Pillow: NULL dereference in ImagingExpand via rank filters on zero-size images

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NULL dereference in ImagingExpand via rank filters on zero-size images
Location: src/libImaging/Filter.c:79  in ImagingExpand()
Commit:   a707c6b21c313880d0771e54965d142dd42358b6
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Applying Pillow's median, minimum, maximum or rank filter to an image with zero width or zero height crashes the whole Python process instead of raising an error. Empty images like this are easy to produce with ordinary calls, for example a crop box whose left and right edges (or top and bottom edges) are equal, or the second page of a crafted multi-page image file. An attacker who can supply crop coordinates or an uploaded file to an application that runs one of these filters can kill its worker process. The effect is a crash only, with no memory disclosure or corruption.

The fault is in ImagingExpand() in src/libImaging/Filter.c. This is the edge-padding step that RankFilter.filter() always runs through the core expand() method, so MedianFilter, MinFilter and MaxFilter go through it too. It fills the margin by copying the input's edge pixels, reading imIn->image[y][0] and rows 0 / ysize-1, and never checks that the input has any pixels. For an empty image, ImagingAllocateArray leaves the line pointers NULL, so the first read dereferences NULL and the process dies with SIGSEGV, in every mode.

Zero-size images reach this point through public API alone:

• Image.crop(), ImageOps.crop() and Image.new();
• a multi-page TIFF opened by path whose second IFD declares ImageLength = 0, on which seek(1) and load() succeed.

Every other ImageFilter returns an empty image for the same inputs. The zero-height case has crashed since the original PIL import, and the zero-width case since Pillow 4.3.0. This is separate from the margin == 0 fix in #9755: the path is the same, but here the degenerate value is the image size rather than the kernel size. It has the same shape as the zero-dimension ImagingOffset crash fixed under GHSA-f87v-9578-p47w.

The attached reproducer shows a one-pixel-wide control crop filtering normally, while four empty crops passed through the rank filters are each killed by SIGSEGV. A patch that returns early in ImagingExpand for empty input is included.

Details

ImagingExpand() (src/libImaging/Filter.c), the edge-padding helper that ImageFilter.RankFilter runs before the actual rank filter, reads the first pixel of each input line without checking that the input has any pixels. For an image with width 0 or height 0 the line pointers are NULL, so Image.filter() with MedianFilter, MinFilter, MaxFilter or RankFilter kills the interpreter with SIGSEGV instead of returning or raising:

from PIL import Image, ImageFilter
im = Image.new("L", (10, 10)).crop((3, 0, 3, 10))   # size (0, 10); crop() accepts left == right
im.filter(ImageFilter.MedianFilter(3))               # SIGSEGV

Under gdb on the stock build the fault is at ImagingExpand (imIn=…, margin=1) at src/libImaging/Filter.c:105 (the EXPAND(UINT8, image8) expansion), called from _expand_image at src/_imaging.c:1151, with imIn->xsize = 0 and imIn->image[0] = 0x0. The same happens for every mode the rank filters accept (L, RGB, RGBA, I, F), and the core expand(m) method with m >= 1 crashes the same way in any mode, including P. The same input through BoxBlur, GaussianBlur, Kernel, ModeFilter and UnsharpMask returns a image; a crop through also returns normally.

The unchecked edge reads date from the PIL import (9a640e315, 2010, so every Pillow release): the zero-height case has crashed since then, and the zero-width case since fd907fbdc ("Fix 0-width and 0-height images other way", Pillow 4.3.0), which stopped backing zero-width images with a dummy block and left their row pointers NULL. It is adjacent to the margin == 0 divide-by-zero fix in #9755 — same expand() → ImagingExpand() path, but the unhandled degenerate value here is the image size rather than the kernel size.

The path:

# src/PIL/ImageFilter.py:111-116   RankFilter.filter  (MedianFilter/MinFilter/MaxFilter subclass it)
        image = image.expand(self.size // 2)
        return image.rankfilter(self.size, self.rank)

expand(m) with m > 0 is ImagingExpand(self->image, m) (src/_imaging.c:1142-1151). It validates only margin, allocates an output of (xsize+2*margin) × (ysize+2*margin) — which is non-empty, so the allocation succeeds — and then fills the margin by replicating edge pixels of the input:

// src/libImaging/Filter.c:70-101
    imOut =
        ImagingNewDirty(imIn->mode, imIn->xsize + 2 * margin, imIn->ysize + 2 * margin);
    …
#define EXPAND_LINE(type, image, yin, yout)                       \
    {                                                             \
        for (x = 0; x < margin; x++) {                            \
            imOut->image[yout][x] = imIn->image[yin][0];          \   // line 79
        }                                                         \
        …                                                         \
        for (x = 0; x < margin; x++) {                            \
            imOut->image[yout][margin + imIn->xsize + x] =        \
                imIn->image[yin][imIn->xsize - 1];                \   // [-1] when xsize == 0
        }                                                         \
    }

#define EXPAND(type, image)                                                      \
    {                                                                            \
        for (y = 0; y < margin; y++) {                                           \
            EXPAND_LINE(type, image, 0, y);                                      \   // line 0 of a 0-line image
        }                                                                        \
        …
        for (y = 0; y < margin; y++) {                                           \
            EXPAND_LINE(type, image, imIn->ysize - 1, margin + imIn->ysize + y); \   // line -1 when ysize == 0
        }                                                                        \
    }

Why the line pointers are NULL: ImagingNewPrologueSubtype callocs the im->image pointer array (src/libImaging/Storage.c:216, at least one entry), and ImagingAllocateArray returns before assigning any of them for an empty image:

// src/libImaging/Storage.c:423-426
    /* 0-width or 0-height image. No need to do anything */
    if (!im->linesize || !im->ysize) {
        return im;
    }

So with xsize == 0 every imIn->image[y] is NULL and imIn->image[yin][0] faults; with ysize == 0 the top-margin pass reads imIn->image[0] (the single calloc'd NULL slot) and faults the same way, and the bottom-margin pass would index imIn->image[-1]. ImagingRankFilter is never reached.

Zero-size images are ordinary objects in Pillow and arrive at filter() through public API only:

• Image.crop(box) rejects only inverted boxes (src/PIL/Image.py:1385-1390); left == right or upper == lower yields a 0×N / N×0 image. ImageOps.crop(im, border) with border == width // 2, and Image.new(mode, (0, n)), do the same.
• From a file: ImageFile.__init__ refuses a zero first-frame size (src/PIL/ImageFile.py:167-173), but TiffImagePlugin._setup() takes ImageWidth/ImageLength of every later IFD as-is on seek() (src/PIL/TiffImagePlugin.py:1468-1481). For an uncompressed page of an image opened by path, ImageFile.load() takes the mmap branch (src/PIL/ImageFile.py:307, 325-347) and Image.core.map_buffer builds an 8×0 core image without complaint (), so succeeds and a following segfaults in exactly as