bpf: check max_entries before allocating memory

For maps of type BPF_MAP_TYPE_CPUMAP memory is allocated first before
checking the max_entries argument. If then max_entries is greater than
NR_CPUS additional work needs to be done to free allocated memory before
an error is returned.
This changes moves the check on max_entries before the allocation
happens.
Signed-off-by: Florian Lehner <dev@der-flo.net>
Link: https://lore.kernel.org/r/20221028183405.59554-1-dev@der-flo.netSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/cpumap.c

@@ -85,7 +85,6 @@ static struct bpf_map *cpu_map_alloc(union bpf_attr *attr)
 {
 	u32 value_size = attr->value_size;
 	struct bpf_cpu_map *cmap;
-	int err = -ENOMEM;
 
 	if (!bpf_capable())
 		return ERR_PTR(-EPERM);
@@ -97,29 +96,26 @@ static struct bpf_map *cpu_map_alloc(union bpf_attr *attr)
 	    attr->map_flags & ~BPF_F_NUMA_NODE)
 		return ERR_PTR(-EINVAL);
 
+	/* Pre-limit array size based on NR_CPUS, not final CPU check */
+	if (attr->max_entries > NR_CPUS)
+		return ERR_PTR(-E2BIG);
+
 	cmap = bpf_map_area_alloc(sizeof(*cmap), NUMA_NO_NODE);
 	if (!cmap)
 		return ERR_PTR(-ENOMEM);
 
 	bpf_map_init_from_attr(&cmap->map, attr);
 
-	/* Pre-limit array size based on NR_CPUS, not final CPU check */
-	if (cmap->map.max_entries > NR_CPUS) {
-		err = -E2BIG;
-		goto free_cmap;
-	}
-
 	/* Alloc array for possible remote "destination" CPUs */
 	cmap->cpu_map = bpf_map_area_alloc(cmap->map.max_entries *
 					   sizeof(struct bpf_cpu_map_entry *),
 					   cmap->map.numa_node);
-	if (!cmap->cpu_map)
-		goto free_cmap;
+	if (!cmap->cpu_map) {
+		bpf_map_area_free(cmap);
+		return ERR_PTR(-ENOMEM);
+	}
 
 	return &cmap->map;
-free_cmap:
-	bpf_map_area_free(cmap);
-	return ERR_PTR(err);
 }
 
 static void get_cpu_map_entry(struct bpf_cpu_map_entry *rcpu)