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

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Message-ID: <CAO4qAq+YJwTgmU=QjAhfxqpKsXJ1sjSz+pniWdW8Yr9zeLGePg@mail.gmail.com>
Date: Wed, 10 Dec 2025 23:20:23 +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 Wed, Dec 10, 2025 at 10:54 PM Jan Kara <jack@...e.cz> wrote:
>
> On Wed 10-12-25 20:59:00, Deepak Karn wrote:
> > On Wed, Dec 10, 2025 at 3:25 PM Jan Kara <jack@...e.cz> wrote:
> > >
> > > On Tue 09-12-25 22:00:04, Deepak Karn wrote:
> > > > 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?
> > >
> > > I'm not strongly opposed to that (although try_to_free_buffers() would seem
> > > like a tad bit better place) but overall I don't think it's a great idea as
> > > it would hide bugs. But perhaps with WARN_ON_ONCE() (to catch sloppy
> > > programming) it would be a sensible hardening.
> > >
> >
> > Thanks Jan for your response. As suggested, adding WARN_ON_ONCE() will be
> > more sensible.
> > I just wanted to clarify my understanding, you are suggesting adding
> > WARN_ON_ONCE() in try_to_free_buffers() as this highlights the issue and
> > also solves the concern. If my understanding is wrong please let me know,
> > I will share the updated patch.
>
> Yes, I meant something like:
>
>         if (WARN_ON_ONCE(!folio_buffers(folio)))
>                 return true;
>
> in try_to_free_buffers().
>

Yes this is what I am also going for, just a minor change that I was
going to return false.

Reason:
>From the flow it should return false whenever failed to free buffers /
operation
couldn't complete. Do you think otherwise.

> > > Also I think mapping_set_release_always() should assert that
> > > mapping->a_ops->release_folio is non-NULL to catch the problem early (once
> > > you fix the AFS bug).
> > >
> >
> > I will address this in another patch for AFS.
>
> Thanks. As a separate patch please :)
>
>                                                                 Honza

Yes.

Regards,
Deepak Karn