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Tue, 23 Sep 2025 06:16:45 +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 5m2WLE070mhCFwAAD6G6ig (envelope-from ); Tue, 23 Sep 2025 06:16:45 +0000 Date: Tue, 23 Sep 2025 08:16:45 +0200 Message-ID: <87plbhn16a.wl-tiwai@suse.de> From: Takashi Iwai To: Alan Stern Cc: Brahmajit Das , syzbot+f02665daa2abeef4a947@syzkaller.appspotmail.com, clemens@ladisch.de, linux-kernel@vger.kernel.org, linux-sound@vger.kernel.org, linux-usb@vger.kernel.org, perex@perex.cz, syzkaller-bugs@googlegroups.com, tiwai@suse.com Subject: Re: [PATCH 1/1] ALSA: usb-audio: Avoid NULL dereference in snd_usbmidi_do_output() In-Reply-To: <43ab3d0e-4b56-4292-aa51-2473e766dca5@rowland.harvard.edu> References: <68d17f44.050a0220.13cd81.05b7.GAE@google.com> <20250922231720.3603805-1-listout@listout.xyz> <43ab3d0e-4b56-4292-aa51-2473e766dca5@rowland.harvard.edu> 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)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; MID_CONTAINS_FROM(1.00)[]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_VIA_SMTP_AUTH(0.00)[]; TAGGED_RCPT(0.00)[f02665daa2abeef4a947]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_SEVEN(0.00)[10]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:mid,syzkaller.appspot.com:url,appspotmail.com:email] X-Spam-Flag: NO X-Spam-Score: -1.80 On Tue, 23 Sep 2025 04:40:13 +0200, Alan Stern wrote: > > On Tue, Sep 23, 2025 at 04:47:20AM +0530, Brahmajit Das wrote: > > Syzkaller reported a general protection fault in snd_usbmidi_do_output(), > > caused by dereferencing a NULL URB pointer when accessing > > ep->urbs[urb_index].urb. > > > > This can happen in rare race conditions where the URB was not initialized > > or was already freed (e.g. during disconnect or after errors), and the > > output timer or other path tries to reuse it. > > > > Fix this by checking if the URB is NULL before accessing it, and skipping > > the current slot if it is. > > > > Reported-by: syzbot+f02665daa2abeef4a947@syzkaller.appspotmail.com > > Link: https://syzkaller.appspot.com/bug?extid=f02665daa2abeef4a947 > > > > Signed-off-by: Brahmajit Das > > --- > > sound/usb/midi.c | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/sound/usb/midi.c b/sound/usb/midi.c > > index acb3bf92857c..7919a39decb4 100644 > > --- a/sound/usb/midi.c > > +++ b/sound/usb/midi.c > > @@ -307,6 +307,10 @@ static void snd_usbmidi_do_output(struct snd_usb_midi_out_endpoint *ep) > > for (;;) { > > if (!(ep->active_urbs & (1 << urb_index))) { > > urb = ep->urbs[urb_index].urb; > > + if (!urb) { > > + // Skip this urb > > + goto next_urb; > > + } > > What prevents the URB from being freed right here? If this happens, > the code below would access memory that was deallocated. > > To prevent races, you have to use some sort of lock or other > synchronization mechanism. A simple test won't work. Yes. The timer instance itself should have been killed before releasing the resources. I guess the patch below could cover it, but since I'm traveling, I can't check it for now. Will check later once after I'm back. thanks, Takashi --- a/sound/usb/midi.c +++ b/sound/usb/midi.c @@ -1511,6 +1511,7 @@ static void snd_usbmidi_free(struct snd_usb_midi *umidi) { int i; + timer_shutdown_sync(&umidi->error_timer); for (i = 0; i < MIDI_MAX_ENDPOINTS; ++i) { struct snd_usb_midi_endpoint *ep = &umidi->endpoints[i]; if (ep->out) @@ -1519,7 +1520,6 @@ static void snd_usbmidi_free(struct snd_usb_midi *umidi) snd_usbmidi_in_endpoint_delete(ep->in); } mutex_destroy(&umidi->mutex); - timer_shutdown_sync(&umidi->error_timer); kfree(umidi); }