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.
>
> 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.
>
> 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.
>
> Thanks to Aleksi Illikainen and Kari Hulkko for uncovering this
> issue.
>
> Reported-by: Ben Ronallo <Benjamin.Ronallo@xxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> ---
> fs/nfsd/nfs3proc.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> 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_rqst *rqstp,
> {
> struct xdr_buf *buf = &resp->dirlist;
> struct xdr_stream *xdr = &resp->xdr;
> -
> - count = clamp(count, (u32)(XDR_UNIT * 2), svc_max_payload(rqstp));
> + unsigned int sendbuf = min_t(unsigned int, rqstp->rq_res.buflen,
> + svc_max_payload(rqstp));
>
> memset(buf, 0, sizeof(*buf));
>
> /* Reserve room for the NULL ptr & eof flag (-2 words) */
> - buf->buflen = count - XDR_UNIT * 2;
> + buf->buflen = clamp(count, (u32)(XDR_UNIT * 2), sendbuf);
> + buf->buflen -= XDR_UNIT * 2;
> buf->pages = rqstp->rq_next_page;
> rqstp->rq_next_page += (buf->buflen + PAGE_SIZE - 1) >> PAGE_SHIFT;
>
>
>
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
