ALSA: hda - Fix incorrect TLV callback check introduced during set_fs() removal

The commit 99b5c5bb9a54 ("ALSA: hda - Remove the use of set_fs()")
converted the get_kctl_0dB_offset() call for killing set_fs() usage in
HD-audio codec code.  The conversion assumed that the TLV callback
used in HD-audio code is only snd_hda_mixer_amp() and applies the TLV
calculation locally.

Although this assumption is correct, and all slave kctls are actually
with that callback, the current code is still utterly buggy; it
doesn't hit this condition and falls back to the next check.  It's
because the function gets called after adding slave kctls to vmaster.
By assigning a slave kctl, the slave kctl object is faked inside
vmaster code, and the whole kctl ops are overridden.  Thus the
callback op points to a different value from what we've assumed.

More badly, as reported by the KERNEXEC and UDEREF features of PaX,
the code flow turns into the unexpected pitfall.  The next fallback
check is SNDRV_CTL_ELEM_ACCESS_TLV_READ access bit, and this always
hits for each kctl with TLV.  Then it evaluates the callback function
pointer wrongly as if it were a TLV array.  Although currently its
side-effect is fairly limited, this incorrect reference may lead to an
unpleasant result.

For addressing the regression, this patch introduces a new helper to
vmaster code, snd_ctl_apply_vmaster_slaves().  This works similarly
like the existing map_slaves() in hda_codec.c: it loops over the slave
list of the given master, and applies the given function to each
slave.  Then the initializer function receives the right kctl object
and we can compare the correct pointer instead of the faked one.

Also, for catching the similar breakage in future, give an error
message when the unexpected TLV callback is found and bail out
immediately.

Fixes: 99b5c5bb9a54 ("ALSA: hda - Remove the use of set_fs()")
Reported-by: PaX Team <pageexec@freemail.hu>
Cc: <stable@vger.kernel.org> # v4.13
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 31 insertions, 0 deletions

diff --git a/sound/core/vmaster.c b/sound/core/vmaster.c
index 6c58e6f73a01..e43af18d4383 100644
--- a/sound/core/vmaster.c
+++ b/sound/core/vmaster.c
@@ -484,3 +484,34 @@ void snd_ctl_sync_vmaster(struct snd_kcontrol *kcontrol, bool hook_only)
 		master->hook(master->hook_private_data, master->val);
 }
 EXPORT_SYMBOL_GPL(snd_ctl_sync_vmaster);
+
+/**
+ * snd_ctl_apply_vmaster_slaves - Apply function to each vmaster slave
+ * @kctl: vmaster kctl element
+ * @func: function to apply
+ * @arg: optional function argument
+ *
+ * Apply the function @func to each slave kctl of the given vmaster kctl.
+ * Returns 0 if successful, or a negative error code.
+ */
+int snd_ctl_apply_vmaster_slaves(struct snd_kcontrol *kctl,
+				 int (*func)(struct snd_kcontrol *, void *),
+				 void *arg)
+{
+	struct link_master *master;
+	struct link_slave *slave;
+	int err;
+
+	master = snd_kcontrol_chip(kctl);
+	err = master_init(master);
+	if (err < 0)
+		return err;
+	list_for_each_entry(slave, &master->slaves, list) {
+		err = func(&slave->slave, arg);
+		if (err < 0)
+			return err;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(snd_ctl_apply_vmaster_slaves);