linux-kernel - Re: [PATCH v2] fs: add NULL check in drop

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Message-ID: <CAO4qAqK-6jpiFXTdpoB-e144N=Ux0Hs+NOouM6cmVDzV8V-Dcw@mail.gmail.com>
Date: Tue, 9 Dec 2025 22:00:04 +0530
From: Deepak Karn <dkarn@...hat.com>
To: Jan Kara <jack@...e.cz>
Cc: djwong@...nel.org, brauner@...nel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	syzbot+e07658f51ca22ab65b4e@...kaller.appspotmail.com, 
	syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk, 
	David Howells <dhowells@...hat.com>, linux-afs@...ts.infradead.org
Subject: Re: [PATCH v2] fs: add NULL check in drop_buffers() to prevent null-ptr-deref

On Tue, Dec 9, 2025 at 4:48 PM Jan Kara <jack@...e.cz> wrote:
>
> On Tue 09-12-25 01:43:33, Deepakkumar Karn wrote:
> > On Mon, 8 Dec 2025 11:30:24 -0800, Darrick J. Wong wrote:
> > > > drop_buffers() dereferences the buffer_head pointer returned by
> > > > folio_buffers() without checking for NULL. This leads to a null pointer
> > > > dereference when called from try_to_free_buffers() on a folio with no
> > > > buffers attached. This happens when filemap_release_folio() is called on
> > > > a folio belonging to a mapping with AS_RELEASE_ALWAYS set but without
> > > > release_folio address_space operation defined. In such case,
> >
> > > What user is that?  All the users of AS_RELEASE_ALWAYS in 6.18 appear to
> > > supply a ->release_folio.  Is this some new thing in 6.19?
> >
> > AFS directories SET AS_RELEASE_ALWAYS but have not .release_folio.
>
> AFAICS AFS sets AS_RELEASE_ALWAYS only for symlinks but not for
> directories? Anyway I agree AFS symlinks will have AS_RELEASE_ALWAYS but no
> .release_folio callback. And this looks like a bug in AFS because AFAICT
> there's no point in setting AS_RELEASE_ALWAYS when you don't have
> .release_folio callback. Added relevant people to CC.
>
>                                                                 Honza

Thank you for your response Jan. As you suggested, the bug is in AFS.
Can we include this current defensive check in drop_buffers() and I can submit
another patch to handle that bug of AFS we discussed?

Regards,
Deepakkumar Karn