Re: [FFmpeg-devel] [PATCH v3] checkasm/h264dsp: Fix stack overflow in check_idct_dequant

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Zhao Zhili:
> 
> 
>> On Jun 16, 2025, at 17:46, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> wrote:
>>
>> Zhao Zhili:
>>> From: Zhao Zhili <zhilizhao@tencent.com>
>>>
>>> ---
>>> tests/checkasm/h264dsp.c | 20 +++++++++++++++-----
>>> 1 file changed, 15 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/tests/checkasm/h264dsp.c b/tests/checkasm/h264dsp.c
>>> index f5f9650224..a0f8fd858a 100644
>>> --- a/tests/checkasm/h264dsp.c
>>> +++ b/tests/checkasm/h264dsp.c
>>> @@ -328,25 +328,35 @@ static void check_idct_multiple(void)
>>> static void check_idct_dequant(void)
>>> {
>>>     static const int depths[5] = { 8, 9, 10, 12, 14 };
>>> -    LOCAL_ALIGNED_16(int16_t, src, [16]);
>>> -    /* Ensure dst buffers are large enough to hold dctcoefs of all bit-depths. */
>>> +    /* Ensure buffers are large enough to hold dctcoefs of all bit-depths. */
>>> +    LOCAL_ALIGNED_16(uint8_t, src_buf, [16 * sizeof(int32_t)]);
>>>     LOCAL_ALIGNED_16(uint8_t, dst0, [16 * 16 * sizeof(int32_t)]);
>>>     LOCAL_ALIGNED_16(uint8_t, dst1, [16 * 16 * sizeof(int32_t)]);
>>> +    int16_t *src = (int16_t *)src_buf;
>>>     int16_t *dst_ref = (int16_t *)dst0;
>>>     int16_t *dst_new = (int16_t *)dst1;
>>>     H264DSPContext h;
>>>     int bit_depth, i, qmul;
>>>     declare_func_emms(AV_CPU_FLAG_MMX | AV_CPU_FLAG_SSE2, void, int16_t *output, int16_t *input, int qmul);
>>>
>>> -    for (int j = 0; j < 16; j++)
>>> -        src[j] = (rnd() % 512) - 256;
>>> -
>>>     qmul = rnd() % 4096;
>>>
>>>     for (i = 0; i < FF_ARRAY_ELEMS(depths); i++) {
>>>         bit_depth = depths[i];
>>>         ff_h264dsp_init(&h, bit_depth, 1);
>>>
>>> +        if (bit_depth == 8) {
>>> +            for (size_t j = 0; j < 16; j++) {
>>> +                int16_t r = (rnd() % 512) - 256;
>>> +                AV_WN16A(&src_buf[j << 1], r);
>>> +            }
>>> +        } else {
>>> +            for (size_t j = 0; j < 16; j++) {
>>> +                int32_t r = (rnd() % (1 << (bit_depth + 1))) - (1 << bit_depth);
>>> +                AV_WN32A(&src_buf[j << 2], r);
>>> +            }
>>> +        }
>>> +
>>>         memset(dst0, 0, 16 * 16 * SIZEOF_COEF);
>>>         memset(dst1, 0, 16 * 16 * SIZEOF_COEF);
>>>
>>
>> This still has an effective-type violation: src_buf is of type uint8_t,
>> yet the ff_h264_luma_dc_dequant_idct functions will read it as
>> int16_t/int32_t. It also still has the downside that buffer overflows
>> for the 8bit case can go undetected.
> 
> A bunch of template has cast like 
> 
>     pixel *dst = (pixel *)_dst;
>     const pixel *src = (const pixel *)_src;
> 
> then read and write as int16_t.
> 
> And a bunch of checkasm use uint8_t[] array on stack as src and dst,
> which leading to UB.
> 
> This patch isn’t specific. And this patch add zero UB (it’s there before the patch,
> both src and dst are accessed as int32_t/int16_t while they are int16_t and uint8_t).
> 

This patch adds UB: src was int16_t before, so that the accesses in the
eight bit function were fine, but are not with this patch. Anyway, it is
irrelevant now.

- Andreas

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