ALSA: seq: More protection for concurrent write and ioctl races

ALSA: seq: More protection for concurrent write and ioctl races

This patch is an attempt for further hardening against races between
the concurrent write and ioctls.  The previous fix d15d662e89fc
("ALSA: seq: Fix racy pool initializations") covered the race of the
pool initialization at writer and the pool resize ioctl by the
client->ioctl_mutex (CVE-2018-1000004).  However, basically this mutex
should be applied more widely to the whole write operation for
avoiding the unexpected pool operations by another thread.

The only change outside snd_seq_write() is the additional mutex
argument to helper functions, so that we can unlock / relock the given
mutex temporarily during schedule() call for blocking write.

Fixes: d15d662e89fc ("ALSA: seq: Fix racy pool initializations")
Reported-by: 范龙飞 <[email protected]>
Reported-by: Nicolai Stange <[email protected]>
Reviewed-and-tested-by: Nicolai Stange <[email protected]>
Cc: <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>