x/crypto/ssh: Server panic on invalid input

[taruti]

The following input makes the x/crypto/ssh server crash:

[]byte("\n\x00\x00\x00\x16\x00\x140000000000000000\xff\xff\xff\xff")

Discovered by fuzzing (see the excellent github.com/dvyukov/go-fuzz), triggerable by

1. run x/crypto/ssh server
2. feed it those bytes (e.g. with nc or go)
3. watch it panic

panic: runtime error: slice bounds out of range

goroutine 5 [running]:
golang.org/x/crypto/ssh.parseString(0xc20801e391, 0x4, 0x4, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc20800a300)
    /go/path/src/golang.org/x/crypto/ssh/messages.go:490 +0x150
golang.org/x/crypto/ssh.parseNameList(0xc20801e391, 0x4, 0x4, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x15)
    /go/path/src/golang.org/x/crypto/ssh/messages.go:502 +0x95
golang.org/x/crypto/ssh.Unmarshal(0xc20801e391, 0x4, 0x4, 0x54e900, 0xc208001320, 0x0, 0x0)
    /go/path/src/golang.org/x/crypto/ssh/messages.go:360 +0xeff
golang.org/x/crypto/ssh.(*handshakeTransport).enterKeyExchange(0xc208056000, 0xc20801e380, 0x15, 0x15, 0x0, 0x0)
    /go/path/src/golang.org/x/crypto/ssh/handshake.go:295 +0x134
golang.org/x/crypto/ssh.(*handshakeTransport).readOnePacket(0xc208056000, 0x0, 0x0, 0x0, 0x0, 0x0)
    /go/path/src/golang.org/x/crypto/ssh/handshake.go:166 +0x1f3
golang.org/x/crypto/ssh.(*handshakeTransport).readLoop(0xc208056000)
    /go/path/src/golang.org/x/crypto/ssh/handshake.go:133 +0x28
created by golang.org/x/crypto/ssh.newServerTransport
    /go/path/src/golang.org/x/crypto/ssh/handshake.go:108 +0xea

goroutine 1 [chan receive]:
golang.org/x/crypto/ssh.(*connection).serverHandshake(0xc20804e200, 0xc208058210, 0xc20801f060, 0x0, 0x0)
    /go/path/src/golang.org/x/crypto/ssh/server.go:193 +0x64f
golang.org/x/crypto/ssh.NewServerConn(0x7f8486836d18, 0xc20801f060, 0xc208058000, 0x7f8486836d18, 0xc208030018, 0x0, 0x0, 0x0)
    /go/path/src/golang.org/x/crypto/ssh/server.go:146 +0x103
main.main()
    /go/path/src/github.com/taruti/sshfuzz/ex1/ssh.go:28 +0x32d

[taruti]

cc @agl

[ebfe]

https://golang.org/cl/11332

[dvyukov]

/cc @dvyukov

[ebfe]

Should be fixed by golang/crypto@cc04154