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Subject: Re: [PATCH] sound/isa/sb/emu8000: Fix a use after free in
 snd_emu8000_create_mixer
To:     Takashi Iwai <tiwai@suse.de>, Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Cc:     tiwai@suse.com, alsa-devel@alsa-project.org,
        linux-kernel@vger.kernel.org
References: <20210426131129.4796-1-lyl2019@mail.ustc.edu.cn>
 <s5ha6pl2kh8.wl-tiwai@suse.de>
From:   Jaroslav Kysela <perex@perex.cz>
Message-ID: <af5a6764-76ee-3184-2622-a756c23fbc98@perex.cz>
Date:   Mon, 26 Apr 2021 16:38:41 +0200
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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.