[PATCH 2/2] ssh_options_set_algo: ensure we only set known algorithms internally

[Mihai Moldovan <ionic@xxxxxxxx>]

From: Nikos Mavrogiannopoulos <nmav@xxxxxxxxxx>

That way, we will not fail later on key exchange phase when something
unknown is negotiated.

Fixes T37

Signed-off-by: Nikos Mavrogiannopoulos <nmav@xxxxxxxxxx>
Reviewed-by: Andreas Schneider <asn@xxxxxxxxx>
(cherry picked from commit 895055ab38e7716390019aae5e11771a88b99d26)
---
 include/libssh/kex.h |  1 +
 src/kex.c            | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/options.c        | 11 ++++----
 3 files changed, 81 insertions(+), 6 deletions(-)

diff --git a/include/libssh/kex.h b/include/libssh/kex.h
index 1a5b6d41..23594985 100644
--- a/include/libssh/kex.h
+++ b/include/libssh/kex.h
@@ -41,6 +41,7 @@ void ssh_list_kex(struct ssh_kex_struct *kex);
 int set_client_kex(ssh_session session);
 int ssh_kex_select_methods(ssh_session session);
 int verify_existing_algo(int algo, const char *name);
+char *keep_known_algos(int algo, const char *list);
 char **space_tokenize(const char *chain);
 int ssh_get_kex1(ssh_session session);
 char *ssh_find_matching(const char *in_d, const char *what_d);
diff --git a/src/kex.c b/src/kex.c
index 519d79ce..f0c9d067 100644
--- a/src/kex.c
+++ b/src/kex.c
@@ -281,6 +281,71 @@ char *ssh_find_matching(const char *available_d, const char *preferred_d){
     return NULL;
 }
 
+static char *ssh_find_all_matching(const char *available_d,
+                                   const char *preferred_d)
+{
+    char **tok_available, **tok_preferred;
+    int i_avail, i_pref;
+    char *ret;
+    unsigned max, len, pos = 0;
+
+    if ((available_d == NULL) || (preferred_d == NULL)) {
+        return NULL; /* don't deal with null args */
+    }
+
+    max = MAX(strlen(available_d), strlen(preferred_d));
+
+    ret = malloc(max+1);
+    if (ret == NULL) {
+      return NULL;
+    }
+    ret[0] = 0;
+
+    tok_available = tokenize(available_d);
+    if (tok_available == NULL) {
+        SAFE_FREE(ret);
+        return NULL;
+    }
+
+    tok_preferred = tokenize(preferred_d);
+    if (tok_preferred == NULL) {
+        SAFE_FREE(ret);
+        SAFE_FREE(tok_available[0]);
+        SAFE_FREE(tok_available);
+        return NULL;
+    }
+
+    for (i_pref = 0; tok_preferred[i_pref] ; ++i_pref) {
+        for (i_avail = 0; tok_available[i_avail]; ++i_avail) {
+            int cmp = strcmp(tok_available[i_avail],tok_preferred[i_pref]);
+            if (cmp == 0) {
+                /* match */
+                if (pos != 0) {
+                    ret[pos] = ',';
+                    pos++;
+                }
+
+                len = strlen(tok_available[i_avail]);
+                memcpy(&ret[pos], tok_available[i_avail], len);
+                pos += len;
+                ret[pos] = '\0';
+            }
+        }
+    }
+
+    if (ret[0] == '\0') {
+        SAFE_FREE(ret);
+        ret = NULL;
+    }
+
+    SAFE_FREE(tok_available[0]);
+    SAFE_FREE(tok_preferred[0]);
+    SAFE_FREE(tok_available);
+    SAFE_FREE(tok_preferred);
+
+    return ret;
+}
+
 /**
  * @internal
  * @brief returns whether the first client key exchange algorithm or
@@ -668,4 +733,14 @@ int verify_existing_algo(int algo, const char *name){
     return 0;
 }
 
+/* returns a copy of the provided list if everything is supported,
+ * otherwise a new list of the supported algorithms */
+char *keep_known_algos(int algo, const char *list)
+{
+    if ((algo > 9) || (algo < 0)) {
+        return NULL;
+    }
+
+    return ssh_find_all_matching(supported_methods[algo], list);
+}
 /* vim: set ts=2 sw=2 et cindent: */
diff --git a/src/options.c b/src/options.c
index aed2dda5..34fe9cc7 100644
--- a/src/options.c
+++ b/src/options.c
@@ -164,7 +164,10 @@ int ssh_options_copy(ssh_session src, ssh_session *dest) {
 
 int ssh_options_set_algo(ssh_session session, int algo,
     const char *list) {
-  if (!verify_existing_algo(algo, list)) {
+  char *p = NULL;
+
+  p = keep_known_algos(algo, list);
+  if (p == NULL) {
     ssh_set_error(session, SSH_REQUEST_DENIED,
         "Setting method: no algorithm for method \"%s\" (%s)\n",
         ssh_kex_get_description(algo), list);
@@ -172,11 +175,7 @@ int ssh_options_set_algo(ssh_session session, int algo,
   }
 
   SAFE_FREE(session->opts.wanted_methods[algo]);
-  session->opts.wanted_methods[algo] = strdup(list);
-  if (session->opts.wanted_methods[algo] == NULL) {
-    ssh_set_error_oom(session);
-    return -1;
-  }
+  session->opts.wanted_methods[algo] = p;
 
   return 0;
 }
-- 
2.15.1