Re: [PATCH] fs/ext4: Initialize new folios before use

[Bartłomiej Kubik]

Hi,

Thank you for your suggestions.

On Wed, 24 Dec 2025 at 02:39, Baokun Li <libaokun1@xxxxxxxxxx> wrote:
>
> Hi Bartlomiej,
>
> On 2025-12-24 05:58, Bartlomiej Kubik wrote:
> > KMSAN reports an uninitialized value in adiantum_crypt, created at
> > write_begin_get_folio(). New folios are allocated with the FGP_CREAT
> > flag and may be returned uninitialized. These uninitialized folios are
> > then used without proper initialization.
> >
> > Fixes: b799474b9aeb ("mm/pagemap: add write_begin_get_folio() helper function")
> > Tested-by: syzbot+703d8a2cd20971854b06@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Reported-by: syzbot+703d8a2cd20971854b06@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=703d8a2cd20971854b06
> >
> > Signed-off-by: Bartlomiej Kubik <kubik.bartlomiej@xxxxxxxxx>
> > ---
> >  include/linux/pagemap.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index 31a848485ad9..31bbc8299e08 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -787,7 +787,8 @@ static inline struct folio *write_begin_get_folio(const struct kiocb *iocb,
> >                  fgp_flags |= FGP_DONTCACHE;
> >
> >          return __filemap_get_folio(mapping, index, fgp_flags,
> > -                                   mapping_gfp_mask(mapping));
> > +                             mapping_gfp_mask(mapping)|
> > +                             __GFP_ZERO);
> We do need to perform some initialization, but doing it in this common
> path is clearly unreasonable. It would introduce unnecessary zeroing
> overhead even for non-crypto scenarios.

Yes. That could introduce unnecessary zeroing in other paths.

> Therefore, I suspect something was missed in certain crypto-related
> initialization paths where the zeroing should have been handled instead.

I will try to fix this in the crypto-path only and send [PATCH v2].

>
> Cheers,
> Baokun
>

Best regards
Bartłomiej Kubik