math/big: index out of range in Float.Add

[catenacyber]

What version of Go are you using (go version)?

$ go version
go version go1.19 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/root/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/root/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/root/.go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/root/.go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.19"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/src/ngolo-fuzzing/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2481516251=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run https://go.dev/play/p/JjDgRJRZXpk

What did you expect to see?

The program finishing and printing Hello

What did you see instead?

panic: runtime error: index out of range [-1]

goroutine 1 [running]:
math/big.fnorm({0xc000092000?, 0x0?, 0x10?})
	/usr/local/go-faketime/src/math/big/float.go:580 +0x9d
math/big.(*Float).uadd(0xc00008e030, 0xa?, 0x3500fffff9?)
	/usr/local/go-faketime/src/math/big/float.go:1253 +0x468
math/big.(*Float).Add(0xc00008e030, 0xc000074f34?, 0xc00006e000?)
	/usr/local/go-faketime/src/math/big/float.go:1466 +0x5a
main.main()
	/tmp/sandbox2865641649/prog.go:13 +0x8b


Program exited.

Found by https://github.com/catenacyber/ngolo-fuzzing with oss-fuzz :
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=54807

Another variant is https://go.dev/play/p/ELqX97rhBPF

cc @rolandshoemaker

Thanks for the report. I've looked at the issue and I think it can be disclosed publicly. The issue appears to be in the Gob decoder, which incorrectly accepts a malformed float which otherwise appears impossible to generate via Float-only operations.

[gopherbot]

Change https://go.dev/cl/463017 mentions this issue: math/big: validate result of Float.GobDecode