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5.3.0 regression: SSL_set_cert_cb(..., NULL, ...) stores a null function pointer instead of clearing, crashing the next handshake

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Dal 15/10/2026.

  • #538 di @dorianverlaine — aperta

Valutazione

Difficoltà
3/5
Tempo stimato
Mezza giornata
Idoneità per principianti
62/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
cpp, rust
Ambito
security

Direzione di ricerca

Start in ssl/ssl_cert.cc at ssl_cert_set_cert_cb (both overloads), where a null callback is stored as an OldCallback instead of resetting to std::monostate, and check CertCb::operator bool() in ssl/internal.h. Then trace do_cert_callback in ssl/handshake_server.cc. A regression test should call SSL_set_cert_cb(ssl, NULL, NULL) and complete a server handshake without invoking a callback. Done when clearing with NULL leaves the callback absent and that test passes, with any boring-sys/patches/ entry kept consistent with the upstream fix.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Summary

In boring / boring-sys 5.3.0 (BoringSSL revision 427ec40c), clearing a certificate callback by passing NULL no longer resets it to the empty state. The null pointer is stored as an OldCallback alternative inside CertCb::cb, so CertCb::operator bool() reports the callback as present, and do_cert_callback then invokes a null function pointer. On our machines this crashes the server process during the resumed TLS handshake and currently keeps us from upgrading to 5.3.0.

Root cause

Introduced by google/boringssl commit 902518a787 ("ssl: Allow certificate selection callback to pick fatal alert"), which turned CERT::cert_cb into struct CertCb { std::variant<std::monostate, OldCallback, NewCallback> }:

 // ssl/ssl_cert.cc, ssl_cert_set_cert_cb (both overloads)
-  cert->cert_cb = cb;
+  cert->cert_cb.cb = cb;

Assigning a null function pointer selects the OldCallback alternative by exact type match rather than monostate, so the "cleared" representation is no longer reachable through this setter. CertCb::operator bool() (ssl/internal.h) only excludes monostate, and do_cert_callback (ssl/handshake_server.cc) calls the callback whenever that conversion is true.

Reproduction (real world)

Pingora's resumable TLS accept uses exactly this pattern: it arms a blocking cert_cb (suspend_when_need_ssl_cert), pauses the handshake, installs the certificate, then clears the callback with SSL_set_cert_cb(ssl, NULL, NULL) before resuming (unblock_ssl_cert in pingora-boringssl). With 5.3.0 every resumed handshake crashes:

bssl::CertCb::operator()(ssl_st*, void*, unsigned char*) const
bssl::do_cert_callback(bssl::SSL_HANDSHAKE*)
bssl::ssl_server_handshake(bssl::SSL_HANDSHAKE*)
...
SSL_do_handshake
SSL_accept

(excerpt from the macOS crash report: EXC_BAD_ACCESS, invalid address 0x0)

Bisecting boring/boring-sys back to 5.2.0 makes all of this disappear: 33 failing integration tests go back to green. SSL_CTX_set_cert_cb(ctx, NULL, arg) alone should be enough to hit the same path on the next server handshake.

Suggested fix

Normalize a null callback to the empty state in ssl_cert_set_cert_cb, restoring the previous "NULL clears" behavior:

if (cb == nullptr) {
  cert->cert_cb.cb = std::monostate();
} else {
  cert->cert_cb.cb = cb;
}

(same for the _ex overload.)

Notes

  • The same code is present on google/boringssl main as of 2026-10-15.
  • Reproduced on macOS 27.0.1 (26A434) aarch64, rustc 1.99.0; the defect in the C++ itself is platform-independent.
  • Happy to send a PR that carries the change as a boring-sys/patches/ entry with a regression test, if that is the preferred route — or to help get it into BoringSSL if you would rather pick it up with the next revision bump.
Lingua principale
Rust
Stelle
526
Fork
178
Merge medio
15h 34m
PR unite (30g)
9

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