Revert BN_copy() flag copy semantics change

Commit 9f9442918a changed the semantics of BN_copy() to additionally
copy the BN_FLG_CONSTTIME flag if it is set. This turns out to be
ill advised as it has unintended consequences. For example calling
BN_mod_inverse_no_branch() can sometimes return a result with the flag
set and sometimes not as a result. This can lead to later failures if we
go down code branches that do not support constant time, but check for
the presence of the flag.

The original commit was made due to an issue in BN_MOD_CTX_set(). The
original PR fixed the problem in that function, but it was changed in
review to fix it in BN_copy() instead. The solution seems to be to revert
the BN_copy() change and go back to the originally proposed way.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/5080)

(cherry picked from commit 7d461736f7bd3af3c2f266f8541034ecf6f41ed9)

diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c
index d7c5fc3c97d3bfff0bcf03866f68a0476c8a651f..b8d44683c194376f6c1fc0005a174f7777485c78 100644 (file)
--- a/crypto/bn/bn_lib.c
+++ b/crypto/bn/bn_lib.c
@@ -406,9 +406,6 @@ BIGNUM *BN_copy(BIGNUM *a, const BIGNUM *b)
     memcpy(a->d, b->d, sizeof(b->d[0]) * b->top);
 #endif
 
-    if (BN_get_flags(b, BN_FLG_CONSTTIME) != 0)
-        BN_set_flags(a, BN_FLG_CONSTTIME);
-
     a->top = b->top;
     a->neg = b->neg;
     bn_check_top(a);

diff --git a/crypto/bn/bn_mont.c b/crypto/bn/bn_mont.c
index faea4684faf70280108cbe9534ed46f98a718167..90e1ba296e23988fa2962dd2f39320fd37f4a6be 100644 (file)
--- a/crypto/bn/bn_mont.c
+++ b/crypto/bn/bn_mont.c
@@ -258,6 +258,8 @@ int BN_MONT_CTX_set(BN_MONT_CTX *mont, const BIGNUM *mod, BN_CTX *ctx)
     R = &(mont->RR);            /* grab RR as a temp */
     if (!BN_copy(&(mont->N), mod))
         goto err;               /* Set N */
+    if (BN_get_flags(mod, BN_FLG_CONSTTIME) != 0)
+        BN_set_flags(&(mont->N), BN_FLG_CONSTTIME);
     mont->N.neg = 0;
 
 #ifdef MONT_WORD