Patch "nfsd: check for oversized NFSv2/v3 arguments" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

nfsd: check for oversized NFSv2/v3 arguments

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
nfsd-check-for-oversized-nfsv2-v3-arguments.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "J. Bruce Fields" <[email protected]>
Date: Fri, 21 Apr 2017 16:10:18 -0400
Subject: nfsd: check for oversized NFSv2/v3 arguments
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From: J. Bruce Fields <[email protected]>

commit e6838a29ecb484c97e4efef9429643b9851fba6e upstream.

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.

---
 fs/nfsd/nfssvc.c |   36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

@@ -733,6 +733,37 @@ static __be32 map_new_errors(u32 vers, _
 	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)
 {

@@ -745,6 +776,11 @@ nfsd_dispatch(struct svc_rqst *rqstp, __
 				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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/nfsd-check-for-oversized-nfsv2-v3-arguments.patch queue-4.9/nfsd4-minor-nfsv2-v3-write-decoding-cleanup.patch queue-4.9/nfsd-stricter-decoding-of-write-like-nfsv2-v3-ops.patch