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Subject: [PATCH v3 5/6] NFSD: Protect against send buffer overflow in NFSv2
 READ
From:   Chuck Lever <chuck.lever@oracle.com>
To:     linux-nfs@vger.kernel.org
Date:   Thu, 01 Sep 2022 15:10:18 -0400
Message-ID: <166205941847.1435.15080240781458940273.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/nfsproc.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
index ddb1902c0a18..4b19cc727ea5 100644
--- a/fs/nfsd/nfsproc.c
+++ b/fs/nfsd/nfsproc.c
@@ -185,6 +185,7 @@ nfsd_proc_read(struct svc_rqst *rqstp)
 		argp->count, argp->offset);
 
 	argp->count = min_t(u32, argp->count, NFSSVC_MAXBLKSIZE_V2);
+	argp->count = min_t(u32, argp->count, rqstp->rq_res.buflen);
 
 	v = 0;
 	len = argp->count;