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gh-155835: fix digest_size and block_size data races on SHA-3 objects #155838

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gh-155835: fix digest_size and block_size data races on SHA-3 objects #155838

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-15-13-12-09.gh-issue-155835.DsbSmy.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
:mod:`hashlib`: Fix data races when accessing
:attr:`~hashlib.hash.digest_size` and :attr:`~hashlib.hash.block_size` on
SHA-3 objects. Patch by Bénédikt Tran.
34 changes: 20 additions & 14 deletions Modules/sha3module.c
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Original file line number Diff line number Diff line change
Expand Up @@ -67,6 +67,12 @@ sha3_get_state(PyObject *module)
typedef struct {
HASHLIB_OBJECT_HEAD
Hacl_Hash_SHA3_state_t *hash_state;
// HACL* update functions entirely replace the state, which can lead
// to races on the free-threaded build. Since the kind of hash is static,
// we can store its corresponding metadata once.
uint32_t digest_size;
uint32_t block_size;
int is_shake;
} SHA3object;

#define _SHA3object_CAST(op) ((SHA3object *)(op))
Expand Down Expand Up @@ -96,7 +102,7 @@ newSHA3object(PyTypeObject *type)
return NULL;
}
HASHLIB_INIT_MUTEX(newobj);

newobj->digest_size = newobj->block_size = 0;
PyObject_GC_Track(newobj);
return newobj;
}
Expand Down Expand Up @@ -179,6 +185,11 @@ py_sha3_new_impl(PyTypeObject *type, PyObject *data_obj, int usedforsecurity,
goto error;
}

// set the metadata once we know that the state is valid
int is_shake = Hacl_Hash_SHA3_is_shake(self->hash_state);
self->digest_size = is_shake ? 0 : Hacl_Hash_SHA3_hash_len(self->hash_state);
self->block_size = Hacl_Hash_SHA3_block_len(self->hash_state);

if (data) {
GET_BUFFER_VIEW_OR_ERROR(data, &buf, goto error);
/* Do not use self->mutex here as this is the constructor
Expand Down Expand Up @@ -253,6 +264,8 @@ _sha3_sha3_224_copy_impl(SHA3object *self, PyTypeObject *cls)
Py_DECREF(newobj);
return PyErr_NoMemory();
}
newobj->digest_size = self->digest_size;
newobj->block_size = self->block_size;
return (PyObject *)newobj;
}

Expand All @@ -273,8 +286,7 @@ _sha3_sha3_224_digest_impl(SHA3object *self)
HASHLIB_ACQUIRE_LOCK(self);
(void)Hacl_Hash_SHA3_digest(self->hash_state, digest);
HASHLIB_RELEASE_LOCK(self);
return PyBytes_FromStringAndSize((const char *)digest,
Hacl_Hash_SHA3_hash_len(self->hash_state));
return PyBytes_FromStringAndSize((const char *)digest, self->digest_size);
}


Expand All @@ -292,8 +304,7 @@ _sha3_sha3_224_hexdigest_impl(SHA3object *self)
HASHLIB_ACQUIRE_LOCK(self);
(void)Hacl_Hash_SHA3_digest(self->hash_state, digest);
HASHLIB_RELEASE_LOCK(self);
return _Py_strhex((const char *)digest,
Hacl_Hash_SHA3_hash_len(self->hash_state));
return _Py_strhex((const char *)digest, self->digest_size);
}


Expand Down Expand Up @@ -334,8 +345,7 @@ static PyObject *
SHA3_get_block_size(PyObject *op, void *Py_UNUSED(closure))
{
SHA3object *self = _SHA3object_CAST(op);
uint32_t rate = Hacl_Hash_SHA3_block_len(self->hash_state);
return PyLong_FromLong(rate);
return PyLong_FromLong(self->block_size);
}


Expand Down Expand Up @@ -371,18 +381,15 @@ SHA3_get_digest_size(PyObject *op, void *Py_UNUSED(closure))
{
// Preserving previous behavior: variable-length algorithms return 0
SHA3object *self = _SHA3object_CAST(op);
if (Hacl_Hash_SHA3_is_shake(self->hash_state))
return PyLong_FromLong(0);
else
return PyLong_FromLong(Hacl_Hash_SHA3_hash_len(self->hash_state));
return PyLong_FromLong(self->digest_size);
}


static PyObject *
SHA3_get_capacity_bits(PyObject *op, void *Py_UNUSED(closure))
{
SHA3object *self = _SHA3object_CAST(op);
uint32_t rate = Hacl_Hash_SHA3_block_len(self->hash_state) * 8;
uint32_t rate = self->block_size * 8;
assert(rate <= 1600);
int capacity = 1600 - rate;
return PyLong_FromLong(capacity);
Expand All @@ -393,8 +400,7 @@ static PyObject *
SHA3_get_rate_bits(PyObject *op, void *Py_UNUSED(closure))
{
SHA3object *self = _SHA3object_CAST(op);
uint32_t rate = Hacl_Hash_SHA3_block_len(self->hash_state) * 8;
return PyLong_FromLong(rate);
return PyLong_FromLong(self->block_size * 8);
}

static PyObject *
Expand Down
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