image/png: fix interlaced palette out-of-bounds

PNG images can be paletted, where each pixel value (a uint8) indexes a
slice of colors. In terms of wire format, the PLTE chunk explicitly
contains the palette length. However, in practice, some arguably
malformed images contain pixel values greater than or equal to the
explicit PLTE length.

Go's image/png decoder accomodates such images by lengthening the
decoded image's palette if the implicit maximum is larger than the
explicit maximum. This was already done, prior to this commit, by the
"if len(paletted.Palette) <= int(idx)" lines in decoder.readImagePass.

Separately, PNG images can also be interlaced, where the final image is
the result of merging multiple partial images, also called passes. Prior
to this commit, we applied the palette lengthening to the pass images
but not the final image. This commit fixes that.

Fixes #48612

Change-Id: I77606538cc9a504fbd726071756ebcd10c9da73f
Reviewed-on: https://go-review.googlesource.com/c/go/+/354709
Trust: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

diff --git a/src/image/png/reader.go b/src/image/png/reader.go
index 910520bd4b343037a2941494626cd00420f6533d..4c65038cb5bd534d147b212fefca158b5e4b1b92 100644 (file)
--- a/src/image/png/reader.go
+++ b/src/image/png/reader.go
@@ -821,9 +821,17 @@ func (d *decoder) mergePassInto(dst image.Image, src image.Image, pass int) {
                dstPix, stride, rect = target.Pix, target.Stride, target.Rect
                bytesPerPixel = 8
        case *image.Paletted:
-               srcPix = src.(*image.Paletted).Pix
+               source := src.(*image.Paletted)
+               srcPix = source.Pix
                dstPix, stride, rect = target.Pix, target.Stride, target.Rect
                bytesPerPixel = 1
+               if len(target.Palette) < len(source.Palette) {
+                       // readImagePass can return a paletted image whose implicit palette
+                       // length (one more than the maximum Pix value) is larger than the
+                       // explicit palette length (what's in the PLTE chunk). Make the
+                       // same adjustment here.
+                       target.Palette = source.Palette
+               }
        case *image.RGBA:
                srcPix = src.(*image.RGBA).Pix
                dstPix, stride, rect = target.Pix, target.Stride, target.Rect