UPSTREAM: nfsd: stricter decoding of write-like NFSv2/v3 ops

The NFSv2/v3 code does not systematically check whether we decode past
the end of the buffer.  This generally appears to be harmless, but there
are a few places where we do arithmetic on the pointers involved and
don't account for the possibility that a length could be negative.  Add
checks to catch these.

Reported-by: Tuomas Haanpää <thaan@synopsys.com>
Reported-by: Ari Kauppi <ari@synopsys.com>
Reviewed-by: NeilBrown <neilb@suse.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Andrey Ulanov <andreyu@google.com>

(cherry picked from commit 13bf9fbff0e5e099e2b6f003a0ab8ae145436309)

BUG=b:38261521
TEST=None

Change-Id: Iba805f76c8e0f6d98374f877f8bfd36ea8dc5718
Reviewed-on: https://chromium-review.googlesource.com/506275
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Commit-Queue: Andrey Ulanov <andreyu@google.com>
Tested-by: Andrey Ulanov <andreyu@google.com>

fs/nfsd/nfs3xdr.c

diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
index 34f4d1b..7162ab7b 100644
--- a/fs/nfsd/nfs3xdr.c
+++ b/fs/nfsd/nfs3xdr.c
@@ -368,6 +368,8 @@
 	args->count = ntohl(*p++);
 	args->stable = ntohl(*p++);
 	len = args->len = ntohl(*p++);
+	if ((void *)p > head->iov_base + head->iov_len)
+		return 0;
 	/*
 	 * The count must equal the amount of data passed.
 	 */
@@ -471,6 +473,8 @@
 	/* first copy and check from the first page */
 	old = (char*)p;
 	vec = &rqstp->rq_arg.head[0];
+	if ((void *)old > vec->iov_base + vec->iov_len)
+		return 0;
 	avail = vec->iov_len - (old - (char*)vec->iov_base);
 	while (len && avail && *old) {
 		*new++ = *old++;

fs/nfsd/nfsxdr.c

diff --git a/fs/nfsd/nfsxdr.c b/fs/nfsd/nfsxdr.c
index 894f2a8f..bf91320 100644
--- a/fs/nfsd/nfsxdr.c
+++ b/fs/nfsd/nfsxdr.c
@@ -302,6 +302,8 @@
 	 * bytes.
 	 */
 	hdr = (void*)p - head->iov_base;
+	if (hdr > head->iov_len)
+		return 0;
 	dlen = head->iov_len + rqstp->rq_arg.page_len - hdr;
 
 	/*