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xz/src/liblzma/common/index_decoder.c
Lasse Collin 30e95bb44c liblzma: Avoid null pointer + 0 (undefined behavior in C).
In the C99 and C17 standards, section 6.5.6 paragraph 8 means that
adding 0 to a null pointer is undefined behavior. As of writing,
"clang -fsanitize=undefined" (Clang 15) diagnoses this. However,
I'm not aware of any compiler that would take advantage of this
when optimizing (Clang 15 included). It's good to avoid this anyway
since compilers might some day infer that pointer arithmetic implies
that the pointer is not NULL. That is, the following foo() would then
unconditionally return 0, even for foo(NULL, 0):

    void bar(char *a, char *b);

    int foo(char *a, size_t n)
    {
        bar(a, a + n);
        return a == NULL;
    }

In contrast to C, C++ explicitly allows null pointer + 0. So if
the above is compiled as C++ then there is no undefined behavior
in the foo(NULL, 0) call.

To me it seems that changing the C standard would be the sane
thing to do (just add one sentence) as it would ensure that a huge
amount of old code won't break in the future. Based on web searches
it seems that a large number of codebases (where null pointer + 0
occurs) are being fixed instead to be future-proof in case compilers
will some day optimize based on it (like making the above foo(NULL, 0)
return 0) which in the worst case will cause security bugs.

Some projects don't plan to change it. For example, gnulib and thus
many GNU tools currently require that null pointer + 0 is defined:

    https://lists.gnu.org/archive/html/bug-gnulib/2021-11/msg00000.html

    https://www.gnu.org/software/gnulib/manual/html_node/Other-portability-assumptions.html

In XZ Utils null pointer + 0 issue should be fixed after this
commit. This adds a few if-statements and thus branches to avoid
null pointer + 0. These check for size > 0 instead of ptr != NULL
because this way bugs where size > 0 && ptr == NULL will likely
get caught quickly. None of them are in hot spots so it shouldn't
matter for performance.

A little less readable version would be replacing

    ptr + offset

with

    offset != 0 ? ptr + offset : ptr

or creating a macro for it:

    #define my_ptr_add(ptr, offset) \
            ((offset) != 0 ? ((ptr) + (offset)) : (ptr))

Checking for offset != 0 instead of ptr != NULL allows GCC >= 8.1,
Clang >= 7, and Clang-based ICX to optimize it to the very same code
as ptr + offset. That is, it won't create a branch. So for hot code
this could be a good solution to avoid null pointer + 0. Unfortunately
other compilers like ICC 2021 or MSVC 19.33 (VS2022) will create a
branch from my_ptr_add().

Thanks to Marcin Kowalczyk for reporting the problem:
https://github.com/tukaani-project/xz/issues/36
2023-02-23 20:41:22 +02:00

363 lines
8.7 KiB
C

///////////////////////////////////////////////////////////////////////////////
//
/// \file index_decoder.c
/// \brief Decodes the Index field
//
// Author: Lasse Collin
//
// This file has been put into the public domain.
// You can do whatever you want with this file.
//
///////////////////////////////////////////////////////////////////////////////
#include "index_decoder.h"
#include "check.h"
typedef struct {
enum {
SEQ_INDICATOR,
SEQ_COUNT,
SEQ_MEMUSAGE,
SEQ_UNPADDED,
SEQ_UNCOMPRESSED,
SEQ_PADDING_INIT,
SEQ_PADDING,
SEQ_CRC32,
} sequence;
/// Memory usage limit
uint64_t memlimit;
/// Target Index
lzma_index *index;
/// Pointer give by the application, which is set after
/// successful decoding.
lzma_index **index_ptr;
/// Number of Records left to decode.
lzma_vli count;
/// The most recent Unpadded Size field
lzma_vli unpadded_size;
/// The most recent Uncompressed Size field
lzma_vli uncompressed_size;
/// Position in integers
size_t pos;
/// CRC32 of the List of Records field
uint32_t crc32;
} lzma_index_coder;
static lzma_ret
index_decode(void *coder_ptr, const lzma_allocator *allocator,
const uint8_t *restrict in, size_t *restrict in_pos,
size_t in_size,
uint8_t *restrict out lzma_attribute((__unused__)),
size_t *restrict out_pos lzma_attribute((__unused__)),
size_t out_size lzma_attribute((__unused__)),
lzma_action action lzma_attribute((__unused__)))
{
lzma_index_coder *coder = coder_ptr;
// Similar optimization as in index_encoder.c
const size_t in_start = *in_pos;
lzma_ret ret = LZMA_OK;
while (*in_pos < in_size)
switch (coder->sequence) {
case SEQ_INDICATOR:
// Return LZMA_DATA_ERROR instead of e.g. LZMA_PROG_ERROR or
// LZMA_FORMAT_ERROR, because a typical usage case for Index
// decoder is when parsing the Stream backwards. If seeking
// backward from the Stream Footer gives us something that
// doesn't begin with Index Indicator, the file is considered
// corrupt, not "programming error" or "unrecognized file
// format". One could argue that the application should
// verify the Index Indicator before trying to decode the
// Index, but well, I suppose it is simpler this way.
if (in[(*in_pos)++] != INDEX_INDICATOR)
return LZMA_DATA_ERROR;
coder->sequence = SEQ_COUNT;
break;
case SEQ_COUNT:
ret = lzma_vli_decode(&coder->count, &coder->pos,
in, in_pos, in_size);
if (ret != LZMA_STREAM_END)
goto out;
coder->pos = 0;
coder->sequence = SEQ_MEMUSAGE;
// Fall through
case SEQ_MEMUSAGE:
if (lzma_index_memusage(1, coder->count) > coder->memlimit) {
ret = LZMA_MEMLIMIT_ERROR;
goto out;
}
// Tell the Index handling code how many Records this
// Index has to allow it to allocate memory more efficiently.
lzma_index_prealloc(coder->index, coder->count);
ret = LZMA_OK;
coder->sequence = coder->count == 0
? SEQ_PADDING_INIT : SEQ_UNPADDED;
break;
case SEQ_UNPADDED:
case SEQ_UNCOMPRESSED: {
lzma_vli *size = coder->sequence == SEQ_UNPADDED
? &coder->unpadded_size
: &coder->uncompressed_size;
ret = lzma_vli_decode(size, &coder->pos,
in, in_pos, in_size);
if (ret != LZMA_STREAM_END)
goto out;
ret = LZMA_OK;
coder->pos = 0;
if (coder->sequence == SEQ_UNPADDED) {
// Validate that encoded Unpadded Size isn't too small
// or too big.
if (coder->unpadded_size < UNPADDED_SIZE_MIN
|| coder->unpadded_size
> UNPADDED_SIZE_MAX)
return LZMA_DATA_ERROR;
coder->sequence = SEQ_UNCOMPRESSED;
} else {
// Add the decoded Record to the Index.
return_if_error(lzma_index_append(
coder->index, allocator,
coder->unpadded_size,
coder->uncompressed_size));
// Check if this was the last Record.
coder->sequence = --coder->count == 0
? SEQ_PADDING_INIT
: SEQ_UNPADDED;
}
break;
}
case SEQ_PADDING_INIT:
coder->pos = lzma_index_padding_size(coder->index);
coder->sequence = SEQ_PADDING;
// Fall through
case SEQ_PADDING:
if (coder->pos > 0) {
--coder->pos;
if (in[(*in_pos)++] != 0x00)
return LZMA_DATA_ERROR;
break;
}
// Finish the CRC32 calculation.
coder->crc32 = lzma_crc32(in + in_start,
*in_pos - in_start, coder->crc32);
coder->sequence = SEQ_CRC32;
// Fall through
case SEQ_CRC32:
do {
if (*in_pos == in_size)
return LZMA_OK;
if (((coder->crc32 >> (coder->pos * 8)) & 0xFF)
!= in[(*in_pos)++]) {
#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
return LZMA_DATA_ERROR;
#endif
}
} while (++coder->pos < 4);
// Decoding was successful, now we can let the application
// see the decoded Index.
*coder->index_ptr = coder->index;
// Make index NULL so we don't free it unintentionally.
coder->index = NULL;
return LZMA_STREAM_END;
default:
assert(0);
return LZMA_PROG_ERROR;
}
out:
// Update the CRC32.
//
// Avoid null pointer + 0 (undefined behavior) in "in + in_start".
// In such a case we had no input and thus in_used == 0.
{
const size_t in_used = *in_pos - in_start;
if (in_used > 0)
coder->crc32 = lzma_crc32(in + in_start,
in_used, coder->crc32);
}
return ret;
}
static void
index_decoder_end(void *coder_ptr, const lzma_allocator *allocator)
{
lzma_index_coder *coder = coder_ptr;
lzma_index_end(coder->index, allocator);
lzma_free(coder, allocator);
return;
}
static lzma_ret
index_decoder_memconfig(void *coder_ptr, uint64_t *memusage,
uint64_t *old_memlimit, uint64_t new_memlimit)
{
lzma_index_coder *coder = coder_ptr;
*memusage = lzma_index_memusage(1, coder->count);
*old_memlimit = coder->memlimit;
if (new_memlimit != 0) {
if (new_memlimit < *memusage)
return LZMA_MEMLIMIT_ERROR;
coder->memlimit = new_memlimit;
}
return LZMA_OK;
}
static lzma_ret
index_decoder_reset(lzma_index_coder *coder, const lzma_allocator *allocator,
lzma_index **i, uint64_t memlimit)
{
// Remember the pointer given by the application. We will set it
// to point to the decoded Index only if decoding is successful.
// Before that, keep it NULL so that applications can always safely
// pass it to lzma_index_end() no matter did decoding succeed or not.
coder->index_ptr = i;
*i = NULL;
// We always allocate a new lzma_index.
coder->index = lzma_index_init(allocator);
if (coder->index == NULL)
return LZMA_MEM_ERROR;
// Initialize the rest.
coder->sequence = SEQ_INDICATOR;
coder->memlimit = my_max(1, memlimit);
coder->count = 0; // Needs to be initialized due to _memconfig().
coder->pos = 0;
coder->crc32 = 0;
return LZMA_OK;
}
extern lzma_ret
lzma_index_decoder_init(lzma_next_coder *next, const lzma_allocator *allocator,
lzma_index **i, uint64_t memlimit)
{
lzma_next_coder_init(&lzma_index_decoder_init, next, allocator);
if (i == NULL)
return LZMA_PROG_ERROR;
lzma_index_coder *coder = next->coder;
if (coder == NULL) {
coder = lzma_alloc(sizeof(lzma_index_coder), allocator);
if (coder == NULL)
return LZMA_MEM_ERROR;
next->coder = coder;
next->code = &index_decode;
next->end = &index_decoder_end;
next->memconfig = &index_decoder_memconfig;
coder->index = NULL;
} else {
lzma_index_end(coder->index, allocator);
}
return index_decoder_reset(coder, allocator, i, memlimit);
}
extern LZMA_API(lzma_ret)
lzma_index_decoder(lzma_stream *strm, lzma_index **i, uint64_t memlimit)
{
lzma_next_strm_init(lzma_index_decoder_init, strm, i, memlimit);
strm->internal->supported_actions[LZMA_RUN] = true;
strm->internal->supported_actions[LZMA_FINISH] = true;
return LZMA_OK;
}
extern LZMA_API(lzma_ret)
lzma_index_buffer_decode(lzma_index **i, uint64_t *memlimit,
const lzma_allocator *allocator,
const uint8_t *in, size_t *in_pos, size_t in_size)
{
// Sanity checks
if (i == NULL || memlimit == NULL
|| in == NULL || in_pos == NULL || *in_pos > in_size)
return LZMA_PROG_ERROR;
// Initialize the decoder.
lzma_index_coder coder;
return_if_error(index_decoder_reset(&coder, allocator, i, *memlimit));
// Store the input start position so that we can restore it in case
// of an error.
const size_t in_start = *in_pos;
// Do the actual decoding.
lzma_ret ret = index_decode(&coder, allocator, in, in_pos, in_size,
NULL, NULL, 0, LZMA_RUN);
if (ret == LZMA_STREAM_END) {
ret = LZMA_OK;
} else {
// Something went wrong, free the Index structure and restore
// the input position.
lzma_index_end(coder.index, allocator);
*in_pos = in_start;
if (ret == LZMA_OK) {
// The input is truncated or otherwise corrupt.
// Use LZMA_DATA_ERROR instead of LZMA_BUF_ERROR
// like lzma_vli_decode() does in single-call mode.
ret = LZMA_DATA_ERROR;
} else if (ret == LZMA_MEMLIMIT_ERROR) {
// Tell the caller how much memory would have
// been needed.
*memlimit = lzma_index_memusage(1, coder.count);
}
}
return ret;
}