[SRU J/K/HWE-5.17/OEM-5.14 4/8] NFSD: Protect against send buffer overflow in NFSv3 READDIR

[Thadeu Lima de Souza Cascardo]

From: Chuck Lever <chuck.lever at oracle.com>

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 at synopsys.com>
Cc: <stable at vger.kernel.org>
Signed-off-by: Chuck Lever <chuck.lever at oracle.com>
Reviewed-by: Jeff Layton <jlayton at kernel.org>
Signed-off-by: Chuck Lever <chuck.lever at oracle.com>
(cherry picked from commit 640f87c190e0d1b2a0fcb2ecf6d2cd53b1c41991)
CVE-2022-43945
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 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 981a3a7a6e16..9e4f065761e6 100644
--- a/fs/nfsd/nfs3proc.c
+++ b/fs/nfsd/nfs3proc.c
@@ -550,13 +550,14 @@ 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;
 
-- 
2.34.1