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Date:   Sat, 8 Aug 2020 10:25:26 +0100
From:   Sean Young <sean@...s.org>
To:     Ezequiel Garcia <ezequiel@...guardiasur.com.ar>
Cc:     syzbot <syzbot+ceef16277388d6f24898@...kaller.appspotmail.com>,
        andreyknvl@...gle.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-media <linux-media@...r.kernel.org>,
        linux-usb <linux-usb@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in rc_dev_uevent

Hi Eze,

On Fri, Aug 07, 2020 at 08:45:12PM -0300, Ezequiel Garcia wrote:
> On Fri, 7 Aug 2020 at 06:15, Sean Young <sean@...s.org> wrote:
> >
> > On Fri, Aug 07, 2020 at 12:26:29AM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    7b4ea945 Revert "x86/mm/64: Do not sync vmalloc/ioremap ma..
> > > git tree:       https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git usb-testing
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=11a7813a900000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=72a84c46d0c668c
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=ceef16277388d6f24898
> > > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+ceef16277388d6f24898@...kaller.appspotmail.com
> > >
> > > ==================================================================
> > > BUG: KASAN: use-after-free in string_nocheck lib/vsprintf.c:611 [inline]
> > > BUG: KASAN: use-after-free in string+0x39c/0x3d0 lib/vsprintf.c:693
> > > Read of size 1 at addr ffff8881ca21cd20 by task systemd-udevd/5147
> > >
> > > CPU: 1 PID: 5147 Comm: systemd-udevd Not tainted 5.8.0-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+0xf6/0x16e lib/dump_stack.c:118
> > >  print_address_description.constprop.0+0x1a/0x210 mm/kasan/report.c:383
> > >  __kasan_report mm/kasan/report.c:513 [inline]
> > >  kasan_report.cold+0x37/0x7c mm/kasan/report.c:530
> > >  string_nocheck lib/vsprintf.c:611 [inline]
> > >  string+0x39c/0x3d0 lib/vsprintf.c:693
> > >  vsnprintf+0x71b/0x14f0 lib/vsprintf.c:2617
> > >  add_uevent_var+0x14d/0x310 lib/kobject_uevent.c:664
> > >  rc_dev_uevent+0x54/0x140 drivers/media/rc/rc-main.c:1616
> > >  dev_uevent+0x30e/0x780 drivers/base/core.c:1916
> > >  uevent_show+0x1bb/0x360 drivers/base/core.c:1963
> > >  dev_attr_show+0x4b/0x90 drivers/base/core.c:1667
> > >  sysfs_kf_seq_show+0x1f8/0x400 fs/sysfs/file.c:60
> > >  seq_read+0x432/0x1070 fs/seq_file.c:208
> > >  kernfs_fop_read+0xe9/0x590 fs/kernfs/file.c:251
> > >  vfs_read+0x1df/0x520 fs/read_write.c:479
> > >  ksys_read+0x12d/0x250 fs/read_write.c:607
> > >  do_syscall_64+0x2d/0x40 arch/x86/entry/common.c:46
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > > RIP: 0033:0x7f6e6c02f910
> > > Code: b6 fe ff ff 48 8d 3d 0f be 08 00 48 83 ec 08 e8 06 db 01 00 66 0f 1f 44 00 00 83 3d f9 2d 2c 00 00 75 10 b8 00 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 de 9b 01 00 48 89 04 24
> > > RSP: 002b:00007fff3cddeae8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > > RAX: ffffffffffffffda RBX: 0000558492caaae0 RCX: 00007f6e6c02f910
> > > RDX: 0000000000001000 RSI: 0000558492cc7530 RDI: 0000000000000007
> > > RBP: 00007f6e6c2ea440 R08: 00007f6e6c2ee298 R09: 0000000000001010
> > > R10: 0000558492caaae0 R11: 0000000000000246 R12: 0000000000001000
> > > R13: 0000000000000d68 R14: 0000558492cc7530 R15: 00007f6e6c2e9900
> >
> > This thread is reading the uevent sysfs file, which reads
> > rc_dev->map.name, and also rc_dev->device_name, but that is not causing
> > problems is this case.
> >
> > >
> > > Allocated by task 5:
> > >  save_stack+0x1b/0x40 mm/kasan/common.c:48
> > >  set_track mm/kasan/common.c:56 [inline]
> > >  __kasan_kmalloc.constprop.0+0xc2/0xd0 mm/kasan/common.c:494
> > >  slab_post_alloc_hook mm/slab.h:586 [inline]
> > >  slab_alloc_node mm/slub.c:2824 [inline]
> > >  slab_alloc mm/slub.c:2832 [inline]
> > >  __kmalloc_track_caller+0xec/0x280 mm/slub.c:4430
> > >  kstrdup+0x36/0x70 mm/util.c:60
> > >  ir_create_table drivers/media/rc/rc-main.c:217 [inline]
> > >  ir_setkeytable drivers/media/rc/rc-main.c:477 [inline]
> > >  rc_prepare_rx_device drivers/media/rc/rc-main.c:1786 [inline]
> > >  rc_register_device+0x464/0x1600 drivers/media/rc/rc-main.c:1914
> > >  igorplugusb_probe+0x7e6/0xc98 drivers/media/rc/igorplugusb.c:209
> > >  usb_probe_interface+0x315/0x7f0 drivers/usb/core/driver.c:374
> > >  really_probe+0x291/0xde0 drivers/base/dd.c:553
> > >  driver_probe_device+0x26b/0x3d0 drivers/base/dd.c:738
> > >  __device_attach_driver+0x1d1/0x290 drivers/base/dd.c:844
> > >  bus_for_each_drv+0x15f/0x1e0 drivers/base/bus.c:431
> > >  __device_attach+0x228/0x4a0 drivers/base/dd.c:912
> > >  bus_probe_device+0x1e4/0x290 drivers/base/bus.c:491
> > >  device_add+0xb51/0x1c70 drivers/base/core.c:2930
> > >  usb_set_configuration+0xf05/0x18a0 drivers/usb/core/message.c:2032
> > >  usb_generic_driver_probe+0xba/0xf2 drivers/usb/core/generic.c:239
> > >  usb_probe_device+0xd9/0x250 drivers/usb/core/driver.c:272
> > >  really_probe+0x291/0xde0 drivers/base/dd.c:553
> > >  driver_probe_device+0x26b/0x3d0 drivers/base/dd.c:738
> > >  __device_attach_driver+0x1d1/0x290 drivers/base/dd.c:844
> > >  bus_for_each_drv+0x15f/0x1e0 drivers/base/bus.c:431
> > >  __device_attach+0x228/0x4a0 drivers/base/dd.c:912
> > >  bus_probe_device+0x1e4/0x290 drivers/base/bus.c:491
> > >  device_add+0xb51/0x1c70 drivers/base/core.c:2930
> > >  usb_new_device.cold+0x71d/0xfd4 drivers/usb/core/hub.c:2554
> > >  hub_port_connect drivers/usb/core/hub.c:5208 [inline]
> > >  hub_port_connect_change drivers/usb/core/hub.c:5348 [inline]
> > >  port_event drivers/usb/core/hub.c:5494 [inline]
> > >  hub_event+0x2361/0x4390 drivers/usb/core/hub.c:5576
> > >  process_one_work+0x94c/0x15f0 kernel/workqueue.c:2269
> > >  worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
> > >  kthread+0x392/0x470 kernel/kthread.c:292
> > >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> >
> > .. this probed the device ..
> >
> > > Freed by task 5:
> > >  save_stack+0x1b/0x40 mm/kasan/common.c:48
> > >  set_track mm/kasan/common.c:56 [inline]
> > >  kasan_set_free_info mm/kasan/common.c:316 [inline]
> > >  __kasan_slab_free+0x116/0x160 mm/kasan/common.c:455
> > >  slab_free_hook mm/slub.c:1474 [inline]
> > >  slab_free_freelist_hook+0x53/0x140 mm/slub.c:1507
> > >  slab_free mm/slub.c:3072 [inline]
> > >  kfree+0xbc/0x2c0 mm/slub.c:4052
> > >  ir_free_table drivers/media/rc/rc-main.c:245 [inline]
> > >  rc_free_rx_device drivers/media/rc/rc-main.c:1875 [inline]
> > >  rc_unregister_device+0x142/0x410 drivers/media/rc/rc-main.c:2014
> > >  igorplugusb_disconnect+0x58/0x110 drivers/media/rc/igorplugusb.c:232
> > >  usb_unbind_interface+0x1d8/0x8d0 drivers/usb/core/driver.c:436
> > >  __device_release_driver+0x3c6/0x6f0 drivers/base/dd.c:1153
> > >  device_release_driver_internal drivers/base/dd.c:1184 [inline]
> > >  device_release_driver+0x26/0x40 drivers/base/dd.c:1207
> > >  bus_remove_device+0x2eb/0x5a0 drivers/base/bus.c:533
> > >  device_del+0x481/0xd90 drivers/base/core.c:3107
> > >  usb_disable_device+0x387/0x930 drivers/usb/core/message.c:1245
> > >  usb_disconnect.cold+0x27d/0x780 drivers/usb/core/hub.c:2217
> > >  hub_port_connect drivers/usb/core/hub.c:5059 [inline]
> > >  hub_port_connect_change drivers/usb/core/hub.c:5348 [inline]
> > >  port_event drivers/usb/core/hub.c:5494 [inline]
> > >  hub_event+0x1c93/0x4390 drivers/usb/core/hub.c:5576
> > >  process_one_work+0x94c/0x15f0 kernel/workqueue.c:2269
> > >  process_scheduled_works kernel/workqueue.c:2331 [inline]
> > >  worker_thread+0x82b/0x1120 kernel/workqueue.c:2417
> > >  kthread+0x392/0x470 kernel/kthread.c:292
> > >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> >
> > This unplugged the device, and freed rc_dev->map->name and sets
> > it to NULL. There is no locking between the two threads so this is
> > a race condition.
> >
> > I think there are worse, related problems here. For example, iguanair
> > driver allocates rc_dev->device_name and frees it in its usb disconnect
> > handler. This field is also read by uevent, and not set to null by
> > the disconnect handler.
> >
> > Not sure what the best solution is yet.
> >
> 
> All USB drivers (and also any kind of driver that can be hotplugged)
> should implement some sort of refcounting, to avoid this kind of
> use-after-free issue.
> 
> Drivers can't free memory that may be associated with an open
> handle, until you remove the device node.

Thank you for trying to be helpful, but I do know all these things; I
added the registered boolean on rc_dev since it was a disaster before that,
there were all sorts of read-after-free on usb disconnect.

With rc-core it can get a little bit tricky between the lifetime of the
usb device, rc_dev device, lirc file descriptor, input device, and sysfs. Even
after spending lots of time trying to make sure this is correct and
thinking through all possible code paths, it is so easy to miss code
paths like these.


Thanks

Sean

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