Restore check of |*xn| against |name| in X509_NAME_set

A previous change of this function introduced a fragility when the
destination happens to be the same as the source.  Such alias isn't
recommended, but could still happen, for example in this kind of code:

    X509_NAME *subject = X509_get_issuer_name(x);

    /* ... some code passes ... */

    X509_set_issuer_name(x, subject);

Fixes #4710

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6280)

(cherry picked from commit c1c1783d45a5e91951e6328a820939d0256c841c)

diff --git a/crypto/x509/x_name.c b/crypto/x509/x_name.c
index 0af5df5cfc4fc5750b01d7a739f1df8b8460f7cb..8e717dcd7e2143837e8c6d90a12829ffca2e1280 100644 (file)
--- a/crypto/x509/x_name.c
+++ b/crypto/x509/x_name.c
@@ -472,6 +472,8 @@ static int i2d_name_canon(STACK_OF(STACK_OF_X509_NAME_ENTRY) * _intname,
 
 int X509_NAME_set(X509_NAME **xn, X509_NAME *name)
 {
+    if (*xn == name)
+        return *xn != NULL;
     if ((name = X509_NAME_dup(name)) == NULL)
         return 0;
     X509_NAME_free(*xn);