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([121.185.186.233]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-78102c0709csm6923531b3a.81.2025.09.27.08.41.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 27 Sep 2025 08:41:14 -0700 (PDT) From: Jeongjun Park To: tiwai@suse.de Cc: aha310510@gmail.com, clemens@ladisch.de, hdanton@sina.com, linux-kernel@vger.kernel.org, linux-sound@vger.kernel.org, perex@perex.cz, syzbot+f02665daa2abeef4a947@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com Subject: Re: [syzbot] [sound?] [usb?] general protection fault in snd_usbmidi_do_output Date: Sun, 28 Sep 2025 00:41:09 +0900 Message-Id: <20250927154109.879926-1-aha310510@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <87o6qw9koa.wl-tiwai@suse.de> References: <87o6qw9koa.wl-tiwai@suse.de> Precedence: bulk X-Mailing-List: linux-sound@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Hi, Takashi Iwai wrote: > > On Sat, 27 Sep 2025 12:36:07 +0200, > Jeongjun Park wrote: > > > > syzbot wrote: > > > > > > Hello, > > > > > > syzbot has tested the proposed patch but the reproducer is still triggering an issue: > > > KASAN: slab-use-after-free Write in snd_usbmidi_in_urb_complete > > > > > > ================================================================== > > > BUG: KASAN: slab-use-after-free in snd_usbmidi_in_urb_complete+0x389/0x3c0 sound/usb/midi.c:251 > > > Write of size 1 at addr ffff888074717943 by task kworker/1:3/5866 > > > > > > > Wow, the UAF bug still occurs? > > > > But... this UAF seems to be a problem with how midi handles urb rather > > than a problem with my patch. > > > > Is there something wrong with the way snd_usbmidi_in_urb_complete() is > > implemented? > > This can be rather a missing kill-and-cleanup in the code path. > So the patch like below. > > Could you check whether this works for you instead of your fix, too? > timer_shutdown_sync() is already called in snd_usbmidi_disconnect(), > and the call in snd_usbmidi_free() should be superfluous after this > change. > Since both my tests and syzbot tests did not cause any bugs when applying this patch, I think the root cause of this vulnerability is the missing kill error_timer + urb. So, I think it would be okay to patch it this way, but in addition, since most of the endpoint delete-related code that existed in snd_usbmidi_free() is now done in snd_usbmidi_disconnect(), I think it would be appropriate to modify it to only perform kfree(ep->out) as in this patch below. --- sound/usb/midi.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-) diff --git a/sound/usb/midi.c b/sound/usb/midi.c index acb3bf92857c..97e7e7662b12 100644 --- a/sound/usb/midi.c +++ b/sound/usb/midi.c @@ -1522,15 +1522,14 @@ static void snd_usbmidi_free(struct snd_usb_midi *umidi) { int i; + if (!umidi->disconnected) + snd_usbmidi_disconnect(&umidi->list); + for (i = 0; i < MIDI_MAX_ENDPOINTS; ++i) { struct snd_usb_midi_endpoint *ep = &umidi->endpoints[i]; - if (ep->out) - snd_usbmidi_out_endpoint_delete(ep->out); - if (ep->in) - snd_usbmidi_in_endpoint_delete(ep->in); + kfree(ep->out); } mutex_destroy(&umidi->mutex); - timer_shutdown_sync(&umidi->error_timer); kfree(umidi); } -- > > thanks, > > Takashi > > --- a/sound/usb/midi.c > +++ b/sound/usb/midi.c > @@ -1522,6 +1522,9 @@ static void snd_usbmidi_free(struct snd_usb_midi *umidi) > { > int i; > > + if (!umidi->disconnected) > + snd_usbmidi_disconnect(&umidi->list); > + > for (i = 0; i < MIDI_MAX_ENDPOINTS; ++i) { > struct snd_usb_midi_endpoint *ep = &umidi->endpoints[i]; > if (ep->out) Regards, Jeongjun Park