From 9a703650cd6afb812a04cbf8167d3b2eb0bd2124 Mon Sep 17 00:00:00 2001 From: Ashish Sharma Date: Tue, 19 May 2026 15:33:33 +0800 Subject: [PATCH] fix(rsa_ds): make PKCS#1 v1.5 unpadding constant-time --- .../psa_crypto_driver_esp_rsa_ds_utilities.c | 116 +++++++++++------- 1 file changed, 74 insertions(+), 42 deletions(-) diff --git a/components/mbedtls/port/psa_driver/esp_rsa_ds/psa_crypto_driver_esp_rsa_ds_utilities.c b/components/mbedtls/port/psa_driver/esp_rsa_ds/psa_crypto_driver_esp_rsa_ds_utilities.c index 270a3fda127..1a491bcc004 100644 --- a/components/mbedtls/port/psa_driver/esp_rsa_ds/psa_crypto_driver_esp_rsa_ds_utilities.c +++ b/components/mbedtls/port/psa_driver/esp_rsa_ds/psa_crypto_driver_esp_rsa_ds_utilities.c @@ -9,6 +9,10 @@ #include "mbedtls/asn1.h" #include "mbedtls/psa_util.h" #include "esp_log.h" +/* The unpadding routines below borrow the audited constant-time primitives + * from upstream mbedtls (the same header is already used by the esp_aes + * driver in this tree). */ +#include "constant_time_internal.h" typedef struct { psa_algorithm_t md_alg; @@ -167,63 +171,91 @@ psa_status_t esp_rsa_ds_pad_v15_unpad(unsigned char *input, size_t output_max_len, size_t *olen) { + /* This implementation mirrors mbedtls_ct_rsaes_pkcs1_v15_unpadding() + * in upstream rsa.c. Below the public-input length check, every + * operation must be constant-time w.r.t. the plaintext contents, + * the position of the 0x00 separator, and padding validity. Failing + * that opens a Bleichenbacher-style padding oracle. The function is + * deliberately longer than the project guideline because each step + * carries an audit comment that has to survive intact. */ + + /* ilen and output_max_len are public; this branch is safe. */ if (ilen < MIN_V15_PADDING_LEN) { return PSA_ERROR_INVALID_ARGUMENT; } - - unsigned char bad = 0; - size_t msg_len = 0; - size_t msg_max_len = 0; - unsigned char pad_done = 0; +#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) size_t pad_count = 0; + size_t plaintext_size = 0; + size_t plaintext_max_size; + mbedtls_ct_condition_t bad; + mbedtls_ct_condition_t pad_done; + mbedtls_ct_condition_t output_too_large; - msg_max_len = (output_max_len > ilen - MIN_V15_PADDING_LEN) ? ilen - MIN_V15_PADDING_LEN : output_max_len; + plaintext_max_size = (output_max_len > ilen - MIN_V15_PADDING_LEN) + ? ilen - MIN_V15_PADDING_LEN : output_max_len; - /* Check the first byte (0x00) */ - bad |= input[0]; + /* EME-PKCS1-v1_5: 0x00 || 0x02 || PS (>= 8 non-zero) || 0x00 || M */ + bad = mbedtls_ct_bool(input[0]); + bad = mbedtls_ct_bool_or(bad, mbedtls_ct_uint_ne(input[1], 2 /* MBEDTLS_RSA_CRYPT */)); - /* Check the padding type */ - bad |= input[1] ^ 2; // MBEDTLS_RSA_CRYPT; - - /* Scan for separator (0x00) and count padding bytes in constant time */ + /* Scan the full buffer; pad_done latches at the first 0x00 found. */ + pad_done = MBEDTLS_CT_FALSE; for (size_t i = 2; i < ilen; i++) { - unsigned char found = (input[i] == 0x00); - pad_done = pad_done | found; - pad_count += (pad_done == 0) ? 1 : 0; + mbedtls_ct_condition_t found = mbedtls_ct_uint_eq(input[i], 0); + pad_done = mbedtls_ct_bool_or(pad_done, found); + pad_count += mbedtls_ct_uint_if_else_0(mbedtls_ct_bool_not(pad_done), 1); } - /* Check if we found a separator and padding is long enough */ - bad |= (pad_done == 0); /* No separator found */ - bad |= (pad_count < 8); /* Padding too short (need at least 8 non-zero bytes) */ + /* No separator found, or PS too short. */ + bad = mbedtls_ct_bool_or(bad, mbedtls_ct_bool_not(pad_done)); + bad = mbedtls_ct_bool_or(bad, mbedtls_ct_uint_gt(8, pad_count)); - /* Calculate message length */ - msg_len = ilen - pad_count - 3; + /* If invalid, substitute plaintext_max_size so the remaining cache + * and timing trace matches the good case. */ + plaintext_size = mbedtls_ct_uint_if(bad, + (unsigned) plaintext_max_size, + (unsigned) (ilen - pad_count - 3)); + output_too_large = mbedtls_ct_uint_gt(plaintext_size, plaintext_max_size); + plaintext_size = mbedtls_ct_uint_if(output_too_large, + (unsigned) plaintext_max_size, + (unsigned) plaintext_size); - /* Check if separator is not at the very end */ - bad |= (msg_len > output_max_len); - if (bad) { - msg_len = msg_max_len; + /* On any failure path (bad padding, or plaintext doesn't fit) zero + * the post-header region of `input` BEFORE the memmove_left + memcpy + * below. Those two operations execute unconditionally to keep the + * memory access trace fixed; this step ensures they propagate zeros + * rather than a failed-decryption plaintext attempt into the + * caller-visible output buffer. Mirrors upstream rsa.c. */ + mbedtls_ct_zeroize_if(mbedtls_ct_bool_or(bad, output_too_large), + input + MIN_V15_PADDING_LEN, + ilen - MIN_V15_PADDING_LEN); + + /* Slide the plaintext to a fixed in-buffer position, then read + * from that fixed position. The slide is CT in the secret offset. */ + mbedtls_ct_memmove_left(input + ilen - plaintext_max_size, + plaintext_max_size, + plaintext_max_size - plaintext_size); + if (output_max_len != 0) { + /* memmove handles input/output aliasing (callers may pass the + * same buffer for both). The length is the public bound, so + * the access pattern reveals nothing secret. */ + memmove(output, input + ilen - plaintext_max_size, plaintext_max_size); } - /* Verify padding bytes are non-zero in constant time */ -#if defined(__clang__) && defined(__xtensa__) - #pragma clang loop vectorize(disable) -#endif - for (size_t i = 2; i < ilen; i++) { - unsigned char in_padding = (i < pad_count + 2); - unsigned char is_zero = (input[i] == 0x00); - bad |= in_padding & is_zero; - } + *olen = plaintext_size; - if (bad) { - return PSA_ERROR_INVALID_ARGUMENT; - } - - *olen = msg_len; - if (*olen > 0) { - memcpy(output, input + ilen - msg_len, msg_len); - } - return PSA_SUCCESS; + /* Collapse both error conditions into the single status we already + * return (PSA_ERROR_INVALID_ARGUMENT); distinguishing them would + * give a Bleichenbacher attacker a finer oracle. */ + return (psa_status_t) mbedtls_ct_error_if_else_0( + mbedtls_ct_bool_or(bad, output_too_large), + PSA_ERROR_INVALID_ARGUMENT); +#else + /* PKCS#1 v1.5 padding is not configured; the driver should not be + * dispatched for this algorithm in the first place. */ + (void) input; (void) output; (void) output_max_len; (void) olen; + return PSA_ERROR_NOT_SUPPORTED; +#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C */ } #if CONFIG_MBEDTLS_SSL_PROTO_TLS1_3