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Mon, 21 Apr 2025 07:23:46 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id tScZAYLyBWgcVwAAD6G6ig (envelope-from ); Mon, 21 Apr 2025 07:23:46 +0000 Date: Mon, 21 Apr 2025 09:23:45 +0200 Message-ID: <87mscat1tq.wl-tiwai@suse.de> From: Takashi Iwai To: Hillf Danton Cc: alsa-devel@alsa-project.org, Mikhail Gavrilov , LKML Subject: Re: [PATCH 2/9] ALSA: usb-audio: Fix possible race at sync of urb completions In-Reply-To: <20250421051832.4179-1-hdanton@sina.com> References: <20210929080844.11583-1-tiwai@suse.de> <20210929080844.11583-3-tiwai@suse.de> <20250418103533.4078-1-hdanton@sina.com> <87a58dvia7.wl-tiwai@suse.de> <20250418144518.4097-1-hdanton@sina.com> <875xj0ve49.wl-tiwai@suse.de> <20250419080410.4148-1-hdanton@sina.com> <871ptnuvad.wl-tiwai@suse.de> <20250421051832.4179-1-hdanton@sina.com> 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-Level: X-Spamd-Result: default: False [-1.80 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; MID_CONTAINS_FROM(1.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; TAGGED_RCPT(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; TO_DN_SOME(0.00)[]; FREEMAIL_TO(0.00)[sina.com]; FREEMAIL_ENVRCPT(0.00)[gmail.com,sina.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[alsa-project.org,gmail.com,vger.kernel.org]; RCPT_COUNT_THREE(0.00)[4]; FROM_EQ_ENVFROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,suse.de:email,suse.de:mid] X-Spam-Score: -1.80 X-Spam-Flag: NO On Mon, 21 Apr 2025 07:18:31 +0200, Hillf Danton wrote: > > On Sun, 20 Apr 2025 09:49:46 +0200 Takashi Iwai wrote: > >On Sat, 19 Apr 2025 10:04:08 +0200, Hillf Danton wrote: > >> On Sat, 19 Apr 2025 08:50:46 +0200 Takashi Iwai wrote: > >> >On Fri, 18 Apr 2025 16:45:17 +0200, Hillf Danton wrote: > >> >> On Fri, 18 Apr 2025 13:08:32 +0200 Takashi Iwai wrote: > >> >> > On Fri, 18 Apr 2025 12:35:32 +0200 Hillf Danton wrote: > >> >> > > On Wed, 29 Sep 2021 10:08:37 +0200 Takashi Iwai wrote: > >> >> > > > USB-audio driver tries to sync with the clear of all pending URBs in > >> >> > > > wait_clear_urbs(), and it waits for all bits in active_mask getting > >> >> > > > cleared. This works fine for the normal operations, but when a stream > >> >> > > > is managed in the implicit feedback mode, there is still a very thin > >> >> > > > race window: namely, in snd_complete_usb(), the active_mask bit for > >> >> > > > the current URB is once cleared before re-submitted in > >> >> > > > queue_pending_output_urbs(). If wait_clear_urbs() is called during > >> >> > > > that period, it may pass the test and go forward even though there may > >> >> > > > be a still pending URB. > >> >> > > > > >> >> > > > For covering it, this patch adds a new counter to each endpoint to > >> >> > > > keep the number of in-flight URBs, and changes wait_clear_urbs() > >> >> > > > checking this number instead. The counter is decremented at the end > >> >> > > > of URB complete, hence the reference is kept as long as the URB > >> >> > > > complete is in process. > >> >> > > > > >> >> > > > Signed-off-by: Takashi Iwai > >> >> > > > --- > >> >> > > > sound/usb/card.h | 1 + > >> >> > > > sound/usb/endpoint.c | 7 ++++++- > >> >> > > > 2 files changed, 7 insertions(+), 1 deletion(-) > >> >> > > > > >> >> > > > diff --git a/sound/usb/card.h b/sound/usb/card.h > >> >> > > > index 3329ce710cb9..746a765b2437 100644 > >> >> > > > --- a/sound/usb/card.h > >> >> > > > +++ b/sound/usb/card.h > >> >> > > > @@ -97,6 +97,7 @@ struct snd_usb_endpoint { > >> >> > > > unsigned int nominal_queue_size; /* total buffer sizes in URBs */ > >> >> > > > unsigned long active_mask; /* bitmask of active urbs */ > >> >> > > > unsigned long unlink_mask; /* bitmask of unlinked urbs */ > >> >> > > > + atomic_t submitted_urbs; /* currently submitted urbs */ > >> >> > > > char *syncbuf; /* sync buffer for all sync URBs */ > >> >> > > > dma_addr_t sync_dma; /* DMA address of syncbuf */ > >> >> > > > > >> >> > > > diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c > >> >> > > > index 29c4865966f5..06241568abf7 100644 > >> >> > > > --- a/sound/usb/endpoint.c > >> >> > > > +++ b/sound/usb/endpoint.c > >> >> > > > @@ -451,6 +451,7 @@ static void queue_pending_output_urbs(struct snd_usb_endpoint *ep) > >> >> > > > } > >> >> > > > > >> >> > > > set_bit(ctx->index, &ep->active_mask); > >> >> > > > + atomic_inc(&ep->submitted_urbs); > >> >> > > > } > >> >> > > > } > >> >> > > > > >> >> > > > @@ -488,6 +489,7 @@ static void snd_complete_urb(struct urb *urb) > >> >> > > > clear_bit(ctx->index, &ep->active_mask); > >> >> > > > spin_unlock_irqrestore(&ep->lock, flags); > >> >> > > > queue_pending_output_urbs(ep); > >> >> > > > >> >> > > smp_mb(); > >> >> > > > >> >> > > > + atomic_dec(&ep->submitted_urbs); /* decrement at last */ > >> >> > > > >> >> > > Does it match the comment to add a mb? > >> >> > > >> >> > How...? I don't understand your intention. > >> >> > > >> >> In addition to the UAF report [1], I saw a customer report of list > >> >> corruption of linux-6.1.99 on arm64 this week without reproducer. > >> >> > >> >> list corruption > >> >> list_add_tail(); > >> >> push_back_to_ready_list(); > >> >> snd_complete_urb(); > >> >> > >> >> And after another look at this patch I wonder if the race can not be > >> >> erased without the certainty that ep will be no longer used after the > >> >> atomic decrement. > >> > > >> > But why adding more barrier if you perform the atomic op...? > >> > > >> Because atomic op != ordering, see 26fbe9772b8c ("USB: core: Fix hang > >> in usb_kill_urb by adding memory barriers") for detail for example. > >> And c5b2cbdbdac5 ("ipc/mqueue.c: update/document memory barriers") as well. > > > > Still don't get it. Which reads and writes are you trying to solve? > > > See a simpler UAF case IIUIC. > > cpu1 cpu2 > atomic_dec(&ep->submitted_urbs); > > if (!atomic_read(&ep->submitted_urbs)) > kfree(ep); > > a = ep->xxx; // UAF That's what I don't get it. Which place does this UAF happen, more specifically? In the whole conversations, the context is missing, and you provided only a snippet of the patch. > > And more importantly, does it actually help at all? If yes, I'd > > Yes and No. Yes because of the fix mentioned in ipc wrt mb. > > > happily take the patch, of course. > > > Provided a repro, a fix like the following diff should have been posted. No idea about repro and the report itself, so hard to judge. Takashi > > Hillf > > --- x/sound/usb/endpoint.c > +++ y/sound/usb/endpoint.c > @@ -569,8 +569,11 @@ static void snd_complete_urb(struct urb > snd_usb_queue_pending_output_urbs(ep, false); > /* decrement at last, and check xrun */ > if (atomic_dec_and_test(&ep->submitted_urbs) && > - !snd_usb_endpoint_implicit_feedback_sink(ep)) > + !snd_usb_endpoint_implicit_feedback_sink(ep)) { > notify_xrun(ep); > + smp_mb(); > + atomic_dec(&ep->submitted_urbs); > + } > return; > } > > @@ -602,7 +605,8 @@ static void snd_complete_urb(struct urb > > exit_clear: > clear_bit(ctx->index, &ep->active_mask); > - atomic_dec(&ep->submitted_urbs); > + if (atomic_dec_and_test(&ep->submitted_urbs)) > + atomic_dec(&ep->submitted_urbs); > } > > /* > @@ -1004,17 +1008,17 @@ static int wait_clear_urbs(struct snd_us > > do { > alive = atomic_read(&ep->submitted_urbs); > - if (!alive) > - break; > + if (alive < 0) > + goto update; > > schedule_timeout_uninterruptible(1); > } while (time_before(jiffies, end_time)); > > - if (alive) > - usb_audio_err(ep->chip, > + usb_audio_err(ep->chip, > "timeout: still %d active urbs on EP #%x\n", > alive, ep->ep_num); > > +update: > if (ep_state_update(ep, EP_STATE_STOPPING, EP_STATE_STOPPED)) { > ep->sync_sink = NULL; > snd_usb_endpoint_set_callback(ep, NULL, NULL, NULL);