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Tue, 26 Mar 2024 07:08:27 +0000 (UTC) Received: from dovecot-director2.suse.de ([10.150.64.162]) by imap1.dmz-prg2.suse.org with ESMTPSA id L5dhNWt0AmaiTAAAD6G6ig (envelope-from ); Tue, 26 Mar 2024 07:08:27 +0000 Date: Tue, 26 Mar 2024 08:08:29 +0100 Message-ID: <875xx9wjv6.wl-tiwai@suse.de> From: Takashi Iwai To: duoming@zju.edu.cn Cc: "Takashi Iwai" , linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org, tiwai@suse.com, perex@perex.cz Subject: Re: [PATCH] ALSA: sh: aica: reorder cleanup operations to avoid UAF bug In-Reply-To: <75af0c57.6e39.18e796e478f.Coremail.duoming@zju.edu.cn> References: <20240325033946.47052-1-duoming@zju.edu.cn> <871q7yybeb.wl-tiwai@suse.de> <43e102f3.61dc.18e7601a2f2.Coremail.duoming@zju.edu.cn> <874jcuxtf9.wl-tiwai@suse.de> <75af0c57.6e39.18e796e478f.Coremail.duoming@zju.edu.cn> User-Agent: Wanderlust/2.15.9 (Almost Unreal) Emacs/27.2 Mule/6.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-Spam-Score: -3.30 X-Spamd-Result: default: False [-3.30 / 50.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; NEURAL_HAM_LONG(-1.00)[-0.999]; MIME_GOOD(-0.10)[text/plain]; RCPT_COUNT_FIVE(0.00)[6]; RCVD_COUNT_THREE(0.00)[3]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-0.995]; MID_CONTAINS_FROM(1.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; BAYES_HAM(-3.00)[100.00%] X-Spam-Level: Authentication-Results: smtp-out1.suse.de; none X-Spam-Flag: NO On Tue, 26 Mar 2024 07:24:14 +0100, duoming@zju.edu.cn wrote: > > On Mon, 25 Mar 2024 15:44:26 +0100 Takashi Iwai wrote: > > > > > The dreamcastcard->timer could schedule the spu_dma_work and the > > > > > spu_dma_work could also arm the dreamcastcard->timer. > > > > > > > > > > When the Yamaha AICA card is closing, the dreamcastcard->channel > > > > > will be deallocated. But it could still be dereferenced in the > > > > > worker thread. The reason is that del_timer() will return directly > > > > > regardless of whether the timer handler is running or not and the > > > > > worker could be rescheduled in the timer handler. As a result, the > > > > > UAF bug will happen. The racy situation is shown below: > > > > > > > > > > (Thread 1) | (Thread 2) > > > > > snd_aicapcm_pcm_close() | > > > > > ... | run_spu_dma() //worker > > > > > | mod_timer() > > > > > flush_work() | > > > > > del_timer() | aica_period_elapsed() //timer > > > > > kfree(dreamcastcard->channel) | schedule_work() > > > > > | run_spu_dma() //worker > > > > > ... | dreamcastcard->channel-> //USE > > > > > > > > > > In order to mitigate this bug, use timer_shutdown_sync() to shutdown > > > > > the timer and then use flush_work() to cancel the worker. > > > > > > > > > > Fixes: 198de43d758c ("[ALSA] Add ALSA support for the SEGA Dreamcast PCM device") > > > > > Signed-off-by: Duoming Zhou > > > > > --- > > > > > sound/sh/aica.c | 2 +- > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > diff --git a/sound/sh/aica.c b/sound/sh/aica.c > > > > > index 320ac792c7f..bc68a3903f2 100644 > > > > > --- a/sound/sh/aica.c > > > > > +++ b/sound/sh/aica.c > > > > > @@ -354,8 +354,8 @@ static int snd_aicapcm_pcm_close(struct snd_pcm_substream > > > > > *substream) > > > > > { > > > > > struct snd_card_aica *dreamcastcard = substream->pcm->private_data; > > > > > + timer_shutdown_sync(&dreamcastcard->timer); > > > > > > > > I thought this call invalidates the timer object, hence it can't be > > > > used again; i.e. it breaks when the stream is re-opened, I suppose? > > > > > > > > In general timer_shutdown*() is used for the code path to clean up the > > > > driver (or the object the timer belongs to). The PCM close is only > > > > about the PCM stream, and it's not the place. > > > > > > > > A proper fix would be rather to implement two things: > > > > - Call mod_timer() conditionally in run_spu_dma() > > > > - Implement PCM sync_stop op to cancel/flush the work > > > > > > > > The former alone should suffice to fix the UAF in your scenario, > > > > though. The latter will cover other possible corner cases. > > > > > > Thank you for your time and reply! I know using timer_shutdown_sync() > > > is not proper. In order to solve the problem, I add a shutdown flag > > > in the struct snd_card_aica and set the flag to true when the PCM > > > stream is closing. Then call mod_timer() conditionally in run_spu_dma(). > > > What's more, use del_timer_sync() to stop the timer and put it before > > > flush_work(). As a result, both timer and worker could be stopped safely. > > > The detail is shown below: > > > > You can use the existing API to check the PCM running state, e.g. > > > > --- a/sound/sh/aica.c > > +++ b/sound/sh/aica.c > > @@ -278,7 +278,8 @@ static void run_spu_dma(struct work_struct *work) > > dreamcastcard->clicks++; > > if (unlikely(dreamcastcard->clicks >= AICA_PERIOD_NUMBER)) > > dreamcastcard->clicks %= AICA_PERIOD_NUMBER; > > - mod_timer(&dreamcastcard->timer, jiffies + 1); > > + if (snd_pcm_running(dreamcastcard->substream)) > > + mod_timer(&dreamcastcard->timer, jiffies + 1); > > } > > } > > Thank you for your suggestions, The following is a new plan using the > existing API to mitigate the bug. > > diff --git a/sound/sh/aica.c b/sound/sh/aica.c > index 320ac792c7fe..bc003dd91a82 100644 > --- a/sound/sh/aica.c > +++ b/sound/sh/aica.c > @@ -278,7 +278,8 @@ static void run_spu_dma(struct work_struct *work) > dreamcastcard->clicks++; > if (unlikely(dreamcastcard->clicks >= AICA_PERIOD_NUMBER)) > dreamcastcard->clicks %= AICA_PERIOD_NUMBER; > - mod_timer(&dreamcastcard->timer, jiffies + 1); > + if (snd_pcm_running(dreamcastcard->substream)) > + mod_timer(&dreamcastcard->timer, jiffies + 1); > } > } > > @@ -316,6 +317,7 @@ static void spu_begin_dma(struct snd_pcm_substream *substream) > struct snd_pcm_runtime *runtime; > runtime = substream->runtime; > dreamcastcard = substream->pcm->private_data; > + __snd_pcm_set_state(runtime, SNDRV_PCM_STATE_RUNNING); Such an explicit state change isn't needed, rather wrong. The above condition check is performed only when kicked off from the timer handler, and that's always after the stream started. > @@ -354,8 +357,9 @@ static int snd_aicapcm_pcm_close(struct snd_pcm_substream > *substream) > { > struct snd_card_aica *dreamcastcard = substream->pcm->private_data; > + __snd_pcm_set_state(substream->runtime, SNDRV_PCM_STATE_DISCONNECTED); This breaks things again! You don't disconnect the device at closing the stream at all. And the state change is handled in PCM core side, not in the driver side. > + del_timer_sync(&dreamcastcard->timer); > flush_work(&(dreamcastcard->spu_dma_work)); > - del_timer(&dreamcastcard->timer); I'd leave this unchanged. The UAF itself is covered by the stream state check. And, if any, we can change more properly: - Add the same PCM state check at the beginning of aica_period_elapsed(), and bail out immediately if not running - Implement PCM sync_stop ops: it should have like static int snd_aicapcm_pcm_sync_stop(struct snd_pcm_substream *substream) { struct snd_card_aica *dreamcastcard = substream->pcm->private_data; del_timer_sync(&dreamcastcard->timer); cancel_work_sync(&dreamcastcard->spu_dma_work); return 0; } and get rid of the corresponding calls from snd_aicapcm_pcm_close() thanks, Takashi