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Date:   Tue, 10 Dec 2019 15:17:01 -0500 (EST)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     syzbot <syzbot+7fa38a608b1075dfd634@syzkaller.appspotmail.com>
cc:     andreyknvl@google.com, <hverkuil@xs4all.nl>,
        <jrdr.linux@gmail.com>,
        Kernel development list <linux-kernel@vger.kernel.org>,
        <linux-media@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>, <mchehab@kernel.org>,
        <rfontana@redhat.com>, <syzkaller-bugs@googlegroups.com>,
        <tglx@linutronix.de>
Subject: Re: Re: KASAN: use-after-free Read in usbvision_v4l2_open
In-Reply-To: <00000000000031a0af05995eca0b@google.com>
Message-ID: <Pine.LNX.4.44L0.1912101513580.1647-100000@iolanthe.rowland.org>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
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On Tue, 10 Dec 2019, syzbot wrote:

> > On Mon, 9 Dec 2019, syzbot wrote:
> 
> >> Hello,
> 
> >> syzbot found the following crash on:
> 
> >> HEAD commit:    1f22d15c usb: gadget: add raw-gadget interface
> >> git tree:       https://github.com/google/kasan.git usb-fuzzer
> >> console output: https://syzkaller.appspot.com/x/log.txt?x=1296f42ae00000
> >> kernel config:   
> >> https://syzkaller.appspot.com/x/.config?x=8ccee2968018adcb
> >> dashboard link:  
> >> https://syzkaller.appspot.com/bug?extid=c7b0ec009a216143df30
> >> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> 
> >> Unfortunately, I don't have any reproducer for this crash yet.
> 
> >> IMPORTANT: if you fix the bug, please add the following tag to the  
> >> commit:
> >> Reported-by: syzbot+c7b0ec009a216143df30@syzkaller.appspotmail.com
> 
> >> ==================================================================
> >> BUG: KASAN: use-after-free in __mutex_lock_common
> >> kernel/locking/mutex.c:1043 [inline]
> >> BUG: KASAN: use-after-free in __mutex_lock+0x124d/0x1360
> >> kernel/locking/mutex.c:1106
> >> Read of size 8 at addr ffff8881cad4d8b8 by task v4l_id/4526
> 
> >> CPU: 0 PID: 4526 Comm: v4l_id Not tainted 5.4.0-syzkaller #0
> >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> >> Google 01/01/2011
> >> Call Trace:
> >>    __dump_stack lib/dump_stack.c:77 [inline]
> >>    dump_stack+0xef/0x16e lib/dump_stack.c:118
> >>    print_address_description.constprop.0+0x36/0x50 mm/kasan/report.c:374
> >>    __kasan_report.cold+0x1a/0x33 mm/kasan/report.c:506
> >>    kasan_report+0xe/0x20 mm/kasan/common.c:638
> >>    __mutex_lock_common kernel/locking/mutex.c:1043 [inline]
> >>    __mutex_lock+0x124d/0x1360 kernel/locking/mutex.c:1106
> >>    usbvision_v4l2_open+0x77/0x340
> >> drivers/media/usb/usbvision/usbvision-video.c:314
> >>    v4l2_open+0x20f/0x3d0 drivers/media/v4l2-core/v4l2-dev.c:423
> >>    chrdev_open+0x219/0x5c0 fs/char_dev.c:414
> >>    do_dentry_open+0x494/0x1120 fs/open.c:797
> >>    do_last fs/namei.c:3412 [inline]
> >>    path_openat+0x142b/0x4030 fs/namei.c:3529
> >>    do_filp_open+0x1a1/0x280 fs/namei.c:3559
> >>    do_sys_open+0x3c0/0x580 fs/open.c:1097
> >>    do_syscall_64+0xb7/0x5b0 arch/x86/entry/common.c:294
> >>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> > This looks like a race in v4l2_open(): The function drops the
> > videodev_lock mutex before calling the video driver's open routine, and
> > the device can be unregistered during the short time between.
> 
> > This patch tries to make the race much more likely to happen, for
> > testing and verification.
> 
> > Andrey, will syzbot run the same test with this patch, even though it
> > says it doesn't have a reproducer?
> 
> > Alan Stern
> 
> > #syz test: https://github.com/google/kasan.git 1f22d15c
> 
> This crash does not have a reproducer. I cannot test it.

Let's try the same patch with a different bug report -- one that has a
reproducer.  I assume that syzbot gets the bug identity from the
email's From: line (which has been updated acoordingly) rather than the 
Subject: line.

#syz test: https://github.com/google/kasan.git 1f22d15c

Index: usb-devel/drivers/media/usb/usbvision/usbvision-video.c
===================================================================
--- usb-devel.orig/drivers/media/usb/usbvision/usbvision-video.c
+++ usb-devel/drivers/media/usb/usbvision/usbvision-video.c
@@ -1585,6 +1585,7 @@ static void usbvision_disconnect(struct
 		wake_up_interruptible(&usbvision->wait_frame);
 		wake_up_interruptible(&usbvision->wait_stream);
 	} else {
+		msleep(100);
 		usbvision_release(usbvision);
 	}
 
Index: usb-devel/drivers/media/v4l2-core/v4l2-dev.c
===================================================================
--- usb-devel.orig/drivers/media/v4l2-core/v4l2-dev.c
+++ usb-devel/drivers/media/v4l2-core/v4l2-dev.c
@@ -419,9 +419,10 @@ static int v4l2_open(struct inode *inode
 	video_get(vdev);
 	mutex_unlock(&videodev_lock);
 	if (vdev->fops->open) {
-		if (video_is_registered(vdev))
+		if (video_is_registered(vdev)) {
+			msleep(200);
 			ret = vdev->fops->open(filp);
-		else
+		} else
 			ret = -ENODEV;
 	}