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Subject: [PATCH v3 6/6] NFSD: Protect against send buffer overflow in NFSv3
 READ
From:   Chuck Lever <chuck.lever@oracle.com>
To:     linux-nfs@vger.kernel.org
Date:   Thu, 01 Sep 2022 15:10:24 -0400
Message-ID: <166205942489.1435.8984764212504461615.stgit@manet.1015granger.net>
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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>
---
 fs/nfsd/nfs3proc.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

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 = rqstp->rq_argp;
 	struct nfsd3_readres *resp = rqstp->rq_resp;
-	u32 max_blocksize = svc_max_payload(rqstp);
 	unsigned int len;
 	int v;
 
@@ -159,7 +158,8 @@ nfsd3_proc_read(struct svc_rqst *rqstp)
 				(unsigned long) argp->count,
 				(unsigned long long) argp->offset);
 
-	argp->count = min_t(u32, argp->count, max_blocksize);
+	argp->count = min_t(u32, argp->count, svc_max_payload(rqstp));
+	argp->count = min_t(u32, argp->count, rqstp->rq_res.buflen);
 	if (argp->offset > (u64)OFFSET_MAX)
 		argp->offset = (u64)OFFSET_MAX;
 	if (argp->offset + argp->count > (u64)OFFSET_MAX)