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Fix out-of-bounds access in motion compensation #10

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Fix out-of-bounds access in motion compensation #10

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26 changes: 25 additions & 1 deletion video.go
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Original file line number Diff line number Diff line change
Expand Up @@ -514,7 +514,7 @@ func (v *Video) decodeMacroblock() {
v.mbRow = v.macroblockAddress / v.mbWidth
v.mbCol = v.macroblockAddress % v.mbWidth

if v.mbCol >= v.mbWidth || v.mbRow >= v.mbHeight {
if v.macroblockAddress < 0 || v.mbCol >= v.mbWidth || v.mbRow >= v.mbHeight {
return // corrupt stream
}

Expand Down Expand Up @@ -605,6 +605,30 @@ func (v *Video) decodeMotionVector(rSize, motion int) int {
return motion
}

// macroblockInBounds reports whether motion compensation stays inside the frame planes.
func macroblockInBounds(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth int, s, d *Frame) bool {
if !blockInBounds(s.Y.Data, d.Y.Data, lumaWidth, 16, motionH, motionV, mbRow, mbCol) {
return false
}
cmH := motionH / 2
cmV := motionV / 2
return blockInBounds(s.Cb.Data, d.Cb.Data, chromaWidth, 8, cmH, cmV, mbRow, mbCol) &&
blockInBounds(s.Cr.Data, d.Cr.Data, chromaWidth, 8, cmH, cmV, mbRow, mbCol)
}

// blockInBounds allows reads into the padding after a plane, but not writes.
func blockInBounds(src, dst []byte, stride, size, motionH, motionV, mbRow, mbCol int) bool {
if mbRow < 0 || mbCol < 0 || mbCol*size+size > stride {
return false
}
di := mbRow*size*stride + mbCol*size
if di+(size-1)*stride+size > len(dst) {
return false
}
si := (mbRow*size+(motionV>>1))*stride + mbCol*size + (motionH >> 1)
return si >= 0 && si+(size-1+(motionV&1))*stride+size+(motionH&1) <= cap(src)
}

func (v *Video) predictMacroblock() {
fwH := v.motionForward.H
fwV := v.motionForward.V
Expand Down
3 changes: 3 additions & 0 deletions video_amd64.go
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,9 @@ func copyMacroblockSSE2(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth i
func copyMacroblockAVX2(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth int, s, d *Frame)

func copyMacroblock(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth int, s, d *Frame) {
if !macroblockInBounds(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth, s, d) {
return // corrupt stream
}
if isAVX2 {
copyMacroblockAVX2(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth, s, d)
} else {
Expand Down
3 changes: 3 additions & 0 deletions video_arm64.go
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Original file line number Diff line number Diff line change
Expand Up @@ -6,5 +6,8 @@ package mpeg
func copyMacroblockNEON(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth int, s, d *Frame)

func copyMacroblock(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth int, s, d *Frame) {
if !macroblockInBounds(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth, s, d) {
return // corrupt stream
}
copyMacroblockNEON(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth, s, d)
}
3 changes: 3 additions & 0 deletions video_noasm.go
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,9 @@ func bilinAvg(a, b, c, d uint64) uint64 {
}

func copyMacroblock(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth int, s, d *Frame) {
if !macroblockInBounds(motionH, motionV, mbRow, mbCol, lumaWidth, chromaWidth, s, d) {
return // corrupt stream
}
hp := motionH >> 1
vp := motionV >> 1
lsi := ((mbRow<<4)+vp)*lumaWidth + (mbCol << 4) + hp
Expand Down
108 changes: 108 additions & 0 deletions video_test.go
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Expand Up @@ -102,6 +102,114 @@ func runParitySweep(t *testing.T, fn copyMacroblockFunc) {
}
}

// newGuardedFrame returns a frame with each plane surrounded by mark bytes.
func newGuardedFrame(lumaWidth, chromaWidth, fill int, mark byte) (*Frame, [][]byte) {
const guard = 4096
f := newTestFrame(lumaWidth, chromaWidth, fill)
var backing [][]byte
carve := func(p *Plane) {
n := len(p.Data)
b := make([]byte, guard+n+guard)
for i := range b {
b[i] = mark
}
copy(b[guard:], p.Data)
p.Data = b[guard : guard+n : guard+n]
backing = append(backing, b)
}
carve(&f.Y)
carve(&f.Cb)
carve(&f.Cr)
return f, backing
}

func guardsIntact(backing [][]byte, mark byte) bool {
const guard = 4096
for _, b := range backing {
for i := 0; i < guard; i++ {
if b[i] != mark || b[len(b)-1-i] != mark {
return false
}
}
}
return true
}

func TestCopyMacroblockOutOfBounds(t *testing.T) {
const lumaWidth, chromaWidth = 64, 32
const mbs = lumaWidth / 16

cases := []struct {
name string
motionH, motionV, mbRow, mbCol int
}{
{"negative column", 0, 0, 0, -1},
{"negative row", 0, 0, -1, 0},
{"column past width", 0, 0, 0, mbs},
{"row past height", 0, 0, mbs, 0},
{"motion above frame", 0, -2048, 1, 1},
{"motion below frame", 0, 2048, 1, 1},
{"motion left of frame", -2048, 0, 0, 0},
{"motion right of frame", 2048, 0, mbs - 1, mbs - 1},
{"half-pel past bottom right", 1, 1, mbs - 1, mbs - 1},
}
for _, c := range cases {
t.Run(c.name, func(t *testing.T) {
src, srcBacking := newGuardedFrame(lumaWidth, chromaWidth, 1, 0x5A)
dst, dstBacking := newGuardedFrame(lumaWidth, chromaWidth, 0, 0xA5)
want := newTestFrame(lumaWidth, chromaWidth, 0)

copyMacroblock(c.motionH, c.motionV, c.mbRow, c.mbCol, lumaWidth, chromaWidth, src, dst)

if !guardsIntact(dstBacking, 0xA5) || !guardsIntact(srcBacking, 0x5A) {
t.Fatal("memory outside the frame planes was modified")
}
if string(dst.Y.Data) != string(want.Y.Data) ||
string(dst.Cb.Data) != string(want.Cb.Data) ||
string(dst.Cr.Data) != string(want.Cr.Data) {
t.Fatal("destination modified by an out of bounds macroblock")
}
})
}
}

// TestCopyMacroblockPaddedEdge checks half-pel reads into the frame padding are allowed.
func TestCopyMacroblockPaddedEdge(t *testing.T) {
const mbWidth, mbHeight = 4, 3
v := &Video{
width: mbWidth * 16,
height: mbHeight * 16,
lumaWidth: mbWidth * 16,
lumaHeight: mbHeight * 16,
chromaWidth: mbWidth * 8,
chromaHeight: mbHeight * 8,
}
var src, got, want Frame
v.initFrame(&src)
v.initFrame(&got)
v.initFrame(&want)
base := src.Y.Data[:cap(src.Y.Data)]
for i := range base {
base[i] = byte(i*131 + 7)
}

wide := src
wide.Y.Data = src.Y.Data[:cap(src.Y.Data)]
wide.Cb.Data = src.Cb.Data[:cap(src.Cb.Data)]
wide.Cr.Data = src.Cr.Data[:cap(src.Cr.Data)]

const motionH, motionV = 3, 3
mbRow, mbCol := mbHeight-1, mbWidth-1
copyMacroblock(motionH, motionV, mbRow, mbCol, v.lumaWidth, v.chromaWidth, &src, &got)
copyMacroblockRef(motionH, motionV, mbRow, mbCol, v.lumaWidth, v.chromaWidth, &wide, &want)

if string(got.Y.Data) != string(want.Y.Data) ||
string(got.Cb.Data) != string(want.Cb.Data) ||
string(got.Cr.Data) != string(want.Cr.Data) {
t.Fatal("bottom right macroblock not predicted from the padded reference")
}
}

func benchmarkCopyMacroblock(b *testing.B, motionH, motionV int) {
const lumaWidth, chromaWidth = 64, 32
src := newTestFrame(lumaWidth, chromaWidth, 1)
Expand Down