[CVE-2017-7895][Y][PATCH 2/2] nfsd: stricter decoding of write-like NFSv2/v3 ops

[Po-Hsu Lin]

From: "J. Bruce Fields" <bfields at redhat.com>

CVE-2017-7895

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 at synopsys.com>
Reported-by: Ari Kauppi <ari at synopsys.com>
Reviewed-by: NeilBrown <neilb at suse.com>
Cc: stable at vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields at redhat.com>
(cherry picked from commit 13bf9fbff0e5e099e2b6f003a0ab8ae145436309)

Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
---
 fs/nfsd/nfs3xdr.c |    4 ++++
 fs/nfsd/nfsxdr.c  |    2 ++
 2 files changed, 6 insertions(+)

diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
index d18cfdd..4523346 100644
--- a/fs/nfsd/nfs3xdr.c
+++ b/fs/nfsd/nfs3xdr.c
@@ -369,6 +369,8 @@ nfs3svc_decode_writeargs(struct svc_rqst *rqstp, __be32 *p,
 	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.
 	 */
@@ -472,6 +474,8 @@ nfs3svc_decode_symlinkargs(struct svc_rqst *rqstp, __be32 *p,
 	/* 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++;
diff --git a/fs/nfsd/nfsxdr.c b/fs/nfsd/nfsxdr.c
index 59bd88a..de07ff6 100644
--- a/fs/nfsd/nfsxdr.c
+++ b/fs/nfsd/nfsxdr.c
@@ -302,6 +302,8 @@ nfssvc_decode_writeargs(struct svc_rqst *rqstp, __be32 *p,
 	 * bytes.
 	 */
 	hdr = (void*)p - head->iov_base;
+	if (hdr > head->iov_len)
+		return 0;
 	dlen = head->iov_len + rqstp->rq_arg.page_len - hdr;
 
 	/*
-- 
1.7.9.5