[PATCH bpf v3 1/2] bpf: Zero-fill other CPUs when BPF_F_CPU creates a per-cpu hash element

[Donggeun Yoo]

pcpu_init_value() initializes the per-cpu area of a newly created
[lru_]percpu_hash element.  The area is recycled, so when the value
comes from a BPF program (onallcpus == false) it writes the running
CPU's slot and zeroes the rest.

bpf_percpu_hash_update() passes onallcpus == true, which delegates to
pcpu_copy_value().  pcpu_copy_value() writes only the CPU named in
map_flags when BPF_F_CPU is set, so on the create path the other slots
keep the recycled element's values:

  update(k1, 0xdeadc0de, BPF_F_ALL_CPUS)  every CPU holds 0xdeadc0de
  delete(k1)                              element back on the freelist
  update(k2, 0xc0ffee, BPF_F_CPU | 0)     creates, writes CPU 0 only
  lookup(k2)                              CPU 0 0xc0ffee, rest 0xdeadc0de

Zero-fill the other CPUs on that arm too.

Fixes: c6936161fd55 ("bpf: Add BPF_F_CPU and BPF_F_ALL_CPUS flags support for percpu_hash and lru_percpu_hash maps")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@xxxxxxxxx>
---
 kernel/bpf/hashtab.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 4f495dcbf670c..f9464e566f109 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1054,14 +1054,17 @@ static void pcpu_init_value(struct bpf_htab *htab, void __percpu *pptr,
 	/* When not setting the initial value on all cpus, zero-fill element
 	 * values for other cpus. Otherwise, bpf program has no way to ensure
 	 * known initial values for cpus other than current one
-	 * (onallcpus=false always when coming from bpf prog).
+	 * (onallcpus=false always when coming from bpf prog,
+	 *  map_flags & BPF_F_CPU when coming from syscall but setting
+	 *  only one cpu).
 	 */
-	if (!onallcpus) {
-		int current_cpu = raw_smp_processor_id();
+	if (!onallcpus || (map_flags & BPF_F_CPU)) {
+		int init_cpu = (map_flags & BPF_F_CPU) ? map_flags >> 32 :
+			       raw_smp_processor_id();
 		int cpu;
 
 		for_each_possible_cpu(cpu) {
-			if (cpu == current_cpu)
+			if (cpu == init_cpu)
 				copy_map_value(&htab->map, per_cpu_ptr(pptr, cpu), value);
 			else /* Since elem is preallocated, we cannot touch special fields */
 				zero_map_value(&htab->map, per_cpu_ptr(pptr, cpu));
-- 
2.53.0