NFSD: Protect against send buffer overflow in NFSv3 READ

jira VULN-155649
cve CVE-2022-50410
commit-author Chuck Lever <chuck.lever@oracle.com>
commit fa6be9c
upstream-diff LTS 8.6 version of the function `nfsd3_proc_read()'
  misses changes from 0cb4d23,
  cc9bcda and
  be63bd2. Preserved the logic of what
  is assigned to `resp->count' despite the missing changes.

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.

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.

	Cc: <stable@vger.kernel.org>
	Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
	Reviewed-by: Jeff Layton <jlayton@kernel.org>
	Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
(cherry picked from commit fa6be9c)
	Signed-off-by: Marcin Wcisło <marcin.wcislo@conclusive.pl>

Author: pvts-mat

fs/nfsd/nfs3proc.c

@@ -146,6 +146,7 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
 	struct nfsd3_readres *resp = rqstp->rq_resp;
 	u32	max_blocksize = svc_max_payload(rqstp);
 	unsigned long cnt = min(argp->count, max_blocksize);
+	cnt = min(cnt, (unsigned long)rqstp->rq_res.buflen);
 
 	dprintk("nfsd: READ(3) %s %lu bytes at %Lu\n",
 				SVCFH_fmt(&argp->fh),