[PATCH] sound/isa/sb/emu8000: Fix a use after free in snd_emu8000_create_mixer

[PATCH] sound/isa/sb/emu8000: Fix a use after free in snd_emu8000_create_mixer

[Lv Yunlong]

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>
---
 sound/isa/sb/emu8000.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/sound/isa/sb/emu8000.c b/sound/isa/sb/emu8000.c
index 0aa545ac6e60..1c90421a88dc 100644
--- a/sound/isa/sb/emu8000.c
+++ b/sound/isa/sb/emu8000.c
@@ -1029,8 +1029,10 @@ 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;
 
-- 
2.25.1

Re: [PATCH] sound/isa/sb/emu8000: Fix a use after free in snd_emu8000_create_mixer

[Takashi Iwai]

On Mon, 26 Apr 2021 15:11:29 +0200,
Lv Yunlong wrote:
> 
> 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>

Thanks, applied now.

The bug was hard to be seen due to the coding style, so we'd need a
cleanup, but it's a different story...


Takashi

Re: [PATCH] sound/isa/sb/emu8000: Fix a use after free in snd_emu8000_create_mixer

[Jaroslav Kysela]

Dne 26. 04. 21 v 16:23 Takashi Iwai napsal(a):
> On Mon, 26 Apr 2021 15:11:29 +0200,
> Lv Yunlong wrote:
>>
>> 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>
> 
> Thanks, applied now.
> 
> The bug was hard to be seen due to the coding style, so we'd need a
> cleanup, but it's a different story...

Yes, it would be better to assign the return value from snd_ctl_new1 to a
local variable and set emu->controls[i] only when everything succeeds.

					Jaroslav

-- 
Jaroslav Kysela <perex@perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.