[SRU J/K/HWE-5.17/OEM-5.14 5/8] NFSD: Protect against send buffer overflow in NFSv2 READ

[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 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 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 401bc1f90874280a80b93f23be33a0e7e2d1f912)
CVE-2022-43945
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 fs/nfsd/nfsproc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
index 346639fea76a..f65eba938a57 100644
--- a/fs/nfsd/nfsproc.c
+++ b/fs/nfsd/nfsproc.c
@@ -182,6 +182,7 @@ 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;
-- 
2.34.1