Integer overflow in `stbi__load_gif_main`

STBI_REALLOC_SIZED [1] in stbi__load_gif_main may overflow when layers is bigger than 1. However it doesn't seem to be exploitable because layers is incremented by one in the loop [2]. The first occurrence of layers == 2 will cause the signed integer overflow resulting in negative number, which casted to unsigned 64 bit number size_t in realloc will cause the realloc to fail and the program to return in [3].

static void *stbi__load_gif_main(stbi__context *s, int **delays, int *x, int *y, int *z, int *comp, int req_comp)
{
...
      do {
         u = stbi__gif_load_next(s, &g, comp, req_comp, two_back);
         if (u == (stbi_uc *) s) u = 0;  // end of animated gif marker

         if (u) {
            *x = g.w;
            *y = g.h;
            ++layers; // [2]
            stride = g.w * g.h * 4;

            if (out) {
               void *tmp = (stbi_uc*) STBI_REALLOC_SIZED( out, out_size, layers * stride ); // [1] int overflow
               if (!tmp)
                  return stbi__load_gif_main_outofmem(&g, out, delays); // [3]
               else {
                   out = (stbi_uc*) tmp;
                   out_size = layers * stride;
               }

               if (delays) {
                  int *new_delays = (int*) STBI_REALLOC_SIZED( *delays, delays_size, sizeof(int) * layers );
                  if (!new_delays)
                     return stbi__load_gif_main_outofmem(&g, out, delays);
                  *delays = new_delays;
                  delays_size = layers * sizeof(int);
               }
            } else {
               out = (stbi_uc*)stbi__malloc( layers * stride );
               if (!out)
                  return stbi__load_gif_main_outofmem(&g, out, delays);
               out_size = layers * stride;
               if (delays) {
                  *delays = (int*) stbi__malloc( layers * sizeof(int) );
                  if (!*delays)
                     return stbi__load_gif_main_outofmem(&g, out, delays);
                  delays_size = layers * sizeof(int);
               }
            }
            memcpy( out + ((layers - 1) * stride), u, stride );
...

Impact

It doesn't look like a potential security issue, but the signed integer overflow behavior is undefined according to C/C++ standard.

Resources

To reproduce the issue:

Make UBSAN 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,0x32,0x03,0x84,0xa9,0x97,
                            0x53,0x43,0x05,0xff,0xbe,0x21,0x00,0x30,0x03,0x01,0x47,0x49,
                            0x46,0x2c,0x00,0x00,0x00,0x00,0xbd,0x00,0x3f,0x71,0x00,0x01,
                            0x00,0x2c};
    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;
}

Set breakpoint at line 6993 in stbi__load_gif_main and run the program to hit the overflow.

/src/stb/tests/../stb_image.h:6993:39: runtime error: signed integer overflow: 2 * 1755853020 cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /src/stb/tests/../stb_image.h:6993:39 in

nothings / stb

Integer overflow in `stbi__load_gif_main` #1531

Impact

Resources