NetworkPkg/IScsiDxe: fix IScsiHexToBin() hex parsing

The IScsiHexToBin() function has the following parser issues:

(1) If the *subject sequence* in "HexStr" is empty, the function returns
    EFI_SUCCESS (with "BinLength" set to 0 on output). Such inputs should
    be rejected.

(2) The function mis-handles a "HexStr" that ends with a stray nibble. For
    example, if "HexStr" is "0xABC", the function decodes it to the bytes
    {0xAB, 0x0C}, sets "BinLength" to 2 on output, and returns
    EFI_SUCCESS. Such inputs should be rejected.

(3) If an invalid hex char is found in "HexStr", the function treats it as
    end-of-hex-string, and returns EFI_SUCCESS. Such inputs should be
    rejected.

All of the above cases are remotely triggerable, as shown in a subsequent
patch, which adds error checking to the IScsiHexToBin() call sites. While
the initiator is not immediately compromised, incorrectly parsing CHAP_R
from the target, in case of mutual authentication, is not great.

Extend the interface contract of IScsiHexToBin() with
EFI_INVALID_PARAMETER, for reporting issues (1) through (3), and implement
the new checks.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3356
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210608121259.32451-9-lersek@redhat.com>

NetworkPkg/IScsiDxe/IScsiMisc.h

@@ -171,6 +171,7 @@ IScsiBinToHex (
 
   @retval EFI_SUCCESS           The hexadecimal string is converted into a
                                 binary encoded buffer.
+  @retval EFI_INVALID_PARAMETER Invalid hex encoding found in HexStr.
   @retval EFI_BUFFER_TOO_SMALL  The binary buffer is too small to hold the
                                 converted data.
 **/