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From:   Miklos Szeredi <mszeredi@redhat.com>
Date:   Wed, 24 Nov 2021 17:21:26 +0100
Message-ID: <CAOssrKd=XAvCKW9t0gk9g_FBqFs6pPNJFsP7pc2EF6Dcj-se7w@mail.gmail.com>
Subject: Re: [PATCH 4.4 109/162] fuse: fix page stealing
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     lkml <linux-kernel@vger.kernel.org>,
        stable <stable@vger.kernel.org>,
        Frank Dinoff <fdinoff@google.com>
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On Wed, Nov 24, 2021 at 1:04 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> From: Miklos Szeredi <mszeredi@redhat.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@google.com>
> Link: https://lore.kernel.org/r/CAAmZXrsGg2xsP1CK+cbuEMumtrqdvD-NKnWzhNcvn71RV3c1yw@mail.gmail.com/
> Fixes: dd3bb14f44a6 ("fuse: support splice() writing to fuse device")
> Cc: <stable@vger.kernel.org> # v2.6.35
> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


Hi Greg,

This patch turned out to have a bug, so stable releases that didn't
yet have it released might be better off backing it out for now and
releasing only together with the fix to avoid regressions.

Thanks,
Miklos