Re: [syzbot] [mm?] KMSAN: uninit-value in swap_writepage

[Andrew Morton]

On Thu, 10 Oct 2024 01:44:27 -0700 syzbot <syzbot+febb2473441bfb8fb380@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    fc20a3e57247 Merge tag 'for-linus-6.12a-rc2-tag' of git://..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11cdfd27980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d302f14701986aa0
> dashboard link: https://syzkaller.appspot.com/bug?extid=febb2473441bfb8fb380
> 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/5b4b9aca7b75/disk-fc20a3e5.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/b22e17636ec0/vmlinux-fc20a3e5.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/5266e625be99/bzImage-fc20a3e5.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+febb2473441bfb8fb380@xxxxxxxxxxxxxxxxxxxxxxxxx

Thanks.  I'm guessing that shmem symlinks aren't initializing the whole
page (folio) and that has tripped up Usama's "store zero pages to be
swapped out in a bitmap" feature - it's checking the uninitialized part
of the page for zeroness.


> =====================================================
> BUG: KMSAN: uninit-value in is_folio_zero_filled mm/page_io.c:189 [inline]
> BUG: KMSAN: uninit-value in swap_writepage+0x536/0x12b0 mm/page_io.c:259
>  is_folio_zero_filled mm/page_io.c:189 [inline]
>  swap_writepage+0x536/0x12b0 mm/page_io.c:259
>  shmem_writepage+0x2117/0x2450 mm/shmem.c:1567
>  pageout mm/vmscan.c:688 [inline]
>  shrink_folio_list+0x5e78/0x7dd0 mm/vmscan.c:1366
>  evict_folios+0x9813/0xbaf0 mm/vmscan.c:4583
>  try_to_shrink_lruvec+0x13a3/0x1750 mm/vmscan.c:4778
>  shrink_one+0x646/0xd20 mm/vmscan.c:4816
>  shrink_many mm/vmscan.c:4879 [inline]
>  lru_gen_shrink_node mm/vmscan.c:4957 [inline] 
>  shrink_node+0x451a/0x50f0 mm/vmscan.c:5937
>  kswapd_shrink_node mm/vmscan.c:6765 [inline]
>  balance_pgdat mm/vmscan.c:6957 [inline]
>  kswapd+0x25e2/0x42f0 mm/vmscan.c:7226
>  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_noprof+0x9d6/0xe70 mm/page_alloc.c:4756
>  alloc_pages_mpol_noprof+0x299/0x990 mm/mempolicy.c:2265
>  folio_alloc_mpol_noprof+0x55/0x180 mm/mempolicy.c:2283
>  shmem_alloc_folio mm/shmem.c:1774 [inline]
>  shmem_alloc_and_add_folio+0xc33/0x1c30 mm/shmem.c:1813
>  shmem_get_folio_gfp+0xacd/0x1f30 mm/shmem.c:2335
>  shmem_get_folio mm/shmem.c:2441 [inline]
>  shmem_symlink+0x528/0xa20 mm/shmem.c:3834
>  vfs_symlink+0x1ed/0x460 fs/namei.c:4615
>  do_symlinkat+0x257/0x8a0 fs/namei.c:4641
>  __do_sys_symlink fs/namei.c:4662 [inline]
>  __se_sys_symlink fs/namei.c:4660 [inline]
>  __x64_sys_symlink+0xe0/0x140 fs/namei.c:4660
>  x64_sys_call+0x30e8/0x3ba0 arch/x86/include/generated/asm/syscalls_64.h:89
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xcd/0x1e0 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> CPU: 1 UID: 0 PID: 80 Comm: kswapd0 Tainted: G        W          6.12.0-rc1-syzkaller-00330-gfc20a3e57247 #0
> Tainted: [W]=WARN
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
> =====================================================
> 
> 
> ---
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