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Subject: Re: [PATCH v3 4/6] NFSD: Protect against send buffer overflow in
 NFSv3 READDIR
From:   Jeff Layton <jlayton@kernel.org>
To:     Chuck Lever <chuck.lever@oracle.com>, linux-nfs@vger.kernel.org
Date:   Fri, 02 Sep 2022 09:12:30 -0400
In-Reply-To: <166205941213.1435.18172275008498406790.stgit@manet.1015granger.net>
References: <166204973526.1435.6068003336048840051.stgit@manet.1015granger.net>
         <166205941213.1435.18172275008498406790.stgit@manet.1015granger.net>
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On Thu, 2022-09-01 at 15:10 -0400, Chuck Lever wrote:
> Since before the git era, NFSD has conserved the number of pages
> held by each nfsd thread by combining the RPC receive and send
> buffers into a single array of pages. This works because there are
> no cases where an operation needs a large RPC Call message and a
> large RPC Reply message at the same time.
>=20
> Once an RPC Call has been received, svc_process() updates
> svc_rqst::rq_res to describe the part of rq_pages that can be
> used for constructing the Reply. This means that the send buffer
> (rq_res) shrinks when the received RPC record containing the RPC
> Call is large.
>=20
> A client can force this shrinkage on TCP by sending a correctly-
> formed RPC Call header contained in an RPC record that is
> excessively large. The full maximum payload size cannot be
> constructed in that case.
>=20
> Thanks to Aleksi Illikainen and Kari Hulkko for uncovering this
> issue.
>=20
> Reported-by: Ben Ronallo <Benjamin.Ronallo@synopsys.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfs3proc.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>=20
> diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
> index a41cca619338..7a159785499a 100644
> --- a/fs/nfsd/nfs3proc.c
> +++ b/fs/nfsd/nfs3proc.c
> @@ -563,13 +563,14 @@ static void nfsd3_init_dirlist_pages(struct svc_rqs=
t *rqstp,
>  {
>  	struct xdr_buf *buf =3D &resp->dirlist;
>  	struct xdr_stream *xdr =3D &resp->xdr;
> -
> -	count =3D clamp(count, (u32)(XDR_UNIT * 2), svc_max_payload(rqstp));
> +	unsigned int sendbuf =3D min_t(unsigned int, rqstp->rq_res.buflen,
> +				     svc_max_payload(rqstp));
> =20
>  	memset(buf, 0, sizeof(*buf));
> =20
>  	/* Reserve room for the NULL ptr & eof flag (-2 words) */
> -	buf->buflen =3D count - XDR_UNIT * 2;
> +	buf->buflen =3D clamp(count, (u32)(XDR_UNIT * 2), sendbuf);
> +	buf->buflen -=3D XDR_UNIT * 2;
>  	buf->pages =3D rqstp->rq_next_page;
>  	rqstp->rq_next_page +=3D (buf->buflen + PAGE_SIZE - 1) >> PAGE_SHIFT;
> =20
>=20
>=20

Reviewed-by: Jeff Layton <jlayton@kernel.org>