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

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Message-ID: <CAPqLRf224VcJJM1rmiJTnFXg+5tNeF4HC+AEBWpBpWZO6VxbiQ@mail.gmail.com>
Date: Sun, 28 Dec 2025 14:00:46 +0100
From: Bartłomiej Kubik <kubik.bartlomiej@...il.com>
To: Baokun Li <libaokun1@...wei.com>
Cc: tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org, 
	linux-kernel@...r.kernel.org, david.hunter.linux@...il.com, 
	skhan@...uxfoundation.org, khalid@...nel.org, 
	linux-kernel-mentees@...ts.linux.dev, 
	syzbot+703d8a2cd20971854b06@...kaller.appspotmail.com
Subject: Re: [PATCH] fs/ext4: Initialize new folios before use

Hi,

Thank you for your suggestions.

On Wed, 24 Dec 2025 at 02:39, Baokun Li <libaokun1@...wei.com> 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@...kaller.appspotmail.com
> > Reported-by: syzbot+703d8a2cd20971854b06@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=703d8a2cd20971854b06
> >
> > Signed-off-by: Bartlomiej Kubik <kubik.bartlomiej@...il.com>
> > ---
> >  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