Re: [PATCH v2] scsi: target: iscsi: validate CHAP_R length before base64 decode

On Wed, May 20, 2026, Dmitry Bogdanov <[email protected]> wrote:
> Yes, the length of Base64 decoded string is not deterministic.
> Moreover, length of Base64 encoded string must be divisible by 4. Which
> is biger that 4/3 of decoded.
>
> | MD5_SIGNATURE_SIZE      | 16   | 21,33333 | 22       | 24                   |
> | SHA256_SIGNATURE_SIZE   | 32   | 42,66667 | 43       | 44                   |
>
> So, that formula is not correct and will break all iscsi authentication.

v3 (sent about an hour before your email) already handles this. Trailing
'=' are stripped before the comparison, so the check is applied only to
the data characters:

	while (r_len > 0 && chap_r[r_len - 1] == '=')
		r_len--;
	if (r_len > DIV_ROUND_UP(chap->digest_size * 4, 3)) {

Using your table as input:

  MD5 padded:     "data==" -> r_len = 24-2 = 22, 22 <= 22 ✓
  SHA-256 padded: "data="  -> r_len = 44-1 = 43, 43 <= 43 ✓

> Alexandru, may be better just to change size of client_diggest variable
> to match it with chap_r like for initiatorchg and initiatorchg_binhex?

That also prevents the overflow. The length check is preferred for
consistency with the HEX branch, which validates input length before
calling the decoder rather than relying on a larger destination buffer.

Alexandru