[PATCH bpf-next 5/8] bpf: Refactor NULL-ness check in check_reg_type().

Alexei Starovoitov
2023-04-04T04:51:09Z

From: Alexei Starovoitov <ast@kernel.org>

check_reg_type() unconditionally disallows PTR_TO_BTF_ID | PTR_MAYBE_NULL.
It's problematic for helpers that allow ARG_PTR_TO_BTF_ID_OR_NULL like
bpf_sk_storage_get(). Allow passing PTR_TO_BTF_ID | PTR_MAYBE_NULL into such
helpers. That technically includes bpf_kptr_xchg() helper, but in practice:
bpf_kptr_xchg(..., bpf_cpumask_create());
is still disallowed because bpf_cpumask_create() returns ref counted pointer
with ref_obj_id > 0.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
kernel/bpf/verifier.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2cd2e0b725cd..4e7d671497f4 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7168,6 +7168,8 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
case PTR_TO_BTF_ID:
case PTR_TO_BTF_ID | PTR_TRUSTED:
case PTR_TO_BTF_ID | MEM_RCU:
+ case PTR_TO_BTF_ID | PTR_MAYBE_NULL:
+ case PTR_TO_BTF_ID | PTR_MAYBE_NULL | MEM_RCU:
{
/* For bpf_sk_release, it needs to match against first member
* 'struct sock_common', hence make an exception for it. This
@@ -7176,6 +7178,12 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
bool strict_type_match = arg_type_is_release(arg_type) &&
meta->func_id != BPF_FUNC_sk_release;

+ if (type_may_be_null(reg->type) &&
+ (!type_may_be_null(arg_type) || arg_type_is_release(arg_type))) {
+ verbose(env, "Possibly NULL pointer passed to helper arg%d\n", regno);
+ return -EACCES;
+ }
+
if (!arg_btf_id) {
if (!compatible->btf_id) {
verbose(env, "verifier internal error: missing arg compatible BTF ID\n");
@@ -7206,10 +7214,6 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
}
break;
}
- case PTR_TO_BTF_ID | PTR_MAYBE_NULL:
- case PTR_TO_BTF_ID | PTR_MAYBE_NULL | MEM_RCU:
- verbose(env, "Possibly NULL pointer passed to helper arg%d\n", regno);
- return -EACCES;
case PTR_TO_BTF_ID | MEM_ALLOC:
if (meta->func_id != BPF_FUNC_spin_lock && meta->func_id != BPF_FUNC_spin_unlock &&
meta->func_id != BPF_FUNC_kptr_xchg) {
--
2.34.1