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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 ffff8800a08ff6f8 by task syz-executor.2/5469

CPU: 0 PID: 5469 Comm: syz-executor.2 Not tainted 4.4.174+ #17
 0000000000000000 04c8eed45b758d7d ffff8800a08ff3a8 ffffffff81aad1a1
 0000000000000000 ffffea0002823fc0 ffff8800a08ff6f8 0000000000000008
 ffff8800a08ff6f0 ffff8800a08ff3e0 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
 [<ffffffff81530c16>] do_splice_from fs/splice.c:1128 [inline]
 [<ffffffff81530c16>] direct_splice_actor+0x126/0x1a0 fs/splice.c:1294
 [<ffffffff8153252e>] splice_direct_to_actor+0x2ce/0x850 fs/splice.c:1247
 [<ffffffff81532c55>] do_splice_direct+0x1a5/0x260 fs/splice.c:1337
 [<ffffffff8149a2fd>] do_sendfile+0x4ed/0xba0 fs/read_write.c:1229
 [<ffffffff8149c474>] C_SYSC_sendfile fs/read_write.c:1311 [inline]
 [<ffffffff8149c474>] compat_SyS_sendfile+0x144/0x160 fs/read_write.c:1294
 [<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:ffffea0002823fc0 count:0 mapcount:0 mapping:          (null) index:0x0
flags: 0x0()
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8800a08ff580: 00 00 00 f2 f2 f2 f2 f2 00 00 00 00 00 00 00 f3
 ffff8800a08ff600: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8800a08ff680: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00 f2 f2
                                                                ^
 ffff8800a08ff700: 00 00 00 00 00 f2 f2 f2 f2 f2 00 00 00 00 00 f3
 ffff8800a08ff780: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
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