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Subject: [PATCH v3 3/6] NFSD: Protect against send buffer overflow in NFSv2
 READDIR
From:   Chuck Lever <chuck.lever@oracle.com>
To:     linux-nfs@vger.kernel.org
Date:   Thu, 01 Sep 2022 15:10:05 -0400
Message-ID: <166205940565.1435.7170548905174362083.stgit@manet.1015granger.net>
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Restore the previous limit on the @count argument to prevent a
buffer overflow attack.

Fixes: 53b1119a6e50 ("NFSD: Fix READDIR buffer overflow")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/nfsd/nfsproc.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
index 7381972f1677..ddb1902c0a18 100644
--- a/fs/nfsd/nfsproc.c
+++ b/fs/nfsd/nfsproc.c
@@ -567,12 +567,11 @@ static void nfsd_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));
-
 	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), (u32)PAGE_SIZE);
+	buf->buflen -= XDR_UNIT * 2;
 	buf->pages = rqstp->rq_next_page;
 	rqstp->rq_next_page++;