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BUG: KASAN: stack-out-of-bounds in iov_iter_advance+0x4b3/0x4f0 lib/iov_iter.c:491
Read of size 8 at addr ffff8800b4e7fa50 by task syz-executor.0/3349

CPU: 1 PID: 3349 Comm: syz-executor.0 Not tainted 4.4.174+ #4
 0000000000000000 3b2eb6c00cdcfe6a ffff8800b4e7f700 ffffffff81aad1a1
 0000000000000000 ffffea0002d39fc0 ffff8800b4e7fa50 0000000000000008
 ffff8800b4e7fa48 ffff8800b4e7f738 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
 [<ffffffff81496ce2>] __kernel_write+0x112/0x370 fs/read_write.c:513
 [<ffffffff81532e6d>] write_pipe_buf+0x15d/0x1f0 fs/splice.c:1074
 [<ffffffff81533b6e>] splice_from_pipe_feed fs/splice.c:776 [inline]
 [<ffffffff81533b6e>] __splice_from_pipe+0x37e/0x7a0 fs/splice.c:901
 [<ffffffff81536be8>] splice_from_pipe+0x108/0x170 fs/splice.c:936
 [<ffffffff81536cdc>] default_file_splice_write+0x3c/0x80 fs/splice.c:1086
 [<ffffffff81537d31>] do_splice_from fs/splice.c:1128 [inline]
 [<ffffffff81537d31>] do_splice fs/splice.c:1404 [inline]
 [<ffffffff81537d31>] SYSC_splice fs/splice.c:1707 [inline]
 [<ffffffff81537d31>] SyS_splice+0xd71/0x13a0 fs/splice.c:1690
 [<ffffffff82718ba1>] entry_SYSCALL_64_fastpath+0x1e/0x9a

The buggy address belongs to the page:
page:ffffea0002d39fc0 count:0 mapcount:0 mapping:          (null) index:0x0
flags: 0x0()
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8800b4e7f900: f2 f2 f2 00 00 00 00 00 00 00 f3 f3 f3 f3 f3 00
 ffff8800b4e7f980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8800b4e7fa00: 00 00 00 f1 f1 f1 f1 00 00 f2 f2 00 00 00 00 00
                                                 ^
 ffff8800b4e7fa80: f2 f2 f2 f2 f2 00 00 00 00 00 f3 f3 f3 f3 f3 00
 ffff8800b4e7fb00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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