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Use time-invariant conditional memcpy to make onionbalance loop safer

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This commit is contained in:
Nick Mathewson 2020-01-16 19:52:01 -05:00
parent 4269ab97c6
commit 942543253a
1 changed files with 20 additions and 31 deletions
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51
src/feature/hs/hs_cell.c
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@ -776,20 +776,20 @@ get_introduce2_keys_and_verify_mac(hs_cell_introduce2_data_t *data,
if (intro_keys == NULL) {
log_info(LD_REND, "Invalid INTRODUCE2 encrypted data. Unable to "
"compute key material");
goto err;
return NULL;
}
/* Make sure we are not about to underflow. */
if (BUG(encrypted_section_len < DIGEST256_LEN)) {
return NULL;
}
/* Validate MAC from the cell and our computed key material. The MAC field
* in the cell is at the end of the encrypted section. */
int found_idx = -1;
intro_keys_result = tor_malloc_zero(sizeof(*intro_keys_result));
for (int i = 0; i < data->n_subcredentials; ++i) {
uint8_t mac[DIGEST256_LEN];
/* Make sure we are not about to underflow. */
if (encrypted_section_len < sizeof(mac)) {
goto err;
}
/* The MAC field is at the very end of the ENCRYPTED section. */
size_t mac_offset = encrypted_section_len - sizeof(mac);
/* Compute the MAC. Use the entire encoded payload with a length up to the
@ -800,33 +800,22 @@ get_introduce2_keys_and_verify_mac(hs_cell_introduce2_data_t *data,
intro_keys[i].mac_key,
sizeof(intro_keys[i].mac_key),
mac, sizeof(mac));
if (tor_memeq(mac, encrypted_section + mac_offset, sizeof(mac))) {
found_idx = i;
break;
}
/* Time-invariant conditional copy: if the MAC is what we expected, then
* set intro_keys_result to intro_keys[i]. Otherwise, don't: but don't
* leak which one it was! */
bool equal = tor_memeq(mac, encrypted_section + mac_offset, sizeof(mac));
memcpy_if_true_timei(equal, intro_keys_result, &intro_keys[i],
sizeof(*intro_keys_result));
}
if (found_idx == -1) {
/* We no longer need intro_keys. */
memwipe(intro_keys, 0,
sizeof(hs_ntor_intro_cell_keys_t) * data->n_subcredentials);
tor_free(intro_keys);
if (safe_mem_is_zero(intro_keys_result, sizeof(*intro_keys_result))) {
log_info(LD_REND, "Invalid MAC validation for INTRODUCE2 cell");
goto err;
}
/* We found a match! */
if (data->n_subcredentials == 1) {
/* There was only one; steal it. */
intro_keys_result = intro_keys;
intro_keys = NULL;
} else {
/* Copy out the one we wanted. */
intro_keys_result = tor_memdup(&intro_keys[found_idx],
sizeof(hs_ntor_intro_cell_keys_t));
}
err:
if (intro_keys) {
memwipe(intro_keys, 0,
sizeof(hs_ntor_intro_cell_keys_t) * data->n_subcredentials);
tor_free(intro_keys);
tor_free(intro_keys_result); /* sets intro_keys_result to NULL */
}
return intro_keys_result;