;
notwrapped = false;
}
iter->pData = (char*)iter->pData + prev_size + iter->current->data_offset;
iter->pSize = (char*)iter->pData + iter->current->size_offset;
return notwrapped;
}
static bool checkreturn pb_field_find(pb_field_iterator_t *iter, uint32_t tag)
{
unsigned start = iter->field_index;
do {
if (iter->current->tag == tag)
return true;
pb_field_next(iter);
} while (iter->field_index != start);
return false;
}
/*************************
* Decode a single field *
*************************/
static bool checkreturn decode_field(pb_istream_t *stream, pb_wire_type_t wire_type, pb_field_iterator_t *iter)
{
pb_decoder_t func = PB_DECODERS[PB_LTYPE(iter->current->type)];
switch (PB_HTYPE(iter->current->type))
{
case PB_HTYPE_REQUIRED:
return func(stream, iter->current, iter->pData);
case PB_HTYPE_OPTIONAL:
*(bool*)iter->pSize = true;
return func(stream, iter->current, iter->pData);
case PB_HTYPE_ARRAY:
if (wire_type == PB_WT_STRING
&& PB_LTYPE(iter->current->type) <= PB_LTYPE_LAST_PACKABLE)
{
/* Packed array */
bool status = true;
size_t *size = (size_t*)iter->pSize;
pb_istream_t substream;
if (!pb_make_string_substream(stream, &substream))
return false;
while (substream.bytes_left && *size < iter->current->array_size)
{
void *pItem = (uint8_t*)iter->pData + iter->current->data_size * (*size);
if (!func(&substream, iter->current, pItem))
{
status = false;
break;
}
(*size)++;
}
pb_close_string_substream(stream, &substream);
if (substream.bytes_left != 0)
PB_RETURN_ERROR(stream, "array overflow");
return status;
}
else
{
/* Repeated field */
size_t *size = (size_t*)iter->pSize;
void *pItem = (uint8_t*)iter->pData + iter->current->data_size * (*size);
if (*size >= iter->current->array_size)
PB_RETURN_ERROR(stream, "array overflow");
(*size)++;
return func(stream, iter->current, pItem);
}
case PB_HTYPE_CALLBACK:
{
pb_callback_t *pCallback = (pb_callback_t*)iter->pData;
if (pCallback->funcs.decode == NULL)
return pb_skip_field(stream, wire_type);
if (wire_type == PB_WT_STRING)
{
pb_istream_t substream;
if (!pb_make_string_substream(stream, &substream))
return false;
while (substream.bytes_left)
{
if (!pCallback->funcs.decode(&substream, iter->current, pCallback->arg))
PB_RETURN_ERROR(stream, "callback failed");
}
pb_close_string_substream(stream, &substream);
return true;
}
else
{
/* Copy the single scalar value to stack.
* This is required so that we can limit the stream length,
* which in turn allows to use same callback for packed and
* not-packed fields. */
pb_istream_t substream;
uint8_t buffer[10];
size_t size = sizeof(buffer);
if (!read_raw_value(stream, wire_type, buffer, &size))
return false;
substream = pb_istream_from_buffer(buffer, size);
return pCallback->funcs.decode(&substream, iter->current, pCallback->arg);
}
}
default:
PB_RETURN_ERROR(stream, "invalid field type");
}
}
/* Initialize message fields to default values, recursively */
static void pb_message_set_to_defaults(const pb_field_t fields[], void *dest_struct)
{
pb_field_iterator_t iter;
pb_field_init(&iter, fields, dest_struct);
/* Initialize size/has fields and apply default values */
do
{
if (iter.current->tag == 0)
continue;
/* Initialize the size field for optional/repeated fields to 0. */
if (PB_HTYPE(iter.current->type) == PB_HTYPE_OPTIONAL)
{
*(bool*)iter.pSize = false;
}
else if (PB_HTYPE(iter.current->type) == PB_HTYPE_ARRAY)
{
*(size_t*)iter.pSize = 0;
continue; /* Array is empty, no need to initialize contents */
}
/* Initialize field contents to default value */
if (PB_HTYPE(iter.current->type) == PB_HTYPE_CALLBACK)
{
continue; /* Don't overwrite callback */
}
else if (PB_LTYPE(iter.current->type) == PB_LTYPE_SUBMESSAGE)
{
pb_message_set_to_defaults((const pb_field_t *) iter.current->ptr, iter.pData);
}
else if (iter.current->ptr != NULL)
{
memcpy(iter.pData, iter.current->ptr, iter.current->data_size);
}
else
{
memset(iter.pData, 0, iter.current->data_size);
}
} while (pb_field_next(&iter));
}
/*********************
* Decode all fields *
*********************/
bool checkreturn pb_decode_noinit(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct)
{
uint8_t fields_seen[(PB_MAX_REQUIRED_FIELDS + 7) / 8] = {0}; /* Used to check for required fields */
pb_field_iterator_t iter;
pb_field_init(&iter, fields, dest_struct);
while (stream->bytes_left)
{
uint32_t tag;
pb_wire_type_t wire_type;
bool eof;
if (!pb_decode_tag(stream, &wire_type, &tag, &eof))
{
if (eof)
break;
else
return false;
}
if (!pb_field_find(&iter, tag))
{
/* No match found, skip data */
if (!pb_skip_field(stream, wire_type))
return false;
continue;
}
if (PB_HTYPE(iter.current->type) == PB_HTYPE_REQUIRED
&& iter.required_field_index < PB_MAX_REQUIRED_FIELDS)
{
fields_seen[iter.required_field_index >> 3] |= (uint8_t)(1 << (iter.required_field_index & 7));
}
if (!decode_field(stream, wire_type, &iter))
return false;
}
/* Check that all required fields were present. */
{
/* First figure out the number of required fields by
* seeking to the end of the field array. Usually we
* are already close to end after decoding.
*/
unsigned req_field_count;
pb_type_t last_type;
unsigned i;
do {
req_field_count = iter.required_field_index;
last_type = iter.current->type;
} while (pb_field_next(&iter));
/* Fixup if last field was also required. */
if (PB_HTYPE(last_type) == PB_HTYPE_REQUIRED)
req_field_count++;
/* Check the whole bytes */
for (i = 0; i < (req_field_count >> 3); i++)
{
if (fields_seen[i] != 0xFF)
PB_RETURN_ERROR(stream, "missing required field");
}
/* Check the remaining bits */
if (fields_seen[req_field_count >> 3] != (0xFF >> (8 - (req_field_count & 7))))
PB_RETURN_ERROR(stream, "missing required field");
}
return true;
}
bool checkreturn pb_decode(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct)
{
pb_message_set_to_defaults(fields, dest_struct);
return pb_decode_noinit(stream, fields, dest_struct);
}
/* Field decoders */
bool pb_decode_svarint(pb_istream_t *stream, int64_t *dest)
{
uint64_t value;
if (!pb_decode_varint(stream, &value))
return false;
if (value & 1)
*dest = (int64_t)(~(value >> 1));
else
*dest = (int64_t)(value >> 1);
return true;
}
bool pb_decode_fixed32(pb_istream_t *stream, void *dest)
{
#ifdef __BIG_ENDIAN__
uint8_t *bytes = (uint8_t*)dest;
uint8_t lebytes[4];
if (!pb_read(stream, lebytes, 4))
return false;
bytes[0] = lebytes[3];
bytes[1] = lebytes[2];
bytes[2] = lebytes[1];
bytes[3] = lebytes[0];
return true;
#else
return pb_read(stream, (uint8_t*)dest, 4);
#endif
}
bool pb_decode_fixed64(pb_istream_t *stream, void *dest)
{
#ifdef __BIG_ENDIAN__
uint8_t *bytes = (uint8_t*)dest;
uint8_t lebytes[8];
if (!pb_read(stream, lebytes, 8))
return false;
bytes[0] = lebytes[7];
bytes[1] = lebytes[6];
bytes[2] = lebytes[5];
bytes[3] = lebytes[4];
bytes[4] = lebytes[3];
bytes[5] = lebytes[2];
bytes[6] = lebytes[1];
bytes[7] = lebytes[0];
return true;
#else
return pb_read(stream, (uint8_t*)dest, 8);
#endif
}
bool checkreturn pb_dec_varint(pb_istream_t *stream, const pb_field_t *field, void *dest)
{
uint64_t value;
bool status = pb_decode_varint(stream, &value);
switch (field->data_size)
{
case 1: *(uint8_t*)dest = (uint8_t)value; break;
case 2: *(uint16_t*)dest = (uint16_t)value; break;
case 4: *(uint32_t*)dest = (uint32_t)value; break;
case 8: *(uint64_t*)dest = value; break;
default: PB_RETURN_ERROR(stream, "invalid data_size");
}
return status;
}
bool checkreturn pb_dec_svarint(pb_istream_t *stream, const pb_field_t *field, void *dest)
{
int64_t value;
bool status = pb_decode_svarint(stream, &value);
switch (field->data_size)
{
case 4: *(int32_t*)dest = (int32_t)value; break;
case 8: *(int64_t*)dest = value; break;
default: PB_RETURN_ERROR(stream, "invalid data_size");
}
return status;
}
bool checkreturn pb_dec_fixed32(pb_istream_t *stream, const pb_field_t *field, void *dest)
{
UNUSED(field);
return pb_decode_fixed32(stream, dest);
}
bool checkreturn pb_dec_fixed64(pb_istream_t *stream, const pb_field_t *field, void *dest)
{
UNUSED(field);
return pb_decode_fixed64(stream, dest);
}
bool checkreturn pb_dec_bytes(pb_istream_t *stream, const pb_field_t *field, void *dest)
{
pb_bytes_array_t *x = (pb_bytes_array_t*)dest;
uint32_t temp;
if (!pb_decode_varint32(stream, &temp))
return false;
x->size = temp;
if (x->size < temp)
PB_RETURN_ERROR(stream, "size too large");
/* Check length, noting the space taken by the size_t header. */
if (x->size > field->data_size - offsetof(pb_bytes_array_t, bytes))
PB_RETURN_ERROR(stream, "bytes overflow");
return pb_read(stream, x->bytes, x->size);
}
bool checkreturn pb_dec_string(pb_istream_t *stream, const pb_field_t *field, void *dest)
{
uint32_t size;
size_t alloc_size;
bool status;
if (!pb_decode_varint32(stream, &size))
return false;
alloc_size = size + 1;
if (alloc_size < size)
PB_RETURN_ERROR(stream, "size too large");
/* Check length, noting the null terminator */
if (alloc_size > field->data_size)
PB_RETURN_ERROR(stream, "string overflow");
status = pb_read(stream, (uint8_t*)dest, size);
*((uint8_t*)dest + size) = 0;
return status;
}
bool checkreturn pb_dec_submessage(pb_istream_t *stream, const pb_field_t *field, void *dest)
{
bool status;
pb_istream_t substream;
const pb_field_t* submsg_fields = (const pb_field_t*)field->ptr;
if (!pb_make_string_substream(stream, &substream))
return false;
if (field->ptr == NULL)
PB_RETURN_ERROR(stream, "invalid field descriptor");
/* New array entries need to be initialized, while required and optional
* submessages have already been initialized in the top-level pb_decode. */
if (PB_HTYPE(field->type) == PB_HTYPE_ARRAY)
status = pb_decode(&substream, submsg_fields, dest);
else
status = pb_decode_noinit(&substream, submsg_fields, dest);
pb_close_string_substream(stream, &substream);
return status;
}
Reference in New Issue View Git Blame Copy Permalink