selftests/bpf: Reject non-percpu values in percpu kptr fields

Add verifier coverage for the two ways a non-percpu pointer can be stored
in a __percpu_kptr field: a program-BTF local allocation returned by
bpf_obj_new(), and a referenced kernel-BTF task_struct pointer.

Without the verifier fix, both programs are unexpectedly accepted and the
negative tests fail. Requiring MEM_PERCPU makes both programs fail
verification with the expected invalid-kptr diagnostic.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 59 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/percpu_alloc_fail.c b/tools/testing/selftests/bpf/progs/percpu_alloc_fail.c
index 08379c3b6a03..3701f4ea58c7 100644
--- a/tools/testing/selftests/bpf/progs/percpu_alloc_fail.c
+++ b/tools/testing/selftests/bpf/progs/percpu_alloc_fail.c
@@ -33,6 +33,20 @@ struct {
 	__type(value, struct elem);
 } array SEC(".maps");
 
+struct kernel_percpu_elem {
+	struct task_struct __percpu_kptr *task;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct kernel_percpu_elem);
+} kernel_percpu_array SEC(".maps");
+
+struct task_struct *bpf_task_from_pid(s32 pid) __ksym;
+void bpf_task_release(struct task_struct *p) __ksym;
+
 long ret;
 
 SEC("?fentry/bpf_fentry_test1")
@@ -137,6 +151,51 @@ int BPF_PROG(test_array_map_5)
 	return 0;
 }
 
+SEC("?syscall")
+__failure __msg("invalid kptr access, R2 type=trusted_ptr_ expected=ptr_task_struct")
+int reject_kernel_ptr_into_percpu_kptr(void *ctx)
+{
+	struct kernel_percpu_elem *e;
+	struct task_struct *p, *old;
+	int index = 0;
+
+	e = bpf_map_lookup_elem(&kernel_percpu_array, &index);
+	if (!e)
+		return 0;
+
+	p = bpf_task_from_pid(1);
+	if (!p)
+		return 0;
+
+	old = bpf_kptr_xchg(&e->task, p);
+	if (old)
+		bpf_task_release(old);
+	return 0;
+}
+
+SEC("?fentry.s/bpf_fentry_test1")
+__failure __msg("invalid kptr access, R2 type=ptr_ expected=ptr_val_t")
+int BPF_PROG(reject_plain_alloc_into_percpu_kptr)
+{
+	struct val_t __percpu_kptr *old;
+	struct val_t *p;
+	struct elem *e;
+	int index = 0;
+
+	e = bpf_map_lookup_elem(&array, &index);
+	if (!e)
+		return 0;
+
+	p = bpf_obj_new(struct val_t);
+	if (!p)
+		return 0;
+
+	old = bpf_kptr_xchg(&e->pc, p);
+	if (old)
+		bpf_percpu_obj_drop(old);
+	return 0;
+}
+
 SEC("?fentry.s/bpf_fentry_test1")
 __failure __msg("bpf_percpu_obj_new type ID argument must be of a struct of scalars")
 int BPF_PROG(test_array_map_6)