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

[Usama Arif]

On 10/10/2024 22:23, Andrew Morton wrote:
> 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.
> 

This is a very similar bug to the one in https://lore.kernel.org/all/000000000000d0f165061a6754c3@xxxxxxxxxx/
(Thanks Nhat for pointing this out!)

As Hugh mentioned in that thread, its likely not a bug in (z)swap.

Its just working with the folio that was given to it, and it should probably be
initialized before swap_writepage is called.

I havent worked on shmem code before, but will try to have a look.
Hugh mentioned in the other thread that shmem can keep uninitialized 
data pages around, but should be zeroing what's still uninitialized before
it can reach the outside world. Maybeshmem_symlink gets a folio
that wasnt initialized?

Thanks,
Usama  

> 
>> =====================================================
>> 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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