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

[syzbot <syzbot+178fff6149127421c2cc@syzkaller.appspotmail.com>]

> On Wed, Dec 24, 2025 at 2:43 PM Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2025/12/24 08:16, Barry Song wrote:
>> > On Wed, Dec 24, 2025 at 12:43 PM Pedro Falcato <pfalcato@suse.de> wrote:
>> >>
>> >> On Wed, Dec 24, 2025 at 11:46:44AM +1300, Barry Song wrote:
>> >>>>
>> >>>> Uninit was created at:
>> >>>>   __alloc_frozen_pages_noprof+0x421/0xab0 mm/page_alloc.c:5233
>> >>>>   alloc_pages_mpol+0x328/0x860 mm/mempolicy.c:2486
>> >>>>   folio_alloc_mpol_noprof+0x56/0x1d0 mm/mempolicy.c:2505
>> >>>>   shmem_alloc_folio mm/shmem.c:1890 [inline]
>> >>>>   shmem_alloc_and_add_folio+0xc56/0x1bd0 mm/shmem.c:1932
>> >>>>   shmem_get_folio_gfp+0xad3/0x1fc0 mm/shmem.c:2556
>> >>>>   shmem_get_folio mm/shmem.c:2662 [inline]
>> >>>>   shmem_symlink+0x562/0xad0 mm/shmem.c:4129
>> >>>>   vfs_symlink+0x42f/0x4c0 fs/namei.c:5514
>> >>>>   do_symlinkat+0x2ae/0xbb0 fs/namei.c:5541
>> >>>
>> >>> +Hugh and Baolin.
>>
>> Thanks for CCing me.
>>
>> >>>
>> >>> This happens in the shmem symlink path, where newly allocated
>> >>> folios are not cleared for some reason. As a result,
>> >>> is_folio_zero_filled() ends up reading uninitialized data.
>> >>>
>> >>
>> >> I'm not Hugh nor Baolin, but I would guess that letting
>> >> is_folio_zero_filled() skip/disable KMSAN would also work. Since all we want
>> >> is to skip writeout if the folio is zero, whether it is incidentally zero, or not,
>> >> does not really matter, I think.
>> >
>> > Hi Pedro, thanks! You’re always welcome to chime in.
>> >
>> > You are probably right. However, I still prefer the remaining
>> > data to be zeroed, as it may be more compression-friendly.
>> >
>> > Random data could potentially lead to larger compressed output,
>> > whereas a large area of zeros would likely result in much smaller
>> > compressed data.
>>
>> Thanks Pedro and Barry. I remember Hugh raised a similar issue before
>> (See [1], but I did not investigate further:(). I agree with Hugh's
>> point that the uninitialized parts should be zeroed before going the
>> outside world.
>>
>> [1]
>> https://lore.kernel.org/all/02a21a55-8fe3-a9eb-f54b-051d75ae8335@google.com/
>>
>> > Not quite sure if the below can fix the issue:
>> >
>> > diff --git a/mm/shmem.c b/mm/shmem.c
>> > index ec6c01378e9d..0ca2d4bffdb4 100644
>> > --- a/mm/shmem.c
>> > +++ b/mm/shmem.c
>> > @@ -4131,6 +4131,7 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
>> >                       goto out_remove_offset;
>> >               inode->i_op = &shmem_symlink_inode_operations;
>> >               memcpy(folio_address(folio), symname, len);
>> > +             memset(folio_address(folio) + len, 0, folio_size(folio) - len);
>> >               folio_mark_uptodate(folio);
>> >               folio_mark_dirty(folio);
>> >               folio_unlock(folio);
>>
>> That looks reasonable to me, though I prefer to use the more readable
>> helper: folio_zero_range(). Barry, could you send out a formal patch？
>> Thanks.
>
> Thanks, Baolin. Let me request a bot test first.
>
> #syz test

This crash does not have a reproducer. I cannot test it.

>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index ec6c01378e9d..835900a08f51 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -4131,6 +4131,7 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
>  			goto out_remove_offset;
>  		inode->i_op = &shmem_symlink_inode_operations;
>  		memcpy(folio_address(folio), symname, len);
> +		folio_zero_range(folio, len, folio_size(folio) - len);
>  		folio_mark_uptodate(folio);
>  		folio_mark_dirty(folio);
>  		folio_unlock(folio);
> -- 
> 2.48.1