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bpo-33632: Avoid signed integer overflow in the _thread module #12729

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bpo-33632: Avoid signed integer overflow in the _thread module #12729

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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2019-04-08-08-29-45.bpo-33632.y-fhEY.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Avoid signed integer overflow in the :mod:`_thread` module.
17 changes: 10 additions & 7 deletions Modules/_threadmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -45,11 +45,12 @@ static PyLockStatus
acquire_timed(PyThread_type_lock lock, _PyTime_t timeout)
{
PyLockStatus r;
_PyTime_t endtime = 0;
_PyTime_t starttime = 0;
_PyTime_t microseconds;

if (timeout > 0)
endtime = _PyTime_GetMonotonicClock() + timeout;
if (timeout > 0) {
starttime = _PyTime_GetMonotonicClock();
}

do {
microseconds = _PyTime_AsMicroseconds(timeout, _PyTime_ROUND_CEILING);
Expand All @@ -73,13 +74,15 @@ acquire_timed(PyThread_type_lock lock, _PyTime_t timeout)
/* If we're using a timeout, recompute the timeout after processing
* signals, since those can take time. */
if (timeout > 0) {
timeout = endtime - _PyTime_GetMonotonicClock();
_PyTime_t elapsed = _PyTime_GetMonotonicClock() - starttime;

/* Check for negative values, since those mean block forever.
*/
if (timeout < 0) {
if (timeout < elapsed) {
r = PY_LOCK_FAILURE;
}
else {
timeout -= elapsed;
starttime += elapsed;
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I'm not sure that this part is really needed. I would prefer to leave starttime and timeout unchanged.
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Thank you, @vstinner, for your review.
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The original code also updates timeout every time it is interrupted. If you don’t adjust the timeout, I think the overall timeout could be too long. I expect you would be able to test this by installing a Python signal handler and sending signals while acquire or whatever method is running.

Perhaps you would prefer to calculate (overall) timeout − (overall) elapsed and pass that to PyThread_acquire_lock_timed.
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starttime += elapsed will cause starttime overflow. Because starttime is closer to _PyTime_GetMonotonicClock() + timeout by accumulating, and _PyTime_GetMonotonicClock() + timeout may overflow.

I think the else block is not needed.
}
}
}
} while (r == PY_LOCK_INTR); /* Retry if we were interrupted. */
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