linux-kernel - Re: [PATCH 5.15 056/917] fuse: fix page stealing

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Date:   Tue, 23 Nov 2021 20:22:32 +0100
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     Justin Forbes <jmforbes@...uxtx.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stable <stable@...r.kernel.org>,
        Frank Dinoff <fdinoff@...gle.com>
Subject: Re: [PATCH 5.15 056/917] fuse: fix page stealing

On Tue, Nov 23, 2021 at 7:29 PM Justin Forbes <jmforbes@...uxtx.org> wrote:
>
> On Mon, Nov 15, 2021 at 7:04 PM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> >
> > From: Miklos Szeredi <mszeredi@...hat.com>
> >
> > commit 712a951025c0667ff00b25afc360f74e639dfabe upstream.
> >
> > It is possible to trigger a crash by splicing anon pipe bufs to the fuse
> > device.
> >
> > The reason for this is that anon_pipe_buf_release() will reuse buf->page if
> > the refcount is 1, but that page might have already been stolen and its
> > flags modified (e.g. PG_lru added).
> >
> > This happens in the unlikely case of fuse_dev_splice_write() getting around
> > to calling pipe_buf_release() after a page has been stolen, added to the
> > page cache and removed from the page cache.
> >
> > Fix by calling pipe_buf_release() right after the page was inserted into
> > the page cache.  In this case the page has an elevated refcount so any
> > release function will know that the page isn't reusable.
> >
> > Reported-by: Frank Dinoff <fdinoff@...gle.com>
> > Link: https://lore.kernel.org/r/CAAmZXrsGg2xsP1CK+cbuEMumtrqdvD-NKnWzhNcvn71RV3c1yw@mail.gmail.com/
> > Fixes: dd3bb14f44a6 ("fuse: support splice() writing to fuse device")
> > Cc: <stable@...r.kernel.org> # v2.6.35
> > Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>
> It appears this patch causes a rather serious regression in flatpacks
> using portals to access files.  Reverting this patch restores expected
> behavior. I have asked users in the Fedora bug to test with 5.16-rc2
> to see if we are just missing a dependent patch in stable, or if this
> is broken there as well, but no response yet.:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=2025285
> https://github.com/flatpak/flatpak/issues/4595

Hi,

Thanks for the report.  Can someone with the reproducer try the attached patch?

I think the race there is unlikely but possible.

Thanks,
Miklos

View attachment "fuse-refcount-fix.patch" of type "text/x-patch" (566 bytes)