From 3711734a0a078f200fbb02edce46ee7f4fd16cf4 Mon Sep 17 00:00:00 2001 From: Andre Goddard Rosa Date: Wed, 1 May 2024 11:47:12 -0500 Subject: [PATCH] Add invalid `padding_len` check in `get_pkcs_padding` When trying to decrypt data with an invalid key, we found that `mbedtls` returned `0x6200` (`-25088`), which means "_CIPHER - Input data contains invalid padding and is rejected_" from `mbedtls_cipher_finish`, but it also set the output len as `18446744073709551516`. In case we detect an error with padding, we leave the output len zero'ed and return `MBEDTLS_ERR_CIPHER_INVALID_PADDING`. Here's a reference for the way `openssl` checks the padding length: - https://github.com/openssl/openssl/blob/1848c561ec39a9ea91ff1bf740a554be274f98b0/crypto/evp/evp_enc.c#L1023 - https://github.com/openssl/openssl/commit/b554eef43b9ac5b92f590da6a120dbfd9ca0582e So add a check ensuring output is set to the least-harmful value in the error cases. With the robustness fix: `PASSED (125 suites, 26644 tests run)` Without the robustness fix: `FAILED (125 suites, 26644 tests run)` Signed-off-by: Andre Goddard Rosa Signed-off-by: Andre Goddard Rosa --- library/cipher.c | 3 +++ tests/suites/test_suite_cipher.function | 8 ++++++++ 2 files changed, 11 insertions(+) diff --git a/library/cipher.c b/library/cipher.c index 0683677ed..7f4c12149 100644 --- a/library/cipher.c +++ b/library/cipher.c @@ -849,6 +849,9 @@ static int get_pkcs_padding(unsigned char *input, size_t input_len, } padding_len = input[input_len - 1]; + if (padding_len == 0 || padding_len > input_len) { + return MBEDTLS_ERR_CIPHER_INVALID_PADDING; + } *data_len = input_len - padding_len; mbedtls_ct_condition_t bad = mbedtls_ct_uint_gt(padding_len, input_len); diff --git a/tests/suites/test_suite_cipher.function b/tests/suites/test_suite_cipher.function index aca415095..8e49d2d3b 100644 --- a/tests/suites/test_suite_cipher.function +++ b/tests/suites/test_suite_cipher.function @@ -549,6 +549,10 @@ void enc_fail(int cipher_id, int pad_mode, int key_len, int length_val, /* encode length number of bytes from inbuf */ TEST_ASSERT(0 == mbedtls_cipher_update(&ctx, inbuf, length, encbuf, &outlen)); TEST_ASSERT(ret == mbedtls_cipher_finish(&ctx, encbuf + outlen, &outlen)); + if (0 != ret) { + /* Check output parameter is set to the least-harmful value on error */ + TEST_ASSERT(0 == outlen); + } /* done */ exit: @@ -826,6 +830,10 @@ void decrypt_test_vec(int cipher_id, int pad_mode, data_t *key, total_len += outlen; TEST_ASSERT(finish_result == mbedtls_cipher_finish(&ctx, output + outlen, &outlen)); + if (0 != finish_result) { + /* Check output parameter is set to the least-harmful value on error */ + TEST_ASSERT(0 == outlen); + } total_len += outlen; #if defined(MBEDTLS_GCM_C) || defined(MBEDTLS_CHACHAPOLY_C) int tag_expected = (ctx.cipher_info->mode == MBEDTLS_MODE_GCM ||