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Subject: Re: [PATCH v3 6/6] NFSD: Protect against send buffer overflow in
 NFSv3 READ
From:   Jeff Layton <jlayton@kernel.org>
To:     Chuck Lever <chuck.lever@oracle.com>, linux-nfs@vger.kernel.org
Date:   Fri, 02 Sep 2022 09:15:21 -0400
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         <166205942489.1435.8984764212504461615.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 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
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfs3proc.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>=20
> diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
> index 7a159785499a..5b1e771238b3 100644
> --- a/fs/nfsd/nfs3proc.c
> +++ b/fs/nfsd/nfs3proc.c
> @@ -150,7 +150,6 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
>  {
>  	struct nfsd3_readargs *argp =3D rqstp->rq_argp;
>  	struct nfsd3_readres *resp =3D rqstp->rq_resp;
> -	u32 max_blocksize =3D svc_max_payload(rqstp);
>  	unsigned int len;
>  	int v;
> =20
> @@ -159,7 +158,8 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
>  				(unsigned long) argp->count,
>  				(unsigned long long) argp->offset);
> =20
> -	argp->count =3D min_t(u32, argp->count, max_blocksize);
> +	argp->count =3D min_t(u32, argp->count, svc_max_payload(rqstp));
> +	argp->count =3D min_t(u32, argp->count, rqstp->rq_res.buflen);
>  	if (argp->offset > (u64)OFFSET_MAX)
>  		argp->offset =3D (u64)OFFSET_MAX;
>  	if (argp->offset + argp->count > (u64)OFFSET_MAX)
>=20
>=20

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