Fix handling of replies to global requests (#112) The current code assumes that all global requests want / need a reply. This isn't always true and the request itself indicates if it wants a reply or not. It causes a specific problem with [email protected] messages. These are sent by OpenSSH after authentication to inform the client of potential other host keys for the host. This can be used to add a new type of host key or to rotate host keys. The initial information message from the server is sent as a global request, but with want_reply set to false. This means that the server doesn't expect an answer to this message. Instead the client needs to send a prove request as a reply if it wants to receive proof of ownership for the host keys. The bug doesn't cause any current problems with due to how OpenSSH treats receiving the failure message. It instead treats it as a keepalive message and further ignores it. Arguably this is a protocol violation though of Dropbear and it is only accidental that it doesn't cause a problem with OpenSSH. The bug was found when adding host keys support to libssh, which is more strict protocol wise and treats the unexpected failure message an error, also see https://gitlab.com/libssh/libssh-mirror/-/merge_requests/145 for more information. The fix here is to honor the want_reply flag in the global request and to only send a reply if the other side expects a reply.

#include "tommath_private.h"
#ifdef BN_MP_GROW_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */

/* grow as required */
mp_err mp_grow(mp_int *a, int size)
{
   int     i;
   mp_digit *tmp;

   /* if the alloc size is smaller alloc more ram */
   if (a->alloc < size) {
      /* reallocate the array a->dp
       *
       * We store the return in a temporary variable
       * in case the operation failed we don't want
       * to overwrite the dp member of a.
       */
      tmp = (mp_digit *) MP_REALLOC(a->dp,
                                    (size_t)a->alloc * sizeof(mp_digit),
                                    (size_t)size * sizeof(mp_digit));
      if (tmp == NULL) {
         /* reallocation failed but "a" is still valid [can be freed] */
         return MP_MEM;
      }

      /* reallocation succeeded so set a->dp */
      a->dp = tmp;

      /* zero excess digits */
      i        = a->alloc;
      a->alloc = size;
      MP_ZERO_DIGITS(a->dp + i, a->alloc - i);
   }
   return MP_OKAY;
}
#endif