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[3.15] gh-149816: Fix SNI callback callable race (GH-150018) by miss-islington · Pull Request #150099 · python/cpython · GitHub

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[3.15] gh-149816: Fix SNI callback callable race (GH-150018) #150099

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Merged
serhiy-storchaka merged 2 commits into from
Jun 8, 2026
+74 −16
Conversation 0 Commits 2 Checks 51 Files changed 3
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[3.15] gh-149816: Fix SNI callback callable race (GH-150018) #150099

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202971f
gh-149816: Fix SNI callback callable race (GH-150018)
kiri11 May 19, 2026
418ad55
Merge branch '3.15' into backport-8b31d08-3.15
encukou May 19, 2026
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53 changes: 53 additions & 0 deletions Lib/test/test_ssl.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1606,6 +1606,59 @@ def dummycallback(sock, servername, ctx, cycle=ctx):
gc.collect()
self.assertIs(wr(), None)

@unittest.skipUnless(support.Py_GIL_DISABLED,
"test is only useful if the GIL is disabled")
@threading_helper.requires_working_threading()
def test_sni_callback_race(self):
# Replacing sni_callback while handshakes are in-flight must not
# crash (use-after-free on the callback in free-threaded builds).
client_ctx, server_ctx, hostname = testing_context()

server_ctx.sni_callback = lambda *a: None
done = threading.Event()

def do_handshakes():
while not done.is_set():
c_in = ssl.MemoryBIO()
c_out = ssl.MemoryBIO()
s_in = ssl.MemoryBIO()
s_out = ssl.MemoryBIO()
client = client_ctx.wrap_bio(
c_in, c_out, server_hostname=hostname)
server = server_ctx.wrap_bio(s_in, s_out, server_side=True)
for _ in range(50):
try:
client.do_handshake()
except ssl.SSLWantReadError:
pass
except ssl.SSLError:
break
if c_out.pending:
s_in.write(c_out.read())
try:
server.do_handshake()
except ssl.SSLWantReadError:
pass
except ssl.SSLError:
break
if s_out.pending:
c_in.write(s_out.read())

def toggle_callback():
while not done.is_set():
server_ctx.sni_callback = lambda *a: None
server_ctx.sni_callback = None

workers = max(4, (os.cpu_count() or 4) * 2)
threads = [threading.Thread(target=do_handshakes)
for _ in range(workers)]
threads.append(threading.Thread(target=toggle_callback))

with threading_helper.catch_threading_exception() as cm:
with threading_helper.start_threads(threads):
done.set()
self.assertIsNone(cm.exc_value)

def test_cert_store_stats(self):
ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
self.assertEqual(ctx.cert_store_stats(),
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2026-05-18-22-45-54.gh-issue-149816.T68vc_.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix race condition in :attr:`ssl.SSLContext.sni_callback`
36 changes: 20 additions & 16 deletions Modules/_ssl.c
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@
#define OPENSSL_NO_DEPRECATED 1

#include "Python.h"
#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION()
#include "pycore_fileutils.h" // _PyIsSelectable_fd()
#include "pycore_long.h" // _PyLong_UnsignedLongLong_Converter()
#include "pycore_pyerrors.h" // _PyErr_ChainExceptions1()
Expand Down Expand Up @@ -5153,12 +5154,15 @@ _servername_callback(SSL *s, int *al, void *args)
PyObject *result;
/* The high-level ssl.SSLSocket object */
PyObject *ssl_socket;
PyObject *sni_cb;
const char *servername = SSL_get_servername(s, TLSEXT_NAMETYPE_host_name);
PyGILState_STATE gstate = PyGILState_Ensure();

if (sslctx->set_sni_cb == NULL) {
/* remove race condition in this the call back while if removing the
* callback is in progress */
Py_BEGIN_CRITICAL_SECTION(sslctx);
sni_cb = Py_XNewRef(sslctx->set_sni_cb);
Py_END_CRITICAL_SECTION();

if (sni_cb == NULL) {
PyGILState_Release(gstate);
return SSL_TLSEXT_ERR_OK;
}
Expand All @@ -5185,7 +5189,7 @@ _servername_callback(SSL *s, int *al, void *args)
goto error;

if (servername == NULL) {
result = PyObject_CallFunctionObjArgs(sslctx->set_sni_cb, ssl_socket,
result = PyObject_CallFunctionObjArgs(sni_cb, ssl_socket,
Py_None, sslctx, NULL);
}
else {
Expand All @@ -5212,7 +5216,7 @@ _servername_callback(SSL *s, int *al, void *args)
}
Py_DECREF(servername_bytes);
result = PyObject_CallFunctionObjArgs(
sslctx->set_sni_cb, ssl_socket, servername_str,
sni_cb, ssl_socket, servername_str,
sslctx, NULL);
Py_DECREF(servername_str);
}
Expand All @@ -5222,7 +5226,7 @@ _servername_callback(SSL *s, int *al, void *args)
PyErr_FormatUnraisable("Exception ignored "
"in ssl servername callback "
"while calling set SNI callback %R",
sslctx->set_sni_cb);
sni_cb);
*al = SSL_AD_HANDSHAKE_FAILURE;
ret = SSL_TLSEXT_ERR_ALERT_FATAL;
}
Expand All @@ -5247,11 +5251,13 @@ _servername_callback(SSL *s, int *al, void *args)
Py_DECREF(result);
}

Py_DECREF(sni_cb);
PyGILState_Release(gstate);
return ret;

error:
Py_XDECREF(ssl_socket);
Py_XDECREF(sni_cb);
*al = SSL_AD_INTERNAL_ERROR;
ret = SSL_TLSEXT_ERR_ALERT_FATAL;
PyGILState_Release(gstate);
Expand Down Expand Up @@ -5301,20 +5307,18 @@ _ssl__SSLContext_sni_callback_set_impl(PySSLContext *self, PyObject *value)
"sni_callback cannot be set on TLS_CLIENT context");
return -1;
}
Py_CLEAR(self->set_sni_cb);
if (value == Py_None) {
if (!PyCallable_Check(value)) {
SSL_CTX_set_tlsext_servername_callback(self->ctx, NULL);
}
else {
if (!PyCallable_Check(value)) {
SSL_CTX_set_tlsext_servername_callback(self->ctx, NULL);
PyErr_SetString(PyExc_TypeError,
"not a callable object");
Py_CLEAR(self->set_sni_cb);
if (value != Py_None) {
PyErr_SetString(PyExc_TypeError, "not a callable object");
return -1;
}
self->set_sni_cb = Py_NewRef(value);
SSL_CTX_set_tlsext_servername_callback(self->ctx, _servername_callback);
}
else {
Py_XSETREF(self->set_sni_cb, Py_NewRef(value));
SSL_CTX_set_tlsext_servername_arg(self->ctx, self);
SSL_CTX_set_tlsext_servername_callback(self->ctx, _servername_callback);
}
return 0;
}
Expand Down
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