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Limit size of modulus for bn_mul_mont and BN_mod_exp_mont_consttime

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Otherwise the alloca can cause an exception.

Issue reported by Jiayi Lin.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/20005)
This commit is contained in:
Bernd Edlinger 2022-11-08 17:43:22 +01:00
parent 92d306b32b
commit 30667f5c30
4 changed files with 77 additions and 8 deletions
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34
crypto/bn/bn_exp.c
View File

@ -36,6 +36,15 @@
/* maximum precomputation table size for *variable* sliding windows */
#define TABLE_SIZE 32
/*
* Beyond this limit the constant time code is disabled due to
* the possible overflow in the computation of powerbufLen in
* BN_mod_exp_mont_consttime.
* When this limit is exceeded, the computation will be done using
* non-constant time code, but it will take very long.
*/
#define BN_CONSTTIME_SIZE_LIMIT (INT_MAX / BN_BYTES / 256)
/* this one works - simple but works */
int BN_exp(BIGNUM *r, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
{
@ -303,12 +312,6 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
BIGNUM *val[TABLE_SIZE];
BN_MONT_CTX *mont = NULL;
if (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
|| BN_get_flags(a, BN_FLG_CONSTTIME) != 0
|| BN_get_flags(m, BN_FLG_CONSTTIME) != 0) {
return BN_mod_exp_mont_consttime(rr, a, p, m, ctx, in_mont);
}
bn_check_top(a);
bn_check_top(p);
bn_check_top(m);
@ -317,6 +320,14 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
ERR_raise(ERR_LIB_BN, BN_R_CALLED_WITH_EVEN_MODULUS);
return 0;
}
if (m->top <= BN_CONSTTIME_SIZE_LIMIT
&& (BN_get_flags(p, BN_FLG_CONSTTIME) != 0
|| BN_get_flags(a, BN_FLG_CONSTTIME) != 0
|| BN_get_flags(m, BN_FLG_CONSTTIME) != 0)) {
return BN_mod_exp_mont_consttime(rr, a, p, m, ctx, in_mont);
}
bits = BN_num_bits(p);
if (bits == 0) {
/* x**0 mod 1, or x**0 mod -1 is still zero. */
@ -615,6 +626,11 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
top = m->top;
if (top > BN_CONSTTIME_SIZE_LIMIT) {
/* Prevent overflowing the powerbufLen computation below */
return BN_mod_exp_mont(rr, a, p, m, ctx, in_mont);
}
/*
* Use all bits stored in |p|, rather than |BN_num_bits|, so we do not leak
* whether the top bits are zero.
@ -694,7 +710,7 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
else
#endif
#if defined(OPENSSL_BN_ASM_MONT5)
if (window >= 5) {
if (window >= 5 && top <= BN_SOFT_LIMIT) {
window = 5; /* ~5% improvement for RSA2048 sign, and even
* for RSA4096 */
/* reserve space for mont->N.d[] copy */
@ -755,6 +771,9 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
if (!bn_to_mont_fixed_top(&am, a, mont, ctx))
goto err;
if (top > BN_SOFT_LIMIT)
goto fallback;
#if defined(SPARC_T4_MONT)
if (t4) {
typedef int (*bn_pwr5_mont_f) (BN_ULONG *tp, const BN_ULONG *np,
@ -1026,6 +1045,7 @@ int BN_mod_exp_mont_consttime(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p,
} else
#endif
{
fallback:
if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&tmp, top, powerbuf, 0, window))
goto err;
if (!MOD_EXP_CTIME_COPY_TO_PREBUF(&am, top, powerbuf, 1, window))

20
crypto/bn/bn_local.h
View File

@ -42,6 +42,26 @@
# include <openssl/rand.h>
# endif
/*
* This should limit the stack usage due to alloca to about 4K.
* BN_SOFT_LIMIT is a soft limit equivalent to 2*OPENSSL_RSA_MAX_MODULUS_BITS.
* Beyond that size bn_mul_mont is no longer used, and the constant time
* assembler code is disabled, due to the blatant alloca and bn_mul_mont usage.
* Note that bn_mul_mont does an alloca that is hidden away in assembly.
* It is not recommended to do computations with numbers exceeding this limit,
* since the result will be highly version dependent:
* While the current OpenSSL version will use non-optimized, but safe code,
* previous versions will use optimized code, that may crash due to unexpected
* stack overflow, and future versions may very well turn this into a hard
* limit.
* Note however, that it is possible to override the size limit using
* "./config -DBN_SOFT_LIMIT=<limit>" if necessary, and the O/S specific
* stack limit is known and taken into consideration.
*/
# ifndef BN_SOFT_LIMIT
# define BN_SOFT_LIMIT (4096 / BN_BYTES)
# endif
# ifndef OPENSSL_SMALL_FOOTPRINT
# define BN_MUL_COMBA
# define BN_SQR_COMBA

2
crypto/bn/bn_mont.c
View File

@ -42,7 +42,7 @@ int bn_mul_mont_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
int num = mont->N.top;
#if defined(OPENSSL_BN_ASM_MONT) && defined(MONT_WORD)
if (num > 1 && a->top == num && b->top == num) {
if (num > 1 && num <= BN_SOFT_LIMIT && a->top == num && b->top == num) {
if (bn_wexpand(r, num) == NULL)
return 0;
if (bn_mul_mont(r->d, a->d, b->d, mont->N.d, mont->n0, num)) {

29
test/exptest.c
View File

@ -50,6 +50,7 @@ static int test_mod_exp_zero(void)
BN_ULONG one_word = 1;
BN_CTX *ctx = BN_CTX_new();
int ret = 0, failed = 0;
BN_MONT_CTX *mont = NULL;
if (!TEST_ptr(m = BN_new())
|| !TEST_ptr(a = BN_new())
@ -94,6 +95,33 @@ static int test_mod_exp_zero(void)
if (!TEST_true(a_is_zero_mod_one("BN_mod_exp_mont_consttime", r, a)))
failed = 1;
if (!TEST_ptr(mont = BN_MONT_CTX_new()))
goto err;
ERR_set_mark();
/* mont is not set but passed in */
if (!TEST_false(BN_mod_exp_mont_consttime(r, p, a, m, ctx, mont)))
goto err;
if (!TEST_false(BN_mod_exp_mont(r, p, a, m, ctx, mont)))
goto err;
ERR_pop_to_mark();
if (!TEST_true(BN_MONT_CTX_set(mont, m, ctx)))
goto err;
/* we compute 0 ** a mod 1 here, to execute code that uses mont */
if (!TEST_true(BN_mod_exp_mont_consttime(r, p, a, m, ctx, mont)))
goto err;
if (!TEST_true(a_is_zero_mod_one("BN_mod_exp_mont_consttime", r, a)))
failed = 1;
if (!TEST_true(BN_mod_exp_mont(r, p, a, m, ctx, mont)))
goto err;
if (!TEST_true(a_is_zero_mod_one("BN_mod_exp_mont", r, a)))
failed = 1;
/*
* A different codepath exists for single word multiplication
* in non-constant-time only.
@ -114,6 +142,7 @@ static int test_mod_exp_zero(void)
BN_free(a);
BN_free(p);
BN_free(m);
BN_MONT_CTX_free(mont);
BN_CTX_free(ctx);
return ret;