==================================================================
BUG: KASAN: stack-out-of-bounds in iov_iter_advance+0x4b3/0x4f0 lib/iov_iter.c:491
Read of size 8 at addr ffff8800b6c6fce8 by task syz-executor.1/15261

CPU: 0 PID: 15261 Comm: syz-executor.1 Not tainted 4.4.174+ #17
 0000000000000000 3e1c9dbecf820db4 ffff8800b6c6f998 ffffffff81aad1a1
 0000000000000000 ffffea0002db1bc0 ffff8800b6c6fce8 0000000000000008
 ffff8800b6c6fce0 ffff8800b6c6f9d0 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
 [<ffffffff8100603d>] do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
 [<ffffffff8100603d>] do_fast_syscall_32+0x32d/0xa90 arch/x86/entry/common.c:397
 [<ffffffff8271a350>] sysenter_flags_fixed+0xd/0x1a

The buggy address belongs to the page:
page:ffffea0002db1bc0 count:0 mapcount:0 mapping:          (null) index:0x0
flags: 0x0()
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8800b6c6fb80: 00 f2 f2 f2 f2 f2 00 00 00 00 00 00 00 f3 f3 f3
 ffff8800b6c6fc00: f3 f3 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8800b6c6fc80: 00 00 00 00 00 00 f1 f1 f1 f1 00 00 f2 f2 00 00
                                                          ^
 ffff8800b6c6fd00: 00 00 00 f2 f2 f2 f2 f2 00 00 00 00 00 f3 f3 f3
 ffff8800b6c6fd80: f3 f3 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================