CVE-2017-7645: Improper Input Validation (CWE-20)
value: The NFSv2/NFSv3 server in the nfsd subsystem in the Linux kernel through 4.10.11 allows remote attackers to cause a denial of service (system crash) via a long RPC reply, related to net/sunrpc/svc.c, fs/nfsd/nfs3xdr.c, and fs/nfsd/nfsxdr.c.
CWE description: The product receives input or data, but it does
        not validate or incorrectly validates that the input has the
        properties that are required to process the data safely and
        correctly.
CVSS v3 base score: 7.5
Published: 2017-04-18T14:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: fs/nfsd/nfssvc.c
Commit date: 2017-04-25 16:34:37 -0400
Commit message: nfsd: check for oversized NFSv2/v3 arguments

A client can append random data to the end of an NFSv2 or NFSv3 RPC call
without our complaining; we'll just stop parsing at the end of the
expected data and ignore the rest.

Encoded arguments and replies are stored together in an array of pages,
and if a call is too large it could leave inadequate space for the
reply.  This is normally OK because NFS RPC's typically have either
short arguments and long replies (like READ) or long arguments and short
replies (like WRITE).  But a client that sends an incorrectly long reply
can violate those assumptions.  This was observed to cause crashes.

Also, several operations increment rq_next_page in the decode routine
before checking the argument size, which can leave rq_next_page pointing
well past the end of the page array, causing trouble later in
svc_free_pages.

So, following a suggestion from Neil Brown, add a central check to
enforce our expectation that no NFSv2/v3 call has both a large call and
a large reply.

As followup we may also want to rewrite the encoding routines to check
more carefully that they aren't running off the end of the page array.

We may also consider rejecting calls that have any extra garbage
appended.  That would be safer, and within our rights by spec, but given
the age of our server and the NFS protocol, and the fact that we've
never enforced this before, we may need to balance that against the
possibility of breaking some oddball client.

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

Diff: diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 31e1f9593457..59979f0bbd4b 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -745,10 +745,41 @@ static __be32 map_new_errors(u32 vers, __be32 nfserr)
 	if (nfserr == nfserr_wrongsec && vers < 4)
 		return nfserr_acces;
 	return nfserr;
 }
 
+/*
+ * A write procedure can have a large argument, and a read procedure can
+ * have a large reply, but no NFSv2 or NFSv3 procedure has argument and
+ * reply that can both be larger than a page.  The xdr code has taken
+ * advantage of this assumption to be a sloppy about bounds checking in
+ * some cases.  Pending a rewrite of the NFSv2/v3 xdr code to fix that
+ * problem, we enforce these assumptions here:
+ */
+static bool nfs_request_too_big(struct svc_rqst *rqstp,
+				struct svc_procedure *proc)
+{
+	/*
+	 * The ACL code has more careful bounds-checking and is not
+	 * susceptible to this problem:
+	 */
+	if (rqstp->rq_prog != NFS_PROGRAM)
+		return false;
+	/*
+	 * Ditto NFSv4 (which can in theory have argument and reply both
+	 * more than a page):
+	 */
+	if (rqstp->rq_vers >= 4)
+		return false;
+	/* The reply will be small, we're OK: */
+	if (proc->pc_xdrressize > 0 &&
+	    proc->pc_xdrressize < XDR_QUADLEN(PAGE_SIZE))
+		return false;
+
+	return rqstp->rq_arg.len > PAGE_SIZE;
+}
+
 int
 nfsd_dispatch(struct svc_rqst *rqstp, __be32 *statp)
 {
 	struct svc_procedure	*proc;
 	kxdrproc_t		xdr;
@@ -757,10 +788,15 @@ nfsd_dispatch(struct svc_rqst *rqstp, __be32 *statp)
 
 	dprintk("nfsd_dispatch: vers %d proc %d\n",
 				rqstp->rq_vers, rqstp->rq_proc);
 	proc = rqstp->rq_procinfo;
 
+	if (nfs_request_too_big(rqstp, proc)) {
+		dprintk("nfsd: NFSv%d argument too large\n", rqstp->rq_vers);
+		*statp = rpc_garbage_args;
+		return 1;
+	}
 	/*
 	 * Give the xdr decoder a chance to change this if it wants
 	 * (necessary in the NFSv4.0 compound case)
 	 */
 	rqstp->rq_cachetype = proc->pc_cachetype;

Fixed code: /*
 * A write procedure can have a large argument, and a read procedure can
 * have a large reply, but no NFSv2 or NFSv3 procedure has argument and
 * reply that can both be larger than a page.  The xdr code has taken
 * advantage of this assumption to be a sloppy about bounds checking in
 * some cases.  Pending a rewrite of the NFSv2/v3 xdr code to fix that
 * problem, we enforce these assumptions here:
 */
static bool nfs_request_too_big(struct svc_rqst *rqstp,
				struct svc_procedure *proc)
{
	/*
	 * The ACL code has more careful bounds-checking and is not
	 * susceptible to this problem:
	 */
	if (rqstp->rq_prog != NFS_PROGRAM)
		return false;
	/*
	 * Ditto NFSv4 (which can in theory have argument and reply both
	 * more than a page):
	 */
	if (rqstp->rq_vers >= 4)
		return false;
	/* The reply will be small, we're OK: */
	if (proc->pc_xdrressize > 0 &&
	    proc->pc_xdrressize < XDR_QUADLEN(PAGE_SIZE))
		return false;

	return rqstp->rq_arg.len > PAGE_SIZE;
}

	if (nfs_request_too_big(rqstp, proc)) {
		dprintk("nfsd: NFSv%d argument too large\n", rqstp->rq_vers);
		*statp = rpc_garbage_args;
		return 1;
	}