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@

1 files changed, 27 insertions, 23 deletions

diff --git a/src/lib/libcrypto/bn/bn_mont.c b/src/lib/libcrypto/bn/bn_mont.c
index bf38eac303..cae8569b8d 100644
--- a/src/lib/libcrypto/bn/bn_mont.c
+++ b/src/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