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Message-ID: <CAO5O-E+iB2dy2BfQrLmCsNjXnuXjxAToPSETBfrJpe38TF7Dag@mail.gmail.com>
Date: Tue, 20 Mar 2018 22:34:25 +0100
From: Guido Vranken <guidovranken@...il.com>
To: oss-security@...ts.openwall.com
Subject: OpenSSL: bug in modular exponentiation
My bignum fuzzer (https://github.com/guidovranken/bignum-fuzzer)
running on Google's oss-fuzz recently found a bug in affecting
constant-time modular exponentiation.
OpenSSL does not treat this as a security vulnerability. This is a
heads-up to developers who rely on the affected code so they can
review the impact on their applications on a case-by-case basis.
The bug is located in a function written in assembly language and the
bug can only manifest on specific processors, most likely the same as
CVE-2017-3738 (see https://www.openssl.org/news/vulnerabilities.html):
"This only affects processors that support the AVX2 but not ADX
extensions like Intel Haswell (4th generation)"
As far as I know BoringSSL and LibreSSL are not affected.
You can use the PoC below the line to see if your system is affected.
A system that is affected:
$ cat /proc/cpuinfo | grep "avx2\|adx" -o | sort -u
avx2
$ ./a.out
result is 0
result is 179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356083471597445343
A system that is not affected:
$ cat /proc/cpuinfo | grep "avx2\|adx" -o | sort -u
adx
avx2
$ ./a.out
result is 0
result is 0
-------------------
#include <openssl/bn.h>
static void do_mod_exp(int consttime)
{
BIGNUM *res, *A = NULL, *B = NULL, *C = NULL;
BN_CTX *ctx = BN_CTX_new();
char* bn_str = NULL;
res = BN_new();
BN_dec2bn(&A,
"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356083471597445343");
BN_dec2bn(&B,
"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000022222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222");
BN_dec2bn(&C,
"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356083471597445343");
if ( consttime ) {
BN_set_flags(A, BN_FLG_CONSTTIME);
}
BN_mod_exp(res, A, B, C, ctx);
bn_str = BN_bn2dec(res);
printf("result is %s\n", bn_str);
OPENSSL_free(bn_str);
BN_CTX_free(ctx);
BN_free(A);
BN_free(B);
BN_free(C);
}
int main(void)
{
do_mod_exp(0);
do_mod_exp(1);
return 0;
}
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