ssl: do not choose auto DH groups that are weaker than the security level

Fixes #15808

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15818)

1 files changed, 24 insertions, 9 deletions

diff --git a/ssl/ssl_cert.c b/ssl/ssl_cert.c
index f78cb99c18..4f3c2f8ee7 100644
--- a/ssl/ssl_cert.c
+++ b/ssl/ssl_cert.c
@@ -960,18 +960,36 @@ int ssl_cert_set_cert_store(CERT *c, X509_STORE *store, int chain, int ref)
     return 1;
 }
 
+int ssl_get_security_level_bits(const SSL *s, const SSL_CTX *ctx, int *levelp)
+{
+    int level;
+    static const int minbits_table[5 + 1] = { 0, 80, 112, 128, 192, 256 };
+
+    if (ctx != NULL)
+        level = SSL_CTX_get_security_level(ctx);
+    else
+        level = SSL_get_security_level(s);
+
+    if (level > 5)
+        level = 5;
+    else if (level < 0)
+        level = 0;
+
+    if (levelp != NULL)
+        *levelp = level;
+
+    return minbits_table[level];
+}
+
 static int ssl_security_default_callback(const SSL *s, const SSL_CTX *ctx,
                                          int op, int bits, int nid, void *other,
                                          void *ex)
 {
     int level, minbits;
-    static const int minbits_table[5] = { 80, 112, 128, 192, 256 };
-    if (ctx)
-        level = SSL_CTX_get_security_level(ctx);
-    else
-        level = SSL_get_security_level(s);
 
-    if (level <= 0) {
+    minbits = ssl_get_security_level_bits(s, ctx, &level);
+
+    if (level == 0) {
         /*
          * No EDH keys weaker than 1024-bits even at level 0, otherwise,
          * anything goes.
@@ -980,9 +998,6 @@ static int ssl_security_default_callback(const SSL *s, const SSL_CTX *ctx,
             return 0;
         return 1;
     }
-    if (level > 5)
-        level = 5;
-    minbits = minbits_table[level - 1];
     switch (op) {
     case SSL_SECOP_CIPHER_SUPPORTED:
     case SSL_SECOP_CIPHER_SHARED: