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Subject: Re: [PATCH v2 3/7] NFSD: Protect against READDIR send buffer
 overflow
From:   Jeff Layton <jlayton@kernel.org>
To:     Chuck Lever <chuck.lever@oracle.com>, linux-nfs@vger.kernel.org
Date:   Mon, 29 Aug 2022 09:43:56 -0400
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On Sun, 2022-08-28 at 14:50 -0400, Chuck Lever wrote:
> For many years, 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. The dividing line between the
> receive and send buffer is pointed to by svc_rqst::rq_respages.
>=20

nit: Given that you don't look at rq_respages in the patch below, the
previous sentence is not particularly relevant. It might be better to
just explain that rq_res describes the part of the array that is the
response buffer, so we want to consult it for the max length.

> Thus the send buffer shrinks when the received RPC record
> containing the RPC Call is large.
>=20
> nfsd3_init_dirlist_pages() needs to account for the space in the
> svc_rqst::rq_pages array already consumed by the RPC receive buffer.
> Otherwise READDIR reply encoding can wander off the end of the page
> array.
>=20
> Thanks to Aleksi Illikainen and Kari Hulkko for discovering this
> issue.
>=20
> Reported-by: Ben Ronallo <Benjamin.Ronallo@synopsys.com>
> Fixes: f5dcccd647da ("NFSD: Update the NFSv2 READDIR entry encoder to use=
 struct xdr_stream")
> Fixes: 7f87fc2d34d4 ("NFSD: Update NFSv3 READDIR entry encoders to use st=
ruct xdr_stream")
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfs3proc.c |    5 ++---
>  fs/nfsd/nfsproc.c  |    5 ++---
>  2 files changed, 4 insertions(+), 6 deletions(-)
>=20
> diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
> index a41cca619338..fab87e9e0b20 100644
> --- a/fs/nfsd/nfs3proc.c
> +++ b/fs/nfsd/nfs3proc.c
> @@ -564,12 +564,11 @@ 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;
> =20
> -	count =3D clamp(count, (u32)(XDR_UNIT * 2), svc_max_payload(rqstp));
> -
>  	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), rqstp->rq_res.buflen)=
;
> +	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
> diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
> index 7381972f1677..23c273cb68a9 100644
> --- a/fs/nfsd/nfsproc.c
> +++ b/fs/nfsd/nfsproc.c
> @@ -567,12 +567,11 @@ static void nfsd_init_dirlist_pages(struct svc_rqst=
 *rqstp,
>  	struct xdr_buf *buf =3D &resp->dirlist;
>  	struct xdr_stream *xdr =3D &resp->xdr;
> =20
> -	count =3D clamp(count, (u32)(XDR_UNIT * 2), svc_max_payload(rqstp));
> -
>  	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), rqstp->rq_res.buflen)=
;
> +	buf->buflen -=3D XDR_UNIT * 2;
>  	buf->pages =3D rqstp->rq_next_page;
>  	rqstp->rq_next_page++;
> =20
>=20
>=20

I wonder if a better fix would be to make svc_max_payload take the
already-consumed arg space into account? We'd need to fix up the other
callers of course.

In any case, the patch itself looks fine:

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