ALSA: seq: Fix possible UAF in snd_seq_check_queue()
Although we've covered the races between concurrent write() and
ioctl() in the previous patch series, there is still a possible UAF in
the following scenario:
So the problem is that a cell is peeked and accessed without any
protection until it's retrieved from the queue again via
snd_seq_prioq_cell_out().
This patch tries to address it, also cleans up the code by a slight
refactoring. snd_seq_prioq_cell_out() now receives an extra pointer
argument. When it's non-NULL, the function checks the event timestamp
with the given pointer. The caller needs to pass the right reference
either to snd_seq_tick or snd_seq_realtime depending on the event
timestamp type.
A good news is that the above change allows us to remove the
snd_seq_prioq_cell_peek(), too, thus the patch actually reduces the
code size.