ALSA: emu8000: Fix a use after free in snd_emu8000_create_mixer

Our code analyzer reported a uaf.

In snd_emu8000_create_mixer, the callee snd_ctl_add(..,emu->controls[i])
calls snd_ctl_add_replace(.., kcontrol,..). Inside snd_ctl_add_replace(),
if error happens, kcontrol will be freed by snd_ctl_free_one(kcontrol).
Then emu->controls[i] points to a freed memory, and the execution comes
to __error branch of snd_emu8000_create_mixer. The freed emu->controls[i]
is used in snd_ctl_remove(card, emu->controls[i]).

My patch set emu->controls[i] to NULL if snd_ctl_add() failed to avoid
the uaf.
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210426131129.4796-1-lyl2019@mail.ustc.edu.cnSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/isa/sb/emu8000.c

@@ -1029,9 +1029,11 @@ snd_emu8000_create_mixer(struct snd_card *card, struct snd_emu8000 *emu)
 
 	memset(emu->controls, 0, sizeof(emu->controls));
 	for (i = 0; i < EMU8000_NUM_CONTROLS; i++) {
-		if ((err = snd_ctl_add(card, emu->controls[i] = snd_ctl_new1(mixer_defs[i], emu))) < 0)
+		if ((err = snd_ctl_add(card, emu->controls[i] = snd_ctl_new1(mixer_defs[i], emu))) < 0) {
+			emu->controls[i] = NULL;
 			goto __error;
 		}
+	}
 	return 0;
 
 __error: