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Date:   Thu, 09 Jun 2022 18:24:25 -0700
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Subject: Re: [syzbot] KASAN: use-after-free Read in copy_page_from_iter_atomic (2)
From:   syzbot <syzbot+d2dd123304b4ae59f1bd@syzkaller.appspotmail.com>
To:     akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        syzkaller-bugs@googlegroups.com, willy@infradead.org
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syzbot has found a reproducer for the following issue on:

HEAD commit:    ff539ac73ea5 Add linux-next specific files for 20220609
git tree:       linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=1627d920080000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a5002042f00a8bce
dashboard link: https://syzkaller.appspot.com/bug?extid=d2dd123304b4ae59f1bd
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10d6d7cff00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1113b2bff00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d2dd123304b4ae59f1bd@syzkaller.appspotmail.com

BTRFS error (device loop0): bad tree block start, want 30490624 have 0
==================================================================
BUG: KASAN: use-after-free in copy_page_from_iter_atomic+0xef6/0x1b30 lib/iov_iter.c:969
Read of size 4096 at addr ffff888170801000 by task kworker/u4:0/8

CPU: 0 PID: 8 Comm: kworker/u4:0 Not tainted 5.19.0-rc1-next-20220609-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: loop0 loop_rootcg_workfn
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0xeb/0x495 mm/kasan/report.c:313
 print_report mm/kasan/report.c:429 [inline]
 kasan_report.cold+0xf4/0x1c6 mm/kasan/report.c:491
 check_region_inline mm/kasan/generic.c:183 [inline]
 kasan_check_range+0x13d/0x180 mm/kasan/generic.c:189
 memcpy+0x20/0x60 mm/kasan/shadow.c:65
 copy_page_from_iter_atomic+0xef6/0x1b30 lib/iov_iter.c:969
 generic_perform_write+0x2b8/0x560 mm/filemap.c:3735
 __generic_file_write_iter+0x2aa/0x4d0 mm/filemap.c:3855
 generic_file_write_iter+0xd7/0x220 mm/filemap.c:3887
 call_write_iter include/linux/fs.h:2057 [inline]
 do_iter_readv_writev+0x3d1/0x640 fs/read_write.c:742
 do_iter_write+0x182/0x700 fs/read_write.c:868
 vfs_iter_write+0x70/0xa0 fs/read_write.c:909
 lo_write_bvec drivers/block/loop.c:249 [inline]
 lo_write_simple drivers/block/loop.c:271 [inline]
 do_req_filebacked drivers/block/loop.c:495 [inline]
 loop_handle_cmd drivers/block/loop.c:1859 [inline]
 loop_process_work+0xd83/0x2050 drivers/block/loop.c:1894
 process_one_work+0x996/0x1610 kernel/workqueue.c:2289
 worker_thread+0x665/0x1080 kernel/workqueue.c:2436
 kthread+0x2e9/0x3a0 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:302
 </TASK>

The buggy address belongs to the physical page:
page:ffffea0005c20040 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x170801
flags: 0x57ff00000000000(node=1|zone=2|lastcpupid=0x7ff)
raw: 057ff00000000000 ffffea0005c20048 ffffea0005c20048 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner info is not present (never set?)

Memory state around the buggy address:
 ffff888170800f00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888170800f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888170801000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff888170801080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888170801100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================