Port, board and/or hardware

unix coverage port

MicroPython version

MicroPython v1.26.0-preview.387.ge4e97f5aa7.dirty on 2025-07-20; linux [GCC 12.2.0] version

Reproduction

Run the following script:

import io
io.BufferedWriter(None, 1).write(b"foo")

Expected behaviour

Some kind of exception like a TypeError

Observed behaviour

Segmentation fault:

Program received signal SIGSEGV, Segmentation fault.
mp_stream_rw (stream=0x6, buf_=buf_@entry=0x7ffff7c30ba0, size=1, 
    errcode=errcode@entry=0x7fffffffd83c, flags=flags@entry=2 '\002')
    at ../../py/stream.c:50
50	    const mp_stream_p_t *stream_p = mp_get_stream(stream);
(gdb) p stream
$3 = (mp_obj_t) 0x6

Additional Information

Minimal validation of the suitablity of the stream object is missing.

This crash was found via automated fuzzing. I minimized the test case the fuzzer found.

Code of Conduct

Yes, I agree