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LibGfx/WebPLossless: Fix out-of-bounds write in color indexing transform

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If a lossless webp has 3 or 4 colors, it uses 2 bits per pixel to
store an offset into a "color index" (which the spec explicitly does
not call palette since it says the 'color cache' is more like that).
This way, it can pack 4 pixels into a single pixel.

If the width of the output image wasn't evenly divisble by 4,
we used to write out-of-bounds in the last few columns of each
row, since we used to always write all 4 pixels.

Found by clusterfuzz. Probably fixes
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=66082

While here, spruce up the comments very slightly.
This commit is contained in:
Nico Weber 2024-03-19 19:28:27 -04:00 committed by Andreas Kling
parent c8b219914e
commit 7cb12e057f
Notes: sideshowbarker 2024-07-17 00:53:02 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/7cb12e057f
Pull-request: https://github.com/SerenityOS/serenity/pull/23645
Reviewed-by: https://github.com/trflynn89 ✅
1 changed files with 6 additions and 6 deletions
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12
Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossless.cpp
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@ -838,11 +838,11 @@ public:
// This returns how many output pixels one input pixel can encode after the color indexing transform.
//
// The spec isn't very explicit about this, but this affects all images after the color indexing transform:
// If a webp file contains a 32x32 image and it contains a color indexing transform with a 4-color palette, then the in-memory size of all images
// after the color indexing transform assume a bitmap size of (32/4)x32 = 8x32.
// If a webp file contains a 29x32 image and it contains a color indexing transform with a 4-color palette, then the in-memory size of all images
// after the color indexing transform assume a bitmap size of ceil_div(29, 4)x32 = 8x32.
// That is, the sizes of transforms after the color indexing transform are computed relative to the size 8x32,
// the main image's meta prefix image's size (if present) is comptued relative to the size 8x32,
// the main image is 8x32, and only applying the color indexing transform resizes the image back to 32x32.
// the main image's meta prefix image's size (if present) is computed relative to the size 8x32,
// the main image is 8x32, and only applying the color indexing transform resizes the image back to 29x32.
int pixels_per_pixel() const { return m_pixels_per_pixel; }
private:
@ -868,7 +868,7 @@ ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> ColorIndexingTransform::read(Litt
IntSize palette_image_size { color_table_size, 1 };
auto palette_bitmap = TRY(decode_webp_chunk_VP8L_image(ImageKind::EntropyCoded, BitmapFormat::BGRA8888, palette_image_size, bit_stream));
// "When the color table is small (equal to or less than 16 colors), several pixels are bundled into a single pixel...."
// "When the color table is small (equal to or less than 16 colors), several pixels are bundled into a single pixel..."
int pixels_per_pixel = 1;
if (color_table_size <= 2)
pixels_per_pixel = 8;
@ -913,7 +913,7 @@ ErrorOr<NonnullRefPtr<Bitmap>> ColorIndexingTransform::transform(NonnullRefPtr<B
for (int x = 0, new_x = 0; x < bitmap->width(); ++x, new_x += pixels_per_pixel()) {
u8 indexes = Color::from_argb(bitmap_scanline[x]).green();
for (int i = 0; i < pixels_per_pixel(); ++i) {
for (int i = 0; i < pixels_per_pixel() && new_x + i < new_bitmap->width(); ++i) {
u8 index = indexes & pixel_mask;
new_bitmap_scanline[new_x + i] = index < m_palette_bitmap->width() ? m_palette_bitmap->scanline(0)[index] : 0;
indexes >>= bits_per_pixel;