[PATCH v2] media: meson: vdec: fix use-after-free of decode work in stop/close path

[Doruk Tan Ozturk]

Please drop v1 and v2 -- both are wrong, and the sashiko review was right
about the deadlock.

The underlying bug is real: vdec_close() does kfree(sess) (and
v4l2_m2m_ctx_release() frees sess->m2m_ctx) without cancelling
sess->esparser_queue_work, whose worker dereferences sess->lock and
sess->m2m_ctx -> UAF if it is pending/running at teardown.

But cancelling on the streamoff/poweroff path can't work:

1) Deadlock. The worker takes sess->lock. For an m2m fh the ioctl core
   takes m2m_ctx->q_lock (== sess->lock) for VIDIOC_STREAMOFF and holds it
   across the handler, so vdec_stop_streaming() -> vdec_poweroff() already
   runs under sess->lock; cancel_work_sync() there waits on a worker blocked
   on that same lock.

2) Use-after-power-down. v2 also cancelled after vdec_ops->stop(), which
   power-gates VDEC1 (__vdec_1_stop()), while the worker still reads a VDEC1
   register (vdec_1_vififo_level() -> VLD_MEM_VIFIFO_LEVEL).

The only deadlock-free point I see is vdec_close() (the ->release fop, not
under sess->lock), cancelling before v4l2_m2m_ctx_release() -- but that
still leaves the threaded VDEC ISR (amvdec_dst_buf_done() ->
schedule_work()) able to re-arm the worker, and there are adjacent teardown
issues (esparser_isr() vs the dos_parser_clk disable;
vdec_decoder_cmd()/esparser_queue_eos() without sess->lock).

I don't have Meson hardware to validate a corrected fix. Is a
vdec_close()-only cancel (plus quiescing the VDEC IRQ outside sess->lock)
the direction you'd want, or would you rather take it given the HW testing
and the surrounding teardown concerns?

Doruk