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rsa/rsa_pk1.c: remove memcpy calls from RSA_padding_check_PKCS1_type_2.

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And make RSAErr call unconditional.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
This commit is contained in:
Andy Polyakov 2018-09-01 12:00:33 +02:00 committed by Matt Caswell
parent 89072e0c2a
commit e875b0cf2f
2 changed files with 58 additions and 44 deletions
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95
crypto/rsa/rsa_pk1.c
View File

@ -158,7 +158,7 @@ int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
int i; int i;
/* |em| is the encoded message, zero-padded to exactly |num| bytes */ /* |em| is the encoded message, zero-padded to exactly |num| bytes */
unsigned char *em = NULL; unsigned char *em = NULL;
unsigned int good, found_zero_byte; unsigned int good, found_zero_byte, mask;
int zero_index = 0, msg_index, mlen = -1; int zero_index = 0, msg_index, mlen = -1;
if (tlen < 0 || flen < 0) if (tlen < 0 || flen < 0)
@ -169,39 +169,41 @@ int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
* section 7.2.2. * section 7.2.2.
*/ */
if (flen > num) if (flen > num || num < 11) {
goto err; RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2,
RSA_R_PKCS_DECODING_ERROR);
if (num < 11) return -1;
goto err;
if (flen != num) {
em = OPENSSL_zalloc(num);
if (em == NULL) {
RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2, ERR_R_MALLOC_FAILURE);
return -1;
}
/*
* Caller is encouraged to pass zero-padded message created with
* BN_bn2binpad, but if it doesn't, we do this zero-padding copy
* to avoid leaking that information. The copy still leaks some
* side-channel information, but it's impossible to have a fixed
* memory access pattern since we can't read out of the bounds of
* |from|.
*/
memcpy(em + num - flen, from, flen);
from = em;
} }
em = OPENSSL_malloc(num);
if (em == NULL) {
RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2, ERR_R_MALLOC_FAILURE);
return -1;
}
/*
* Caller is encouraged to pass zero-padded message created with
* BN_bn2binpad. Trouble is that since we can't read out of |from|'s
* bounds, it's impossible to have an invariant memory access pattern
* in case |from| was not zero-padded in advance.
*/
for (from += flen, em += num, i = 0; i < num; i++) {
mask = ~constant_time_is_zero(flen);
flen -= 1 & mask;
from -= 1 & mask;
*--em = *from & mask;
}
from = em;
good = constant_time_is_zero(from[0]); good = constant_time_is_zero(from[0]);
good &= constant_time_eq(from[1], 2); good &= constant_time_eq(from[1], 2);
/* scan over padding data */
found_zero_byte = 0; found_zero_byte = 0;
for (i = 2; i < num; i++) { for (i = 2; i < num; i++) {
unsigned int equals0 = constant_time_is_zero(from[i]); unsigned int equals0 = constant_time_is_zero(from[i]);
zero_index =
constant_time_select_int(~found_zero_byte & equals0, i, zero_index = constant_time_select_int(~found_zero_byte & equals0,
zero_index); i, zero_index);
found_zero_byte |= equals0; found_zero_byte |= equals0;
} }
@ -210,7 +212,7 @@ int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
* If we never found a 0-byte, then |zero_index| is 0 and the check * If we never found a 0-byte, then |zero_index| is 0 and the check
* also fails. * also fails.
*/ */
good &= constant_time_ge((unsigned int)(zero_index), 2 + 8); good &= constant_time_ge(zero_index, 2 + 8);
/* /*
* Skip the zero byte. This is incorrect if we never found a zero-byte * Skip the zero byte. This is incorrect if we never found a zero-byte
@ -220,27 +222,34 @@ int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
mlen = num - msg_index; mlen = num - msg_index;
/* /*
* For good measure, do this check in constant time as well; it could * For good measure, do this check in constant time as well.
* leak something if |tlen| was assuming valid padding.
*/ */
good &= constant_time_ge((unsigned int)(tlen), (unsigned int)(mlen)); good &= constant_time_ge(tlen, mlen);
/* /*
* We can't continue in constant-time because we need to copy the result * Even though we can't fake result's length, we can pretend copying
* and we cannot fake its length. This unavoidably leaks timing * |tlen| bytes where |mlen| bytes would be real. Last |tlen| of |num|
* information at the API boundary. * bytes are viewed as circular buffer with start at |tlen|-|mlen'|,
* where |mlen'| is "saturated" |mlen| value. Deducing information
* about failure or |mlen| would take attacker's ability to observe
* memory access pattern with byte granularity *as it occurs*. It
* should be noted that failure is indistinguishable from normal
* operation if |tlen| is fixed by protocol.
*/ */
if (!good) { tlen = constant_time_select_int(constant_time_lt(num, tlen), num, tlen);
mlen = -1; msg_index = constant_time_select_int(good, msg_index, num - tlen);
goto err; mlen = num - msg_index;
for (from += msg_index, mask = good, i = 0; i < tlen; i++) {
unsigned int equals = constant_time_eq(i, mlen);
from -= tlen & equals; /* if (i == mlen) rewind */
mask &= mask ^ equals; /* if (i == mlen) mask = 0 */
to[i] = constant_time_select_8(mask, from[i], to[i]);
} }
memcpy(to, from + msg_index, mlen);
err:
OPENSSL_clear_free(em, num); OPENSSL_clear_free(em, num);
if (mlen == -1) RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2, RSA_R_PKCS_DECODING_ERROR);
RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2, err_clear_last_constant_time(1 & good);
RSA_R_PKCS_DECODING_ERROR);
return mlen; return constant_time_select_int(good, mlen, -1);
} }

7
doc/man3/RSA_padding_add_PKCS1_type_1.pod
View File

@ -110,7 +110,12 @@ L<ERR_get_error(3)>.
The RSA_padding_check_PKCS1_type_2() padding check leaks timing The RSA_padding_check_PKCS1_type_2() padding check leaks timing
information which can potentially be used to mount a Bleichenbacher information which can potentially be used to mount a Bleichenbacher
padding oracle attack. This is an inherent weakness in the PKCS #1 padding oracle attack. This is an inherent weakness in the PKCS #1
v1.5 padding design. Prefer PKCS1_OAEP padding. v1.5 padding design. Prefer PKCS1_OAEP padding. Otherwise it can
be recommended to pass zero-padded B<f>, so that B<fl> equals to
B<rsa_len>, and if fixed by protocol, B<tlen> being set to the
expected length. In such case leakage would be minimal, it would
take attacker's ability to observe memory access pattern with byte
granilarity as it occurs, post-factum timing analysis won't do.
=head1 SEE ALSO =head1 SEE ALSO