Commit 43d53fa1 authored by Andy Polyakov's avatar Andy Polyakov Committed by Richard Levitte
Browse files

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



And make RSAErr call unconditional.

Reviewed-by: default avatarRichard Levitte <levitte@openssl.org>
Reviewed-by: default avatarMatt Caswell <matt@openssl.org>
(cherry picked from commit e875b0cf)

(Merged from https://github.com/openssl/openssl/pull/7735)
parent b58c44a8

crypto/rsa/rsa_pk1.c

+51 −42
Original line number Diff line number Diff line
@@ -158,7 +158,7 @@ int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen, 
    int i;

    /* |em| is the encoded message, zero-padded to exactly |num| bytes */

    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;



    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.

     */



    if (flen > num)

        goto err;



    if (num < 11)

        goto err;

    if (flen > num || num < 11) {

        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2,

               RSA_R_PKCS_DECODING_ERROR);

        return -1;

    }



    if (flen != num) {

        em = OPENSSL_zalloc(num);

    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, 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|.

     * 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.

     */

        memcpy(em + num - flen, from, flen);

        from = em;

    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_eq(from[1], 2);



    /* scan over padding data */

    found_zero_byte = 0;

    for (i = 2; i < num; i++) {

        unsigned int equals0 = constant_time_is_zero(from[i]);

        zero_index =

            constant_time_select_int(~found_zero_byte & equals0, i,

                                     zero_index);



        zero_index = constant_time_select_int(~found_zero_byte & equals0,

                                              i, zero_index);

        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

     * 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
@@ -220,27 +222,34 @@ int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen, 
    mlen = num - msg_index;



    /*

     * For good measure, do this check in constant time as well; it could

     * leak something if |tlen| was assuming valid padding.

     * For good measure, do this check in constant time as well.

     */

    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

     * and we cannot fake its length. This unavoidably leaks timing

     * information at the API boundary.

     * Even though we can't fake result's length, we can pretend copying

     * |tlen| bytes where |mlen| bytes would be real. Last |tlen| of |num|

     * 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) {

        mlen = -1;

        goto err;

    }

    tlen = constant_time_select_int(constant_time_lt(num, tlen), num, tlen);

    msg_index = constant_time_select_int(good, msg_index, num - tlen);

    mlen = num - msg_index;

    for (from += msg_index, mask = good, i = 0; i < tlen; i++) {

        unsigned int equals = constant_time_eq(i, mlen);



    memcpy(to, from + msg_index, 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]);

    }



 err:

    OPENSSL_clear_free(em, num);

    if (mlen == -1)

        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2,

               RSA_R_PKCS_DECODING_ERROR);

    return mlen;

    RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2, RSA_R_PKCS_DECODING_ERROR);

    err_clear_last_constant_time(1 & good);



    return constant_time_select_int(good, mlen, -1);

}

doc/crypto/RSA_padding_add_PKCS1_type_1.pod

+6 −1
Original line number Diff line number Diff line
@@ -109,7 +109,12 @@ L<ERR_get_error(3)>. 
The RSA_padding_check_PKCS1_type_2() padding check leaks timing

information which can potentially be used to mount a Bleichenbacher

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