NFSD: Clamp WRITE offsets

Ensure that a client cannot specify a WRITE range that falls in a
byte range outside what the kernel's internal types (such as loff_t,
which is signed) can represent. The kiocb iterators, invoked in
nfsd_vfs_write(), should properly limit write operations to within
the underlying file system's s_maxbytes.

Cc: stable@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs3proc.c

@@ -203,6 +203,11 @@ nfsd3_proc_write(struct svc_rqst *rqstp)
 				(unsigned long long) argp->offset,
 				argp->stable? " stable" : "");
 
+	resp->status = nfserr_fbig;
+	if (argp->offset > (u64)OFFSET_MAX ||
+	    argp->offset + argp->len > (u64)OFFSET_MAX)
+		return rpc_success;
+
 	fh_copy(&resp->fh, &argp->fh);
 	resp->committed = argp->stable;
 	nvecs = svc_fill_write_vector(rqstp, &argp->payload);

fs/nfsd/nfs4proc.c

@@ -1022,8 +1022,9 @@ nfsd4_write(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	unsigned long cnt;
 	int nvecs;
 
-	if (write->wr_offset >= OFFSET_MAX)
-		return nfserr_inval;
+	if (write->wr_offset > (u64)OFFSET_MAX ||
+	    write->wr_offset + write->wr_buflen > (u64)OFFSET_MAX)
+		return nfserr_fbig;
 
 	cnt = write->wr_buflen;
 	trace_nfsd_write_start(rqstp, &cstate->current_fh,