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openssl3 integration: use EVP_PKEY_get1_RSA to avoid modifying const RSA key #3473

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openssl3 integration: use EVP_PKEY_get1_RSA to avoid modifying const RSA key #3473

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use EVP_PKEY_get1_RSA to avoid modifying const RSA key and free copy …
toidiu Aug 30, 2022
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toidiu Aug 30, 2022
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toidiu Aug 31, 2022
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toidiu Aug 31, 2022
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16 changes: 11 additions & 5 deletions crypto/s2n_rsa_pss.c
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Original file line number Diff line number Diff line change
Expand Up @@ -144,7 +144,7 @@ static int s2n_rsa_validate_params_match(const struct s2n_pkey *pub, const struc
POSIX_ENSURE_REF(priv);

/* OpenSSL Documentation Links:
* - https://www.openssl.org/docs/manmaster/man3/EVP_PKEY_get0_RSA.html
* - https://www.openssl.org/docs/manmaster/man3/EVP_PKEY_get1_RSA.html
* - https://www.openssl.org/docs/manmaster/man3/RSA_get0_key.html
*/
RSA *pub_rsa_key = pub->key.rsa_key.rsa;
Expand Down Expand Up @@ -176,14 +176,20 @@ static int s2n_rsa_pss_keys_match(const struct s2n_pkey *pub, const struct s2n_p

static int s2n_rsa_pss_key_free(struct s2n_pkey *pkey)
{
/* This object does not own the reference to the key --
* s2n_pkey handles it. */
POSIX_ENSURE_REF(pkey);
POSIX_ENSURE_REF(&pkey->key.rsa_key);
struct s2n_rsa_key *rsa_key = &pkey->key.rsa_key;
if (rsa_key->rsa == NULL) { return 0; }

RSA_free(rsa_key->rsa);
rsa_key->rsa = NULL;

return 0;
}

int s2n_evp_pkey_to_rsa_pss_public_key(struct s2n_rsa_key *rsa_key, EVP_PKEY *pkey) {
RSA *pub_rsa_key = EVP_PKEY_get0_RSA(pkey);
RSA *pub_rsa_key = EVP_PKEY_get1_RSA(pkey);
POSIX_ENSURE_REF(pub_rsa_key);

S2N_ERROR_IF(s2n_rsa_is_private_key(pub_rsa_key), S2N_ERR_KEY_MISMATCH);

Expand All @@ -193,7 +199,7 @@ int s2n_evp_pkey_to_rsa_pss_public_key(struct s2n_rsa_key *rsa_key, EVP_PKEY *pk

int s2n_evp_pkey_to_rsa_pss_private_key(struct s2n_rsa_key *rsa_key, EVP_PKEY *pkey)
{
RSA *priv_rsa_key = EVP_PKEY_get0_RSA(pkey);
RSA *priv_rsa_key = EVP_PKEY_get1_RSA(pkey);
POSIX_ENSURE_REF(priv_rsa_key);
Comment on lines +200 to 201
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Are you sure we don't want RSA* to be const? Now that the behavior has changed and modifying the RSA key isn't reflected back in the EVP_PKEY, isn't this likely to cause bugs? How do you know we're not relying on that behavior?

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This usage is consistent with rsa and ecdsa. That means that we make similar assumptions in all the other implementations also:

The assumptions in ossl1:

  • we are getting a raw ref to the key
  • the ref count is incremented so we need to call free
  • since its a raw ref, we can mutate it

The assumptions in ossl3:

  • we are getting a ref to a pre-cached copy of the key
  • the ref count of the copy is incremented so we need to call free
  • we can can mutate the copy, but the changes are not reflected back to the original key
    • this is ok since we dont modify the key (except free) (based on my investigation of methods allowed on keys)
    • this is ok if we store the copy in s2n_rsa_key(the copy) and dont actually use the original

In conclusion, why PR impl is Safe but brittle:

  • get1 allows us to get around the api changes introduced with get0
    • this forces us to call free on the key
  • ossl3 now returns a pre-cached copy, but that is ok since the key is not mutated
  • if we do mutate then we violate the above behavior

Alternative and reasons why I didnt go that route:

  • use PKEY directly: this is a large refactor change (if we are really uncomfortable with the current behavior then this would be the recommended approach. This can/should also be follow up work since the above is not a 1-way door)
  • use get0 and const: this is a large refactor change since s2n_rsa.c and rsa_signing will need to change. its also less prefered to using PKEY directly imo

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As discussed offline, "if we do mutate then we violate the above behavior" is what I'm concerned about. The new Openssl 3.0 behavior effectively makes the RSA* object const, but that's not reflected anywhere in our code. Nothing will stop a future dev from trying to modify the key, and things will not behave as someone reading this code would expect them to. This seems like a recipe for bugs.

If we're not going to make RSA* const, then we minimally need very scary comments everywhere that they key is referenced.

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specifically here is an impl which stores PKEY and converts to RSA when needed. Its also the recommended ossl approach: b9fd783


/* Documentation: https://www.openssl.org/docs/man1.1.1/man3/RSA_check_key.html */
Expand Down
23 changes: 14 additions & 9 deletions tests/unit/s2n_rsa_pss_rsae_test.c
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@
#include <openssl/evp.h>
#include <openssl/rsa.h>

#include "error/s2n_errno.h"
#include "s2n_test.h"
#include "testlib/s2n_testlib.h"

Expand All @@ -25,6 +26,7 @@
#include "stuffer/s2n_stuffer.h"
#include "tls/s2n_connection.h"
#include "tls/s2n_config.h"
#include "utils/s2n_safety.h"
#include "utils/s2n_random.h"

#define HASH_ALG S2N_HASH_SHA256
Expand Down Expand Up @@ -195,7 +197,7 @@ int main(int argc, char **argv)
EXPECT_SUCCESS(s2n_pkey_free(&rsa_public_key));
}

#if RSA_PSS_CERTS_SUPPORTED
#if RSA_PSS_CERTS_SUPPORTED

struct s2n_cert_chain_and_key *rsa_pss_cert_chain;
EXPECT_SUCCESS(s2n_test_cert_chain_and_key_new(&rsa_pss_cert_chain,
Expand Down Expand Up @@ -245,11 +247,10 @@ int main(int argc, char **argv)
EXPECT_SUCCESS(s2n_asn1der_to_public_key_and_type(&rsa_pss_public_key, &rsa_pss_pkey_type, &rsa_pss_cert_chain->cert_chain->head->raw));
EXPECT_EQUAL(rsa_pss_pkey_type, S2N_PKEY_TYPE_RSA_PSS);

/* Set the keys equal */
const BIGNUM *n, *e, *d;
RSA_get0_key(EVP_PKEY_get0_RSA(rsa_public_key.pkey), &n, &e, &d);
EXPECT_SUCCESS(RSA_set0_key(EVP_PKEY_get0_RSA(rsa_pss_public_key.pkey),
BN_dup(n), BN_dup(e), BN_dup(d)));
/* Set the keys equal. */
RSA *rsa_key_copy = EVP_PKEY_get1_RSA(rsa_public_key.pkey);
POSIX_GUARD_OSSL(EVP_PKEY_set1_RSA(rsa_pss_public_key.pkey, rsa_key_copy), S2N_ERR_KEY_INIT);
RSA_free(rsa_key_copy);

/* RSA signed with PSS, RSA_PSS verified with PSS */
{
Expand Down Expand Up @@ -285,12 +286,15 @@ int main(int argc, char **argv)
&rsa_cert_chain->cert_chain->head->raw));
EXPECT_EQUAL(rsa_pkey_type, S2N_PKEY_TYPE_RSA);

RSA *rsa_key = EVP_PKEY_get0_RSA(rsa_public_key.pkey);
/* Modify the rsa_public_key for each test_case. */
RSA *rsa_key_copy = EVP_PKEY_get1_RSA(rsa_public_key.pkey);
BIGNUM *n = BN_new(), *e = BN_new(), *d = BN_new();
EXPECT_SUCCESS(BN_hex2bn(&n, test_case.key_param_n));
EXPECT_SUCCESS(BN_hex2bn(&e, test_case.key_param_e));
EXPECT_SUCCESS(BN_hex2bn(&d, test_case.key_param_d));
EXPECT_SUCCESS(RSA_set0_key(rsa_key, n, e, d));
EXPECT_SUCCESS(RSA_set0_key(rsa_key_copy, n, e, d));
POSIX_GUARD_OSSL(EVP_PKEY_set1_RSA(rsa_public_key.pkey, rsa_key_copy), S2N_ERR_KEY_INIT);
RSA_free(rsa_key_copy);

struct s2n_stuffer message_stuffer = { 0 }, signature_stuffer = { 0 };
s2n_stuffer_alloc_ro_from_hex_string(&message_stuffer, test_case.message);
Expand All @@ -311,7 +315,8 @@ int main(int argc, char **argv)
}

EXPECT_SUCCESS(s2n_cert_chain_and_key_free(rsa_pss_cert_chain));
#endif
#endif

EXPECT_SUCCESS(s2n_cert_chain_and_key_free(rsa_cert_chain));
END_TEST();
}