Rewrite BN_MONT_CTX_set_locked()

Rewrite and simplify BN_MONT_CTX_set_locked - in particular, only hold the
lock for a short period of time, rather than holding a write lock for a
module across an expensive operation.

ok tb@

diff --git a/lib/libcrypto/bn/bn_mont.c b/lib/libcrypto/bn/bn_mont.c
index bf38eac..cae8569 100644 (file)
--- a/lib/libcrypto/bn/bn_mont.c
+++ b/lib/libcrypto/bn/bn_mont.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_mont.c,v 1.40 2023/02/19 13:44:29 jsing Exp $ */
+/* $OpenBSD: bn_mont.c,v 1.41 2023/02/19 13:51:00 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -330,35 +330,39 @@ err:
 }
 
 BN_MONT_CTX *
-BN_MONT_CTX_set_locked(BN_MONT_CTX **pmont, int lock, const BIGNUM *mod,
+BN_MONT_CTX_set_locked(BN_MONT_CTX **pmctx, int lock, const BIGNUM *mod,
     BN_CTX *ctx)
 {
-       int got_write_lock = 0;
-       BN_MONT_CTX *ret;
+       BN_MONT_CTX *mctx = NULL;
 
        CRYPTO_r_lock(lock);
-       if (!*pmont) {
-               CRYPTO_r_unlock(lock);
-               CRYPTO_w_lock(lock);
-               got_write_lock = 1;
-
-               if (!*pmont) {
-                       ret = BN_MONT_CTX_new();
-                       if (ret && !BN_MONT_CTX_set(ret, mod, ctx))
-                               BN_MONT_CTX_free(ret);
-                       else
-                               *pmont = ret;
-               }
-       }
+       mctx = *pmctx;
+       CRYPTO_r_unlock(lock);
 
-       ret = *pmont;
+       if (mctx != NULL)
+               goto done;
 
-       if (got_write_lock)
-               CRYPTO_w_unlock(lock);
-       else
-               CRYPTO_r_unlock(lock);
+       if ((mctx = BN_MONT_CTX_new()) == NULL)
+               goto err;
+       if (!BN_MONT_CTX_set(mctx, mod, ctx))
+               goto err;
 
-       return ret;
+       CRYPTO_w_lock(lock);
+       if (*pmctx != NULL) {
+               /* Someone else raced us... */
+               BN_MONT_CTX_free(mctx);
+               mctx = *pmctx;
+       } else {
+               *pmctx = mctx;
+       }
+       CRYPTO_w_unlock(lock);
+
+       goto done;
+ err:
+       BN_MONT_CTX_free(mctx);
+       mctx = NULL;
+ done:
+       return mctx;
 }
 
 #ifdef OPENSSL_NO_ASM