bpf: zero-fill re-used per-cpu map element

Zero-fill element values for all cpus, just as when not using
prealloc. This is the only way the bpf program can ensure known
initial values for cpus other than the current one ('onallcpus'
cannot be set when coming from the bpf program).

The scenario is: bpf program inserts some elements in a per-cpu
map, then deletes some (or userspace does). When later adding
new elements using bpf_map_update_elem(), the bpf program can
only set the value of the new elements for the current cpu.
When prealloc is enabled, previously deleted elements are re-used.
Without the fix, values for other cpus remain whatever they were
when the re-used entry was previously freed.

Fixes: 6c90598 ("bpf: pre-allocate hash map elements")
Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David Verbeiren <david.verbeiren@tessares.net>

Author: dverbeir

kernel/bpf/hashtab.c

@@ -836,6 +836,7 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 	bool prealloc = htab_is_prealloc(htab);
 	struct htab_elem *l_new, **pl_new;
 	void __percpu *pptr;
+	int cpu;
 
 	if (prealloc) {
 		if (old_elem) {
@@ -880,6 +881,17 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 		size = round_up(size, 8);
 		if (prealloc) {
 			pptr = htab_elem_get_ptr(l_new, key_size);
+
+			/* zero-fill element values for all cpus, just as when
+			 * not using prealloc. Only way for bpf program to
+			 * ensure known initial values for cpus other than
+			 * current one (onallcpus=false when coming from bpf
+			 * prog).
+			 */
+			if (!onallcpus)
+				for_each_possible_cpu(cpu)
+					memset((void *)per_cpu_ptr(pptr, cpu),
+					       0, size);
 		} else {
 			/* alloc_percpu zero-fills */
 			pptr = __alloc_percpu_gfp(size, 8,