internal/rand: RandStr and (*Rand).Str should have limits to their lengths lest cause memory hogs and if used can caused a Denial-of-Service

[odeke-em]

Found by fuzzing and also by plain auditing, if I pass in a large integer into (*Rand).Str, like in this test

func TestRandStrHuge(t *testing.T) {
        _ = RandStr(8455134228352958140)
}

it'll consume boundless amounts of memory until it is killed by my kernel

Explanation

Due to its logic that just tries to get alphanumeric strings of the input length, with no length limits, it'll keep doing lots of work and consume endless amounts of memory

iavl/internal/rand/random.go

Lines 90 to 110 in d0241db

	func (r *Rand) Str(length int) string {
	chars := []byte{}
	MAIN_LOOP:
	for {
	val := r.Int63()
	for i := 0; i < 10; i++ {
	v := int(val & 0x3f) // rightmost 6 bits
	if v >= 62 { // only 62 characters in strChars
	val >>= 6
	continue
	}
	chars = append(chars, strChars[v])
	if len(chars) == length {
	break MAIN_LOOP
	}
	val >>= 6
	}
	}
	return string(chars)
	}

Suggestion

Please make a reasonable limit to the number of characters that can be max produced or if this function isn't used, mark it as deprecated and delete it eventually

/cc @elias-orijtech

[tac0turtle]

this is used in testing, please check how its used before opening issues please