Date: Mon, 10 Jun 2024 12:01:14 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: syzbot <syzbot+9c1fe13fcb51574b249b@...kaller.appspotmail.com>
Cc: akpm@...ux-foundation.org, chengming.zhou@...ux.dev, hannes@...xchg.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, nphamcs@...il.com, 
	syzkaller-bugs@...glegroups.com, Hugh Dickins <hughd@...gle.com>
Subject: Re: [syzbot] [mm?] KMSAN: uninit-value in zswap_store

On Sat, Jun 8, 2024 at 2:09 PM syzbot
<syzbot+9c1fe13fcb51574b249b@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    614da38e2f7a Merge tag 'hid-for-linus-2024051401' of git:/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=113f3fd6980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f5d2cbf33633f507
> dashboard link: https://syzkaller.appspot.com/bug?extid=9c1fe13fcb51574b249b
> 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/89eafb874b71/disk-614da38e.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/356000512ad9/vmlinux-614da38e.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/839c73939115/bzImage-614da38e.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9c1fe13fcb51574b249b@...kaller.appspotmail.com

This doesn't look like a zswap bug, and perhaps not a bug at all but I
am not sure. Zswap is reading the data in a folio to check if it is
filled with a repeated pattern, and it is encountering uninitialized
data. I am not sure if this is expected behavior or not.

Could it just be that the size of the data written by syzbot is not
divisible by PAGE_SIZE, so part of the final page is left
uninitialized? Do we keep the unwritten parts of a shmem page
uninitialized by any chance?

Hugh, do you mind taking a quick look at whether this is a real bug?

If this is expected behavior, perhaps there is some annotation we can
use in zswap that it is fine to encounter uninitialized data when
reading the folio.

>
> =====================================================
> BUG: KMSAN: uninit-value in zswap_is_page_same_filled mm/zswap.c:1481 [inline]
> BUG: KMSAN: uninit-value in zswap_store+0x1008/0x2ca0 mm/zswap.c:1553
>  zswap_is_page_same_filled mm/zswap.c:1481 [inline]
>  zswap_store+0x1008/0x2ca0 mm/zswap.c:1553
>  swap_writepage+0x126/0x4c0 mm/page_io.c:198
>  shmem_writepage+0x1826/0x1f70 mm/shmem.c:1518
>  pageout mm/vmscan.c:660 [inline]
>  shrink_folio_list+0x4a55/0x7910 mm/vmscan.c:1323
>  evict_folios+0x9d7f/0xcc20 mm/vmscan.c:4537
>  try_to_shrink_lruvec+0x160e/0x1a50 mm/vmscan.c:4733
>  shrink_one+0x66f/0xd40 mm/vmscan.c:4772
>  shrink_many mm/vmscan.c:4835 [inline]
>  lru_gen_shrink_node mm/vmscan.c:4935 [inline]
>  shrink_node+0x4856/0x55f0 mm/vmscan.c:5894
>  kswapd_shrink_node mm/vmscan.c:6704 [inline]
>  balance_pgdat mm/vmscan.c:6895 [inline]
>  kswapd+0x1eba/0x4460 mm/vmscan.c:7164
>  kthread+0x3e2/0x540 kernel/kthread.c:389
>  ret_from_fork+0x6d/0x90 arch/x86/kernel/process.c:147
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
>
> Uninit was stored to memory at:
>  memcpy_from_iter lib/iov_iter.c:73 [inline]
>  iterate_bvec include/linux/iov_iter.h:122 [inline]
>  iterate_and_advance2 include/linux/iov_iter.h:249 [inline]
>  iterate_and_advance include/linux/iov_iter.h:271 [inline]
>  __copy_from_iter lib/iov_iter.c:249 [inline]
>  copy_page_from_iter_atomic+0x12b7/0x2ae0 lib/iov_iter.c:481
>  generic_perform_write+0x4c1/0xc60 mm/filemap.c:3982
>  shmem_file_write_iter+0x2bd/0x2f0 mm/shmem.c:2920
>  do_iter_readv_writev+0x7e6/0x960
>  vfs_iter_write+0x459/0xd00 fs/read_write.c:895
>  lo_write_bvec drivers/block/loop.c:246 [inline]
>  lo_write_simple drivers/block/loop.c:267 [inline]
>  do_req_filebacked drivers/block/loop.c:491 [inline]
>  loop_handle_cmd drivers/block/loop.c:1907 [inline]
>  loop_process_work+0x1502/0x3440 drivers/block/loop.c:1942
>  loop_rootcg_workfn+0x2b/0x40 drivers/block/loop.c:1973
>  process_one_work kernel/workqueue.c:3267 [inline]
>  process_scheduled_works+0xa81/0x1bd0 kernel/workqueue.c:3348
>  worker_thread+0xea5/0x1560 kernel/workqueue.c:3429
>  kthread+0x3e2/0x540 kernel/kthread.c:389
>  ret_from_fork+0x6d/0x90 arch/x86/kernel/process.c:147
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
>
> Uninit was created at:
>  __alloc_pages+0x9d6/0xe70 mm/page_alloc.c:4598
>  alloc_pages_mpol+0x299/0x990 mm/mempolicy.c:2264
>  alloc_pages mm/mempolicy.c:2335 [inline]
>  folio_alloc+0x1d0/0x230 mm/mempolicy.c:2342
>  filemap_alloc_folio+0xa6/0x440 mm/filemap.c:984
>  __filemap_get_folio+0xa10/0x14b0 mm/filemap.c:1926
>  grow_dev_folio fs/buffer.c:1042 [inline]
>  grow_buffers fs/buffer.c:1108 [inline]
>  __getblk_slow fs/buffer.c:1134 [inline]
>  bdev_getblk+0x39b/0xc80 fs/buffer.c:1429
>  __getblk include/linux/buffer_head.h:355 [inline]
>  sb_getblk include/linux/buffer_head.h:361 [inline]
>  ext4_getblk+0x3da/0xe00 fs/ext4/inode.c:843
>  ext4_xattr_inode_write fs/ext4/xattr.c:1421 [inline]
>  ext4_xattr_inode_lookup_create fs/ext4/xattr.c:1596 [inline]
>  ext4_xattr_set_entry+0x574d/0x6880 fs/ext4/xattr.c:1718
>  ext4_xattr_block_set+0xb94/0x4fb0 fs/ext4/xattr.c:2037
>  ext4_xattr_move_to_block fs/ext4/xattr.c:2654 [inline]
>  ext4_xattr_make_inode_space fs/ext4/xattr.c:2729 [inline]
>  ext4_expand_extra_isize_ea+0x20bd/0x3560 fs/ext4/xattr.c:2821
>  __ext4_expand_extra_isize+0x5dc/0x680 fs/ext4/inode.c:5789
>  ext4_try_to_expand_extra_isize fs/ext4/inode.c:5832 [inline]
>  __ext4_mark_inode_dirty+0x70d/0xa10 fs/ext4/inode.c:5910
>  ext4_delete_inline_entry+0x650/0x7d0 fs/ext4/inline.c:1753
>  ext4_delete_entry+0x13f/0x7d0 fs/ext4/namei.c:2719
>  __ext4_unlink+0x9b8/0x11b0 fs/ext4/namei.c:3273
>  ext4_unlink+0x226/0x630 fs/ext4/namei.c:3321
>  vfs_unlink+0x676/0xa30 fs/namei.c:4343
>  do_unlinkat+0x823/0xe10 fs/namei.c:4407
>  __do_sys_unlinkat fs/namei.c:4450 [inline]
>  __se_sys_unlinkat fs/namei.c:4443 [inline]
>  __x64_sys_unlinkat+0x17c/0x230 fs/namei.c:4443
>  x64_sys_call+0x846/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:264
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> CPU: 1 PID: 88 Comm: kswapd1 Not tainted 6.9.0-syzkaller-02707-g614da38e2f7a #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
> =====================================================
>
>
> ---
> 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@...glegroups.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:
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> (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

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