Enforce acceptable range for Diffie-Hellman server value.

Florent Daigniere of Matta points out that RFC 4253 actually
_requires_ us to refuse to accept out-of-range values, though it isn't
completely clear to me why this should be a MUST on the receiving end.

Matta considers this to be a security vulnerability, on the grounds
that if a server should accidentally send an obviously useless value
such as 1 then we will fail to reject it and agree a key that an
eavesdropper could also figure out. Their id for this vulnerability is
MATTA-2015-002.

ssh.c

@@ -6645,6 +6645,13 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
         }
         ssh_pkt_getstring(pktin, &s->sigdata, &s->siglen);
 
+        {
+            const char *err = dh_validate_f(ssh->kex_ctx, s->f);
+            if (err) {
+                bombout(("key exchange reply failed validation: %s", err));
+                crStopV;
+            }
+        }
         s->K = dh_find_K(ssh->kex_ctx, s->f);
 
         /* We assume everything from now on will be quick, and it might

ssh.h

@@ -536,6 +536,7 @@ void *dh_setup_group(const struct ssh_kex *kex);
 void *dh_setup_gex(Bignum pval, Bignum gval);
 void dh_cleanup(void *);
 Bignum dh_create_e(void *, int nbits);
+const char *dh_validate_f(void *handle, Bignum f);
 Bignum dh_find_K(void *, Bignum f);
 
 int loadrsakey(const Filename *filename, struct RSAKey *key,

sshdh.c

@@ -218,6 +218,29 @@ Bignum dh_create_e(void *handle, int nbits)
     return ctx->e;
 }
 
+/*
+ * DH stage 2-epsilon: given a number f, validate it to ensure it's in
+ * range. (RFC 4253 section 8: "Values of 'e' or 'f' that are not in
+ * the range [1, p-1] MUST NOT be sent or accepted by either side."
+ * Also, we rule out 1 and p-1 too, since that's easy to do and since
+ * they lead to obviously weak keys that even a passive eavesdropper
+ * can figure out.)
+ */
+const char *dh_validate_f(void *handle, Bignum f)
+{
+    struct dh_ctx *ctx = (struct dh_ctx *)handle;
+    if (bignum_cmp(f, One) <= 0) {
+        return "f value received is too small";
+    } else {
+        Bignum pm1 = bigsub(ctx->p, One);
+        int cmp = bignum_cmp(f, pm1);
+        freebn(pm1);
+        if (cmp >= 0)
+            return "f value received is too large";
+    }
+    return NULL;
+}
+
 /*
  * DH stage 2: given a number f, compute K = f^x mod p.
  */