On 7/14/24 10:51, Amery Hung wrote:
Allow a struct_ops program to return a referenced kptr if the struct_ops
operator has pointer to struct as the return type. To make sure the
returned pointer continues to be valid in the kernel, several
constraints are required:
1) The type of the pointer must matches the return type
2) The pointer originally comes from the kernel (not locally allocated)
3) The pointer is in its unmodified form
In addition, since the first user, Qdisc_ops::dequeue, allows a NULL
pointer to be returned when there is no skb to be dequeued, we will allow
a scalar value with value equals to NULL to be returned.
In the future when there is a struct_ops user that always expects a valid
pointer to be returned from an operator, we may extend tagging to the
return value. We can tell the verifier to only allow NULL pointer return
if the return value is tagged with MAY_BE_NULL.
The check is split into two parts since check_reference_leak() happens
before check_return_code(). We first allow a reference object to leak
through return if it is in the return register and the type matches the
return type. Then, we check whether the pointer to-be-returned is valid in
check_return_code().
Signed-off-by: Amery Hung <amery.hung@xxxxxxxxxxxxx>
---
kernel/bpf/verifier.c | 50 +++++++++++++++++++++++++++++++++++++++----
1 file changed, 46 insertions(+), 4 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f614ab283c37..e7f356098902 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10188,16 +10188,36 @@ record_func_key(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
static int check_reference_leak(struct bpf_verifier_env *env, bool exception_exit)
{
+ enum bpf_prog_type type = resolve_prog_type(env->prog);
+ u32 regno = exception_exit ? BPF_REG_1 : BPF_REG_0;
+ struct bpf_reg_state *reg = reg_state(env, regno);
struct bpf_func_state *state = cur_func(env);
+ const struct bpf_prog *prog = env->prog;
+ const struct btf_type *ret_type = NULL;
bool refs_lingering = false;
+ struct btf *btf;
int i;
if (!exception_exit && state->frameno && !state->in_callback_fn)
return 0;
+ if (type == BPF_PROG_TYPE_STRUCT_OPS &&
+ reg->type & PTR_TO_BTF_ID && reg->ref_obj_id) {
+ btf = bpf_prog_get_target_btf(prog);
+ ret_type = btf_type_by_id(btf, prog->aux->attach_func_proto->type);
+ if (reg->btf_id != ret_type->type) {
+ verbose(env, "Return kptr type, struct %s, doesn't match function prototype, struct %s\n",
+ btf_type_name(reg->btf, reg->btf_id),
+ btf_type_name(btf, ret_type->type));
+ return -EINVAL;
+ }
+ }
+
for (i = 0; i < state->acquired_refs; i++) {
if (!exception_exit && state->in_callback_fn && state->refs[i].callback_ref != state->frameno)
continue;
+ if (ret_type && reg->ref_obj_id == state->refs[i].id)
+ continue;
Is it possible having two kptrs that both are in the returned type
passing into a function?
verbose(env, "Unreleased reference id=%d alloc_insn=%d\n",
state->refs[i].id, state->refs[i].insn_idx);
refs_lingering = true;
@@ -15677,12 +15697,15 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
const char *exit_ctx = "At program exit";
struct tnum enforce_attach_type_range = tnum_unknown;
const struct bpf_prog *prog = env->prog;
- struct bpf_reg_state *reg;
+ struct bpf_reg_state *reg = reg_state(env, regno);
struct bpf_retval_range range = retval_range(0, 1);
enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
int err;
struct bpf_func_state *frame = env->cur_state->frame[0];
const bool is_subprog = frame->subprogno;
+ struct btf *btf = bpf_prog_get_target_btf(prog);
+ bool st_ops_ret_is_kptr = false;
+ const struct btf_type *t;
/* LSM and struct_ops func-ptr's return type could be "void" */
if (!is_subprog || frame->in_exception_callback_fn) {
@@ -15691,10 +15714,26 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
if (prog->expected_attach_type == BPF_LSM_CGROUP)
/* See below, can be 0 or 0-1 depending on hook. */
break;
- fallthrough;
+ if (!prog->aux->attach_func_proto->type)
+ return 0;
+ break;
case BPF_PROG_TYPE_STRUCT_OPS:
if (!prog->aux->attach_func_proto->type)
return 0;
+
+ t = btf_type_by_id(btf, prog->aux->attach_func_proto->type);
+ if (btf_type_is_ptr(t)) {
+ /* Allow struct_ops programs to return kptr or null if
+ * the return type is a pointer type.
+ * check_reference_leak has ensured the returning kptr
+ * matches the type of the function prototype and is
+ * the only leaking reference. Thus, we can safely return
+ * if the pointer is in its unmodified form
+ */
+ if (reg->type & PTR_TO_BTF_ID)
+ return __check_ptr_off_reg(env, reg, regno, false);
+ st_ops_ret_is_kptr = true;
+ }
break;
default:
break;
@@ -15716,8 +15755,6 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
return -EACCES;
}
- reg = cur_regs(env) + regno;
-
if (frame->in_async_callback_fn) {
/* enforce return zero from async callbacks like timer */
exit_ctx = "At async callback return";
@@ -15804,6 +15841,11 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
case BPF_PROG_TYPE_NETFILTER:
range = retval_range(NF_DROP, NF_ACCEPT);
break;
+ case BPF_PROG_TYPE_STRUCT_OPS:
+ if (!st_ops_ret_is_kptr)
+ return 0;
+ range = retval_range(0, 0);
+ break;
case BPF_PROG_TYPE_EXT:
/* freplace program can return anything as its return value
* depends on the to-be-replaced kernel func or bpf program.
