ALSA: hda: fix a possible null-pointer dereference due to data race in snd_hdac_regmap_sync()

In the Linux kernel, the following vulnerability has been resolved:

ALSA: hda: fix a possible null-pointer dereference due to data race in snd_hdac_regmap_sync()

The variable codec->regmap is often protected by the lock

codec->regmap_lock when is accessed. However, it is accessed without

holding the lock when is accessed in snd_hdac_regmap_sync():

if (codec->regmap)

In my opinion, this may be a harmful race, because if codec->regmap is

set to NULL right after the condition is checked, a null-pointer

dereference can occur in the called function regcache_sync():

map->lock(map->lock_arg); --> Line 360 in drivers/base/regmap/regcache.c

To fix this possible null-pointer dereference caused by data race, the

mutex_lock coverage is extended to protect the if statement as well as the

function call to regcache_sync().

[ Note: the lack of the regmap_lock itself is harmless for the current

codec driver implementations, as snd_hdac_regmap_sync() is only for

PM runtime resume that is prohibited during the codec probe.

But the change makes the whole code more consistent, so it's merged

as is -- tiwai ]