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Thu, 3 Sep 2026 17:55:40 +0800 (CST) X-Sender: hdanton@sina.com X-Auth-ID: hdanton@sina.com Authentication-Results: sina.com; spf=none smtp.mailfrom=hdanton@sina.com; dkim=none header.i=none; dmarc=none action=none header.from=hdanton@sina.com X-SMAIL-MID: 8448236685242 X-SMAIL-UIID: 8D7F7C77A2FA4BD4ACDB80B6024E9B0F-20260903-175540-1 From: Hillf Danton To: Michal Pecio Cc: syzbot , linux-media@vger.kernel.org, linux-usb@vger.kernel.org, mchehab@kernel.org, syzkaller-bugs@googlegroups.com, Daniel Mack , Takashi Iwai , linux-sound@vger.kernel.org Subject: Re: [syzbot] [usb?] KASAN: slab-use-after-free Write in usb_free_urb Date: Thu, 3 Sep 2026 17:55:25 +0800 Message-ID: <20260903095527.2048-1-hdanton@sina.com> In-Reply-To: <20260903112417.21b76017.michal.pecio@gmail.com> References: Precedence: bulk X-Mailing-List: linux-media@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Thu, 3 Sep 2026 11:24:17 +0200 Michal Pecio wrote: >On Tue, 01 Sep 2026 22:09:33 -0700, syzbot wrote: >> syzbot has found a reproducer for the following issue on: >> >> HEAD commit: 786262be6048 Merge tag 'edac_updates_for_v7.3_rc2' of git:.. >> git tree: upstream >> console+strace: https://syzkaller.appspot.com/x/log.txt?x=12a28125580000 >> kernel config: https://syzkaller.appspot.com/x/.config?x=b454dc6b1b7acd30 >> dashboard link: https://syzkaller.appspot.com/bug?extid=832ce9fa3face1b7d44d >> compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8 >> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1735af79580000 >> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=135b7f79580000 >> >> Downloadable assets: >> disk image: https://storage.googleapis.com/syzbot-assets/c3e528ebf709/disk-786262be.raw.xz >> vmlinux: https://storage.googleapis.com/syzbot-assets/46f3418d2828/vmlinux-786262be.xz >> kernel image: https://storage.googleapis.com/syzbot-assets/60ab8a27c4f7/bzImage-786262be.xz >> >> IMPORTANT: if you fix the issue, please add the following tag to the commit: >> Reported-by: syzbot+832ce9fa3face1b7d44d@syzkaller.appspotmail.com >> >> usb 1-1: received EP1 urb->status = -2 >> ================================================================== >> BUG: KASAN: slab-use-after-free in instrument_atomic_read_write include/linux/instrumented.h:112 [inline] >> BUG: KASAN: slab-use-after-free in atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline] >> BUG: KASAN: slab-use-after-free in __refcount_sub_and_test include/linux/refcount.h:389 [inline] >> BUG: KASAN: slab-use-after-free in __refcount_dec_and_test include/linux/refcount.h:432 [inline] >> BUG: KASAN: slab-use-after-free in refcount_dec_and_test include/linux/refcount.h:450 [inline] >> BUG: KASAN: slab-use-after-free in kref_put include/linux/kref.h:64 [inline] >> BUG: KASAN: slab-use-after-free in usb_free_urb+0x24/0x120 drivers/usb/core/urb.c:96 >> Write of size 4 at addr ffff88803cee1050 by task ktimers/1/29 >> >> CPU: 1 UID: 0 PID: 29 Comm: ktimers/1 Not tainted syzkaller #0 PREEMPT_{RT,(full)} >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026 >> Call Trace: >> >> dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120 >> print_address_description+0x55/0x1e0 mm/kasan/report.c:378 >> print_report+0x58/0x70 mm/kasan/report.c:482 >> kasan_report+0x117/0x150 mm/kasan/report.c:595 >> check_region_inline mm/kasan/generic.c:-1 [inline] >> kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200 >> instrument_atomic_read_write include/linux/instrumented.h:112 [inline] >> atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline] >> __refcount_sub_and_test include/linux/refcount.h:389 [inline] >> __refcount_dec_and_test include/linux/refcount.h:432 [inline] >> refcount_dec_and_test include/linux/refcount.h:450 [inline] >> kref_put include/linux/kref.h:64 [inline] >> usb_free_urb+0x24/0x120 drivers/usb/core/urb.c:96 >> dummy_timer+0xaac/0x4d50 drivers/usb/gadget/udc/dummy_hcd.c:2019 >> __run_hrtimer kernel/time/hrtimer.c:2067 [inline] >> __hrtimer_run_queues+0x3eb/0xaf0 kernel/time/hrtimer.c:2124 >> hrtimer_run_softirq+0x1e1/0x2e0 kernel/time/hrtimer.c:2141 >> handle_softirqs+0x1da/0x6d0 kernel/softirq.c:645 >> __do_softirq kernel/softirq.c:679 [inline] >> run_ktimerd+0x6a/0x100 kernel/softirq.c:1183 >> smpboot_thread_fn+0x565/0xa70 kernel/smpboot.c:160 >> kthread+0x38b/0x470 kernel/kthread.c:436 >> ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158 >> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245 >> > >[Adding sound people] > > So what happens here is that USB core continues to use a URB after > completion to implement things like usb_kill_urb(), so URBs are > reference counted. Then core decrements the count - one more use. > > If a driver waits for completion or even usb_kill_urb() to return > and then proceeds to free the URB's storage, this becomes a UAF. > This driver embeds 2 URBs in its priv and does just that. > Given dummy_urb_dequeue(), the dequeued urb should be tagged not to be handled again in the dummy_timer callback instead, just my $0.02