From 3655e35b78df095b367b3334b6515f6d9436fd95 Mon Sep 17 00:00:00 2001 From: Nicola Tuveri Date: Tue, 21 Jan 2020 17:08:16 +0200 Subject: [PATCH] [BN] harden `BN_copy()` against leaks from memory accesses `BN_copy()` (and indirectly `BN_dup()`) do not propagate the `BN_FLG_CONSTTIME` flag: the propagation has been turned on and off a few times in the past years, because in some conditions it has shown unintended consequences in some code paths. Without turning the propagation on once more, we can still improve `BN_copy()` by avoiding to leak `src->top` in case `src` is flagged with `BN_FLG_CONSTTIME`. In this case we can instead use `src->dmax` as the number of words allocated for `dst` and for the `memcpy` operation. Barring compiler or runtime optimizations, if the caller provides `src` flagged as const time and preallocated to a public size, no leak should happen due to the copy operation. (cherry picked from commit 2d9167ed0b588dacbdd0303fb6041ffe1d8b3a92) Reviewed-by: Matt Caswell (Merged from https://github.com/openssl/openssl/pull/11127) --- crypto/bn/bn_lib.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c index 86d4956c8a..759d4c70ed 100644 --- a/crypto/bn/bn_lib.c +++ b/crypto/bn/bn_lib.c @@ -322,15 +322,19 @@ BIGNUM *BN_dup(const BIGNUM *a) BIGNUM *BN_copy(BIGNUM *a, const BIGNUM *b) { + int bn_words; + bn_check_top(b); + bn_words = BN_get_flags(b, BN_FLG_CONSTTIME) ? b->dmax : b->top; + if (a == b) return a; - if (bn_wexpand(a, b->top) == NULL) + if (bn_wexpand(a, bn_words) == NULL) return NULL; if (b->top > 0) - memcpy(a->d, b->d, sizeof(b->d[0]) * b->top); + memcpy(a->d, b->d, sizeof(b->d[0]) * bn_words); a->neg = b->neg; a->top = b->top; -- 2.25.1