From:   Chuck Lever III <chuck.lever@oracle.com>
To:     Jeff Layton <jlayton@kernel.org>
CC:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        "dcritch@redhat.com" <dcritch@redhat.com>,
        "d.lesca@solinos.it" <d.lesca@solinos.it>,
        Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v2 1/2] nfsd: don't replace page in rq_pages if it's a
 continuation of last page
Thread-Topic: [PATCH v2 1/2] nfsd: don't replace page in rq_pages if it's a
 continuation of last page
Thread-Index: AQHZWPPJ3uW/378GqUmc8Oe+CyIiW67/O2GA
Date:   Fri, 17 Mar 2023 17:33:53 +0000
Message-ID: <28E207C2-B47D-42CF-A531-5EF184D4DED6@oracle.com>
References: <20230317171309.73607-1-jlayton@kernel.org>
In-Reply-To: <20230317171309.73607-1-jlayton@kernel.org>
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> On Mar 17, 2023, at 1:13 PM, Jeff Layton <jlayton@kernel.org> wrote:
>=20
> The splice read calls nfsd_splice_actor to put the pages containing file
> data into the svc_rqst->rq_pages array. It's possible however to get a
> splice result that only has a partial page at the end, if (e.g.) the
> filesystem hands back a short read that doesn't cover the whole page.
>=20
> nfsd_splice_actor will plop the partial page into its rq_pages array and
> return. Then later, when nfsd_splice_actor is called again, the
> remainder of the page may end up being filled out. At this point,
> nfsd_splice_actor will put the page into the array _again_ corrupting
> the reply. If this is done enough times, rq_next_page will overrun the
> array and corrupt the trailing fields -- the rq_respages and
> rq_next_page pointers themselves.
>=20
> If we've already added the page to the array in the last pass, don't add
> it to the array a second time when dealing with a splice continuation.
> This was originally handled properly in nfsd_splice_actor, but commit
> 91e23b1c3982 removed the check for it.
>=20
> Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()")
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Reported-by: Dario Lesca <d.lesca@solinos.it>
> Tested-by: David Critch <dcritch@redhat.com>
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=3D2150630
> Signed-off-by: Jeff Layton <jlayton@kernel.org>

I've applied this one internally for due-diligence testing.
I'll wait a couple of days for review comments, so it's
not likely to make v6.3-rc3 on Sunday.


> ---
> fs/nfsd/vfs.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>=20
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 502e1b7742db..97b38b47c563 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -941,8 +941,14 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, stru=
ct pipe_buffer *buf,
> 	struct page *last_page;
>=20
> 	last_page =3D page + (offset + sd->len - 1) / PAGE_SIZE;
> -	for (page +=3D offset / PAGE_SIZE; page <=3D last_page; page++)
> -		svc_rqst_replace_page(rqstp, page);
> +	for (page +=3D offset / PAGE_SIZE; page <=3D last_page; page++) {
> +		/*
> +		 * Skip page replacement when extending the contents
> +		 * of the current page.
> +		 */
> +		if (page !=3D *(rqstp->rq_next_page - 1))
> +			svc_rqst_replace_page(rqstp, page);
> +	}
> 	if (rqstp->rq_res.page_len =3D=3D 0)	// first call
> 		rqstp->rq_res.page_base =3D offset % PAGE_SIZE;
> 	rqstp->rq_res.page_len +=3D sd->len;
> --=20
> 2.39.2
>=20

--
Chuck Lever