GHSA-654x-cwwv-5j9c

Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT > 1

### Summary When [`PILLOW_ALIGNMENT`](https://pillow.readthedocs.io/en/stable/reference/block_allocator.html#memory-pools) is set above 1, exporting an image to Arrow returns uninitialized heap memory mixed in with the pixel data. That memory can include pixels from images the process freed earlier. ### Details With an alignment above 1, the default allocator pads each row up to `aligned_linesize` and starts the first row at an aligned offset into the block. It then exports `im->blocks[0].ptr` as `xsize * ysize` tightly packed pixels, so exported data includes the padding bytes, and can include a few bytes from before the first row. Tested vulnerable on 11.2.1, 12.3.0, 4c1ed27a5. ### PoC Whether things leak may depend on allocator configuration. This does show a leak on my Mac. ```python import os os.environ["PILLOW_ALIGNMENT"] = "64" from PIL import Image # Image A: 64 px wide, so its rows fill the whole aligned stride a = Image.new("L", (64, 40), 0x41).transpose(Image.Transpose.FLIP_LEFT_RIGHT) del a # Image B: 61 px wide, so each row has 3 bytes of padding; its pixels are all 0 b = Image.new("L", (61, 40), 0).transpose(Image.Transpose.FLIP_LEFT_RIGHT) try: exported = Image.fromarrow(b, b.mode, b.size).tobytes() except ValueError as e: print("Not vulnerable:", e) else: if leaked := len(exported) - exported.count(0): raise RuntimeError(f"{leaked} of {len(exported)} exported bytes are not B's pixels") ``` ### Impact Information disclosure for applications that set `PILLOW_ALIGNMENT` and export images to Arrow. Likely any image whose row length isn't a multiple of the alignment will do. In a service handling several users' images, one user's exported data can contain pixels from another user's image, or other stale heap contents.