auth: change return type of passwords::check to future

Introduce a new `passwords::hash_with_salt_async` and change the return
type of `passwords::check` to `future<bool>`. This enables yielding
during password computations later in this patch series.

The old method, `hash_with_salt`, is marked as deprecated because
new code should use the new `hash_with_salt_async` function.
We are not removing `hash_with_salt` now to reduce the regression risk
of changing the hashing implementation—at least the methods that change
persistent hashes (CREATE, ALTER) will continue to use the old hashing
method. However, in the future, `hash_with_salt` should be entirely
removed.

Refs: scylladb/scylladb#26859

auth/password_authenticator.cc

@@ -328,7 +328,7 @@ future<authenticated_user> password_authenticator::authenticate(
             }
             salted_hash = role->salted_hash;
         }
-        const bool password_match = passwords::check(password, *salted_hash);
+        const bool password_match = co_await passwords::check(password, *salted_hash);
         if (!password_match) {
             throw exceptions::authentication_exception("Username and/or password are incorrect");
         }

auth/passwords.cc

@@ -7,6 +7,8 @@
  */
 
 #include "auth/passwords.hh"
+#include "utils/crypt_sha512.hh"
+#include <seastar/core/coroutine.hh>
 
 #include <cerrno>
 
@@ -45,6 +47,24 @@ sstring hash_with_salt(const sstring& pass, const sstring& salt) {
     return res;
 }
 
+seastar::future<sstring> hash_with_salt_async(const sstring& pass, const sstring& salt) {
+    sstring res;
+    // Only SHA-512 hashes for passphrases shorter than 256 bytes can be computed using
+    // the __crypt_sha512 method. For other computations, we fall back to the
+    // crypt_r implementation from `<crypt.h>`, which can stall.
+    if (salt.starts_with(prefix_for_scheme(scheme::sha_512)) && pass.size() <= 255) {
+        char buf[128];
+        const char * output_ptr = co_await __crypt_sha512(pass.c_str(), salt.c_str(), buf);
+        verify_hashing_output(output_ptr);
+        res = output_ptr;
+    } else {
+        const char * output_ptr = crypt_r(pass.c_str(), salt.c_str(), &tlcrypt);
+        verify_hashing_output(output_ptr);
+        res = output_ptr;
+    }
+    co_return res;
+}
+
 std::string_view prefix_for_scheme(scheme c) noexcept {
     switch (c) {
     case scheme::bcrypt_y: return "$2y$";
@@ -61,8 +81,9 @@ no_supported_schemes::no_supported_schemes()
         : std::runtime_error("No allowed hashing schemes are supported on this system") {
 }
 
-bool check(const sstring& pass, const sstring& salted_hash) {
-    return detail::hash_with_salt(pass, salted_hash) == salted_hash;
+seastar::future<bool> check(const sstring& pass, const sstring& salted_hash) {
+    const auto pwd_hash = co_await detail::hash_with_salt_async(pass, salted_hash);
+    co_return pwd_hash == salted_hash;
 }
 
 } // namespace auth::passwords

auth/passwords.hh

@@ -11,6 +11,7 @@
 #include <random>
 #include <stdexcept>
 
+#include <seastar/core/future.hh>
 #include <seastar/core/sstring.hh>
 
 #include "seastarx.hh"
@@ -75,10 +76,18 @@ sstring generate_salt(RandomNumberEngine& g, scheme scheme) {
 
 ///
 /// Hash a password combined with an implementation-specific salt string.
+/// Deprecated in favor of `hash_with_salt_async`.
 ///
 /// \throws \ref std::system_error when an unexpected implementation-specific error occurs.
 ///
-sstring hash_with_salt(const sstring& pass, const sstring& salt);
+[[deprecated("Use hash_with_salt_async instead")]] sstring hash_with_salt(const sstring& pass, const sstring& salt);
+
+///
+/// Async version of `hash_with_salt` that returns a future.
+///
+/// \throws \ref std::system_error when an unexpected implementation-specific error occurs.
+///
+seastar::future<sstring> hash_with_salt_async(const sstring& pass, const sstring& salt);
 
 } // namespace detail
 
@@ -107,6 +116,6 @@ sstring hash(const sstring& pass, RandomNumberEngine& g, scheme scheme) {
 ///
 /// \throws \ref std::system_error when an unexpected implementation-specific error occurs.
 ///
-bool check(const sstring& pass, const sstring& salted_hash);
+seastar::future<bool> check(const sstring& pass, const sstring& salted_hash);
 
 } // namespace auth::passwords

test/boost/auth_passwords_test.cc

@@ -54,9 +54,8 @@ SEASTAR_TEST_CASE(correct_passwords_authenticate) {
     };
 
     for (const char* p : passwords) {
-        BOOST_REQUIRE(auth::passwords::check(p, auth::passwords::hash(p, rng_for_salt, auth::passwords::scheme::sha_512)));
+        BOOST_REQUIRE(co_await auth::passwords::check(p, auth::passwords::hash(p, rng_for_salt, auth::passwords::scheme::sha_512)));
     }
-    co_return;
 }
 
 //
@@ -64,6 +63,5 @@ SEASTAR_TEST_CASE(correct_passwords_authenticate) {
 //
 SEASTAR_TEST_CASE(incorrect_passwords_do_not_authenticate) {
     const sstring hashed_password = auth::passwords::hash("actual_password", rng_for_salt,auth::passwords::scheme::sha_512);
-    BOOST_REQUIRE(!auth::passwords::check("password_guess", hashed_password));
-    co_return;
+    BOOST_REQUIRE(!co_await auth::passwords::check("password_guess", hashed_password));
 }