==================================================================
BUG: KASAN: stack-out-of-bounds in iov_iter_advance+0x4b3/0x4f0 lib/iov_iter.c:491
Read of size 8 at addr ffff8800b622fd60 by task syz-executor.3/7568

CPU: 1 PID: 7568 Comm: syz-executor.3 Not tainted 4.4.174+ #4
 0000000000000000 f27ad69a373f3759 ffff8800b622fa10 ffffffff81aad1a1
 0000000000000000 ffffea0002d88bc0 ffff8800b622fd60 0000000000000008
 ffff8800b622fd58 ffff8800b622fa48 ffffffff81490120 0000000000000000
Call Trace:
 [<ffffffff81aad1a1>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81aad1a1>] dump_stack+0xc1/0x120 lib/dump_stack.c:51
 [<ffffffff81490120>] print_address_description+0x6f/0x21b mm/kasan/report.c:252
 [<ffffffff81490358>] kasan_report_error mm/kasan/report.c:351 [inline]
 [<ffffffff81490358>] kasan_report mm/kasan/report.c:408 [inline]
 [<ffffffff81490358>] kasan_report.cold+0x8c/0x2be mm/kasan/report.c:393
 [<ffffffff81484ed4>] __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:429
 [<ffffffff81ae82f3>] iov_iter_advance+0x4b3/0x4f0 lib/iov_iter.c:491
 [<ffffffff81e22476>] tun_get_user+0x2c6/0x2640 drivers/net/tun.c:1124
 [<ffffffff81e24a0a>] tun_chr_write_iter+0xda/0x190 drivers/net/tun.c:1283
 [<ffffffff81496ae8>] new_sync_write fs/read_write.c:480 [inline]
 [<ffffffff81496ae8>] __vfs_write+0x2e8/0x3d0 fs/read_write.c:493
 [<ffffffff81498612>] vfs_write+0x182/0x4e0 fs/read_write.c:540
 [<ffffffff8149ac4c>] SYSC_write fs/read_write.c:587 [inline]
 [<ffffffff8149ac4c>] SyS_write+0xdc/0x1c0 fs/read_write.c:579
 [<ffffffff82718ba1>] entry_SYSCALL_64_fastpath+0x1e/0x9a

The buggy address belongs to the page:
page:ffffea0002d88bc0 count:0 mapcount:0 mapping:          (null) index:0x0
flags: 0x0()
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8800b622fc00: f2 f2 f2 f2 f2 00 00 00 00 00 00 00 f3 f3 f3 f3
 ffff8800b622fc80: f3 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8800b622fd00: 00 00 00 00 00 f1 f1 f1 f1 00 00 f2 f2 00 00 00
                                                       ^
 ffff8800b622fd80: 00 00 f2 f2 f2 f2 f2 00 00 00 00 00 f3 f3 f3 f3
 ffff8800b622fe00: f3 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================
audit_printk_skb: 9 callbacks suppressed
audit: type=1400 audit(1574343149.457:201): avc:  denied  { create } for  pid=7575 comm="syz-executor.0" scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 tcontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 tclass=netlink_generic_socket permissive=1
audit: type=1400 audit(1574343149.677:202): avc:  denied  { write } for  pid=7575 comm="syz-executor.0" scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 tcontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 tclass=netlink_generic_socket permissive=1