NFS4: Avoid NULL reference or double free in nfsd4_fslocs_free()

If fsloc_parse() failed at kzalloc(), fs/nfsd/export.c
 411
 412         fsloc->locations = kzalloc(fsloc->locations_count
 413                         * sizeof(struct nfsd4_fs_location), GFP_KERNEL);
 414         if (!fsloc->locations)
 415                 return -ENOMEM;

svc_export_parse() will call nfsd4_fslocs_free() with fsloc->locations = NULL,
so that, "kfree(fsloc->locations[i].path);" will cause a crash.

If fsloc_parse() failed after that, fsloc_parse() will call nfsd4_fslocs_free(),
and svc_export_parse() will call it again, so that, a double free is caused.

This patch checks the fsloc->locations, and set to NULL after it be freed.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/export.c

@@ -295,13 +295,19 @@ svc_expkey_update(struct cache_detail *cd, struct svc_expkey *new,
 
 static void nfsd4_fslocs_free(struct nfsd4_fs_locations *fsloc)
 {
+	struct nfsd4_fs_location *locations = fsloc->locations;
 	int i;
 
+	if (!locations)
+		return;
+
 	for (i = 0; i < fsloc->locations_count; i++) {
-		kfree(fsloc->locations[i].path);
-		kfree(fsloc->locations[i].hosts);
+		kfree(locations[i].path);
+		kfree(locations[i].hosts);
 	}
-	kfree(fsloc->locations);
+
+	kfree(locations);
+	fsloc->locations = NULL;
 }
 
 static void svc_export_put(struct kref *ref)