Date:   Sun, 18 Oct 2020 15:01:17 -0700
From:   Rustam Kovhaev <rkovhaev@...il.com>
To:     mchehab@...nel.org, linux-media@...r.kernel.org
Cc:     andreyknvl@...gle.com, hans.verkuil@...co.com,
        laurent.pinchart@...asonboard.com, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, sakari.ailus@...ux.intel.com,
        syzkaller-bugs@...glegroups.com, gregkh@...uxfoundation.org
Subject: Re: KASAN: use-after-free Read in v4l2_fh_init

On Fri, Apr 19, 2019 at 07:36:05AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    d34f9519 usb-fuzzer: main usb gadget fuzzer driver
> git tree:       https://github.com/google/kasan/tree/usb-fuzzer
> console output: https://syzkaller.appspot.com/x/log.txt?x=125bbb5b200000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c73d1bb5aeaeae20
> dashboard link: https://syzkaller.appspot.com/bug?extid=c025d34b8eaa54c571b8
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1513ac1d200000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13555c1d200000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+c025d34b8eaa54c571b8@...kaller.appspotmail.com
> 
> em28xx 1-1:5.176: failed to create media graph
> em28xx 1-1:5.176: V4L2 device video32 deregistered
> em28xx 1-1:5.176: Binding DVB extension
> ==================================================================
> em28xx 1-1:5.176: no endpoint for DVB mode and transfer type 0
> BUG: KASAN: use-after-free in v4l2_fh_init+0x24c/0x290
> drivers/media/v4l2-core/v4l2-fh.c:33
> Read of size 8 at addr ffff8880a4f149d0 by task v4l_id/5313
> 
> CPU: 1 PID: 5313 Comm: v4l_id Not tainted 5.1.0-rc5-319617-gd34f951 #4
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> em28xx 1-1:5.176: failed to pre-allocate USB transfer buffers for DVB.
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0xe8/0x16e lib/dump_stack.c:113
>  print_address_description+0x6c/0x236 mm/kasan/report.c:187
> em28xx 1-1:5.176: Registering input extension
>  kasan_report.cold+0x1a/0x3c mm/kasan/report.c:317
>  v4l2_fh_init+0x24c/0x290 drivers/media/v4l2-core/v4l2-fh.c:33
>  v4l2_fh_open+0x8d/0xd0 drivers/media/v4l2-core/v4l2-fh.c:71
>  em28xx_v4l2_open+0x11f/0x470 drivers/media/usb/em28xx/em28xx-video.c:2184
>  v4l2_open+0x1b6/0x360 drivers/media/v4l2-core/v4l2-dev.c:427
>  chrdev_open+0x220/0x5d0 fs/char_dev.c:417
>  do_dentry_open+0x49c/0x1130 fs/open.c:777
>  do_last fs/namei.c:3416 [inline]
>  path_openat+0x147d/0x40b0 fs/namei.c:3533
>  do_filp_open+0x1a6/0x280 fs/namei.c:3563
>  do_sys_open+0x3c5/0x590 fs/open.c:1069
>  do_syscall_64+0xcf/0x4f0 arch/x86/entry/common.c:290
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x7fdb23b8d120
> Code: 48 8b 15 1b 4d 2b 00 f7 d8 64 89 02 83 c8 ff c3 90 90 90 90 90 90 90
> 90 90 90 83 3d d5 a4 2b 00 00 75 10 b8 02 00 00 00 0f 05 <48> 3d 01 f0 ff ff
> 73 31 c3 48 83 ec 08 e8 5e 8c 01 00 48 89 04 24
> RSP: 002b:00007ffc1aebde18 EFLAGS: 00000246 ORIG_RAX: 0000000000000002
> RAX: ffffffffffffffda RBX: 00007ffc1aebdf78 RCX: 00007fdb23b8d120
> RDX: 00007fdb23e42138 RSI: 0000000000000000 RDI: 00007ffc1aebef17
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000400884
> R13: 00007ffc1aebdf70 R14: 0000000000000000 R15: 0000000000000000
> 
> Allocated by task 12:
>  set_track mm/kasan/common.c:87 [inline]
>  __kasan_kmalloc mm/kasan/common.c:497 [inline]
>  __kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:470
>  kmalloc include/linux/slab.h:547 [inline]
>  kzalloc include/linux/slab.h:742 [inline]
>  em28xx_v4l2_init drivers/media/usb/em28xx/em28xx-video.c:2563 [inline]
>  em28xx_v4l2_init.cold+0x93/0x3112
> drivers/media/usb/em28xx/em28xx-video.c:2541
>  em28xx_init_extension+0x13a/0x200
> drivers/media/usb/em28xx/em28xx-core.c:1128
>  request_module_async+0x62/0x70 drivers/media/usb/em28xx/em28xx-cards.c:3300
>  process_one_work+0x90f/0x1580 kernel/workqueue.c:2269
>  worker_thread+0x9b/0xe20 kernel/workqueue.c:2415
>  kthread+0x313/0x420 kernel/kthread.c:253
>  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
> 
> Freed by task 12:
>  set_track mm/kasan/common.c:87 [inline]
>  __kasan_slab_free+0x130/0x180 mm/kasan/common.c:459
>  slab_free_hook mm/slub.c:1429 [inline]
>  slab_free_freelist_hook+0x5e/0x140 mm/slub.c:1456
>  slab_free mm/slub.c:3003 [inline]
>  kfree+0xce/0x280 mm/slub.c:3958
>  em28xx_free_v4l2 drivers/media/usb/em28xx/em28xx-video.c:2149 [inline]
>  kref_put include/linux/kref.h:67 [inline]
>  em28xx_v4l2_init drivers/media/usb/em28xx/em28xx-video.c:2920 [inline]
>  em28xx_v4l2_init.cold+0x2cf/0x3112
> drivers/media/usb/em28xx/em28xx-video.c:2541
>  em28xx_init_extension+0x13a/0x200
> drivers/media/usb/em28xx/em28xx-core.c:1128
>  request_module_async+0x62/0x70 drivers/media/usb/em28xx/em28xx-cards.c:3300
>  process_one_work+0x90f/0x1580 kernel/workqueue.c:2269
>  worker_thread+0x9b/0xe20 kernel/workqueue.c:2415
>  kthread+0x313/0x420 kernel/kthread.c:253
>  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
> 
> The buggy address belongs to the object at ffff8880a4f14200
>  which belongs to the cache kmalloc-8k of size 8192
> The buggy address is located 2000 bytes inside of
>  8192-byte region [ffff8880a4f14200, ffff8880a4f16200)
> The buggy address belongs to the page:
> page:ffffea000293c400 count:1 mapcount:0 mapping:ffff88812c3f4400 index:0x0
> compound_mapcount: 0
> flags: 0xfff00000010200(slab|head)
> raw: 00fff00000010200 dead000000000100 dead000000000200 ffff88812c3f4400
> raw: 0000000000000000 0000000080030003 00000001ffffffff 0000000000000000
> page dumped because: kasan: bad access detected
> 
> Memory state around the buggy address:
>  ffff8880a4f14880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff8880a4f14900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > ffff8880a4f14980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                                                  ^
>  ffff8880a4f14a00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff8880a4f14a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> 
> 
> ---
> This bug is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
> 
> syzbot will keep track of this bug report. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> syzbot can test patches for this bug, for details see:
> https://goo.gl/tpsmEJ#testing-patches

in v4l2_open() calling video_get() has no effect when vdev is a nested
struct inside some driver's struct (em28xx_v4l2 in this case), and referencing
vdev again, after a call to vdev->fops->open(), might cause UAF if
the driver's struct has already been freed

the following patch fixes both syzbot issue with v4l2_fh_init() and side
issue with v4l2_open() described above:
https://syzkaller.appspot.com/text?tag=Patch&x=15fbba30500000
1. it adds check for already freed struct in em28xx_v4l2_open(), it fixes
this syzbot UAF report about v4l2_fh_init(), but it moves UAF up to v4l2_open()
2. it delays freeing of em28xx_v4l2, hence allows v4l2_open() to
safely reference vdev and exit without UAF

fix for #2 does not look great, and it seems that proper fix would
be changing em28xx driver and having vdev allocated separateley and not
nested in struct em28xx_v4l2:
	struct video_device *vdev 
instead of:
	struct video_device vdev
and having em28xx utilize video_device_release() instead of
video_device_release_empty()

any advice or hints in fixing this would be highly appreciated, tyvm!

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