Re: [syzbot] [fs?] [mm?] KCSAN: data-race in xas_create / xas_find (8)

[Sabyrzhan Tasbolatov <snovitoll@gmail.com>]

On Mon, Oct 14, 2024 at 11:08 AM syzbot
<syzbot+b79be83906cd9bab16ff@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    2f91ff27b0ee Merge tag 'sound-6.12-rc2' of git://git.kerne..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=155c879f980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=95098faba89c70c9
> dashboard link: https://syzkaller.appspot.com/bug?extid=b79be83906cd9bab16ff
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/14933c4ac457/disk-2f91ff27.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/6725831fc1a1/vmlinux-2f91ff27.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/98d64e038e72/bzImage-2f91ff27.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b79be83906cd9bab16ff@syzkaller.appspotmail.com
>
> loop4: detected capacity change from 0 to 4096
> EXT4-fs: Ignoring removed nobh option
> EXT4-fs: Ignoring removed i_version option
> ==================================================================
> BUG: KCSAN: data-race in xas_create / xas_find
>
> write to 0xffff888106819919 of 1 bytes by task 3435 on cpu 0:
>  xas_expand lib/xarray.c:613 [inline]
>  xas_create+0x666/0xbd0 lib/xarray.c:654
>  xas_store+0x6f/0xc90 lib/xarray.c:788

AFAIU, xas_store() itself, doesn't have a locking mechanism,
but is locked in xa_* functions. Example:

void *xa_store_range(...)
{
   XA_STATE(xas, xa, 0);
   ...
   do {
      xas_lock(&xas);
      if (entry) {
      ...
         xas_create(&xas, true);
   }
...
unlock:
   xas_unlock(&xas);
}

Same thing is for the another racing xas_find() function:

void *xa_find(...)
{
   XA_STATE(xas, xa, *indexp);
   void *entry;
   rcu_read_lock();
   do {
      if (...)
         entry = xas_find_marked(&xas, max, filter);
      else
         entry = xas_find(&xas, max);
      ...
   rcu_read_unlock();
}

In this KCSAN report, xas_create() and xas_find() are racing for `offset` field.

>  __filemap_add_folio+0x3cc/0x6f0 mm/filemap.c:916
>  filemap_add_folio+0x9c/0x1b0 mm/filemap.c:972
>  page_cache_ra_unbounded+0x175/0x310 mm/readahead.c:268
>  do_page_cache_ra mm/readahead.c:320 [inline]
>  force_page_cache_ra mm/readahead.c:349 [inline]
>  page_cache_sync_ra+0x252/0x670 mm/readahead.c:562
>  page_cache_sync_readahead include/linux/pagemap.h:1394 [inline]
>  filemap_get_pages+0x2c1/0x10e0 mm/filemap.c:2547
>  filemap_read+0x216/0x680 mm/filemap.c:2645
>  blkdev_read_iter+0x20e/0x2c0 block/fops.c:765
>  new_sync_read fs/read_write.c:488 [inline]
>  vfs_read+0x5f6/0x720 fs/read_write.c:569
>  ksys_read+0xeb/0x1b0 fs/read_write.c:712
>  __do_sys_read fs/read_write.c:722 [inline]
>  __se_sys_read fs/read_write.c:720 [inline]
>  __x64_sys_read+0x42/0x50 fs/read_write.c:720
>  x64_sys_call+0x27d3/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:1
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> read to 0xffff888106819919 of 1 bytes by task 9109 on cpu 1:
>  xas_find+0x372/0x3f0 lib/xarray.c:1278
>  find_get_entry+0x66/0x390 mm/filemap.c:1992
>  find_get_entries+0xa4/0x220 mm/filemap.c:2047
>  truncate_inode_pages_range+0x4ac/0x6b0 mm/truncate.c:378
>  truncate_inode_pages+0x24/0x30 mm/truncate.c:423
>  kill_bdev block/bdev.c:91 [inline]
>  set_blocksize+0x258/0x270 block/bdev.c:173
>  sb_set_blocksize block/bdev.c:182 [inline]
>  sb_min_blocksize+0x63/0xe0 block/bdev.c:198
>  ext4_load_super fs/ext4/super.c:4992 [inline]
>  __ext4_fill_super fs/ext4/super.c:5213 [inline]
>  ext4_fill_super+0x38b/0x3a10 fs/ext4/super.c:5686
>  get_tree_bdev+0x256/0x2e0 fs/super.c:1635
>  ext4_get_tree+0x1c/0x30 fs/ext4/super.c:5718
>  vfs_get_tree+0x56/0x1e0 fs/super.c:1800
>  do_new_mount+0x227/0x690 fs/namespace.c:3507
>  path_mount+0x49b/0xb30 fs/namespace.c:3834
>  do_mount fs/namespace.c:3847 [inline]
>  __do_sys_mount fs/namespace.c:4055 [inline]
>  __se_sys_mount+0x27c/0x2d0 fs/namespace.c:4032
>  __x64_sys_mount+0x67/0x80 fs/namespace.c:4032
>  x64_sys_call+0x203e/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:166
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> value changed: 0x0e -> 0x00
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 UID: 0 PID: 9109 Comm: syz.4.1794 Not tainted 6.12.0-rc1-syzkaller-00257-g2f91ff27b0ee #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
> ==================================================================
> EXT4-fs (loop4): mounted filesystem 00000000-0000-0000-0000-000000000000 r/w without journal. Quota mode: writeback.
> EXT4-fs (loop4): unmounting filesystem 00000000-0000-0000-0000-000000000000.
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
>
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
>
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
>
> If you want to undo deduplication, reply with:
> #syz undup
>