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BUG: KFENCE: use-after-free read in dvb_device_open+0xc4/0x350

Use-after-free read at 0xffff88823c5b6f18 (in kfence-#218):
 dvb_device_open+0xc4/0x350
 chrdev_open+0x4cd/0x5e0
 do_dentry_open+0x785/0x14e0
 vfs_open+0x3b/0x340
 path_openat+0x2e08/0x3860
 do_file_open+0x23e/0x4a0
 do_sys_openat2+0x113/0x200
 __x64_sys_openat+0x138/0x170
 do_syscall_64+0x15f/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

kfence-#218: 0xffff88823c5b6f00-0xffff88823c5b6fff, size=256, cache=vm_area_struct

allocated by task 6346 on cpu 1 at 99.863075s (11.210045s ago):
 vm_area_alloc+0x24/0x140
 mmap_region+0x11c6/0x2290
 do_mmap+0xc39/0x10c0
 vm_mmap_pgoff+0x2c9/0x4f0
 ksys_mmap_pgoff+0x51e/0x760
 do_syscall_64+0x15f/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

freed by task 6346 on cpu 1 at 99.931934s (11.158103s ago):
 tear_down_vmas+0x312/0x520
 exit_mmap+0x4b6/0x9e0
 __mmput+0x118/0x430
 exit_mm+0x18e/0x250
 do_exit+0x6a2/0x22c0
 do_group_exit+0x21b/0x2d0
 __x64_sys_exit_group+0x3f/0x40
 x64_sys_call+0x221a/0x2240
 do_syscall_64+0x15f/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 1 UID: 0 PID: 6569 Comm: syz.3.196 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:dvb_device_open+0xc4/0x350
Code: e8 71 57 4b fa 4d 8b 3f 4d 85 ff 0f 84 64 01 00 00 4d 8d 67 18 4c 89 e0 48 c1 e8 03 80 3c 18 00 74 08 4c 89 e7 e8 4c 57 4b fa <4d> 8b 2c 24 4d 85 ed 0f 84 45 01 00 00 4c 89 e8 48 c1 e8 03 80 3c
RSP: 0018:ffffc900067c7770 EFLAGS: 00010246
RAX: 1ffff110478b6de3 RBX: dffffc0000000000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: 0000000000000008 RDI: 00000000ffffffff
RBP: ffff8881bbc70e00 R08: ffffffff8f8cad0f R09: 1ffffffff1f195a1
R10: dffffc0000000000 R11: fffffbfff1f195a2 R12: ffff88823c5b6f18
R13: dffffc0000000000 R14: ffff88810d215bb8 R15: ffff88823c5b6f00
FS:  00007f4a25d8c6c0(0000) GS:ffff8882a9291000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff88823c5b6f18 CR3: 000000016ab2e000 CR4: 00000000000006f0
Call Trace:
 <TASK>
 chrdev_open+0x4cd/0x5e0
 do_dentry_open+0x785/0x14e0
 vfs_open+0x3b/0x340
 path_openat+0x2e08/0x3860
 do_file_open+0x23e/0x4a0
 do_sys_openat2+0x113/0x200
 __x64_sys_openat+0x138/0x170
 do_syscall_64+0x15f/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f4a24f5d60e
Code: 08 0f 85 a5 a8 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 80 00 00 00 00 48 83 ec 08
RSP: 002b:00007f4a25d8bb28 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007f4a25d8c6c0 RCX: 00007f4a24f5d60e
RDX: 0000000000000002 RSI: 00007f4a25d8bc00 RDI: ffffffffffffff9c
RBP: 00007f4a25d8bc00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: cccccccccccccccd
R13: 00007f4a25216038 R14: 00007f4a25215fa0 R15: 00007ffdff6d0de8
 </TASK>
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