On Sat, Mar 05, 2022 at 01:58:30AM IST, Martin KaFai Lau wrote:
> On Fri, Mar 04, 2022 at 05:35:04AM +0530, Kumar Kartikeya Dwivedi wrote:
> > Let's ensure that the PTR_TO_BTF_ID reg being passed in to release BPF
> > helpers and kfuncs always has its offset set to 0. While not a real
> > problem now, there's a very real possibility this will become a problem
> > when more and more kfuncs are exposed, and more BPF helpers are added
> > which can release PTR_TO_BTF_ID.
> >
> > Previous commits already protected against non-zero var_off. One of the
> > case we are concerned about now is when we have a type that can be
> > returned by e.g. an acquire kfunc:
> >
> > struct foo {
> > int a;
> > int b;
> > struct bar b;
> > };
> >
> > ... and struct bar is also a type that can be returned by another
> > acquire kfunc.
> >
> > Then, doing the following sequence:
> >
> > struct foo *f = bpf_get_foo(); // acquire kfunc
> > if (!f)
> > return 0;
> > bpf_put_bar(&f->b); // release kfunc
> >
> > ... would work with the current code, since the btf_struct_ids_match
> > takes reg->off into account for matching pointer type with release kfunc
> > argument type, but would obviously be incorrect, and most likely lead to
> > a kernel crash. A test has been included later to prevent regressions in
> > this area.
> >
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> > ---
> > include/linux/bpf_verifier.h | 3 ++-
> > kernel/bpf/btf.c | 13 +++++++++----
> > kernel/bpf/verifier.c | 27 +++++++++++++++++++++++----
> > 3 files changed, 34 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> > index 38b24ee8d8c2..7a684050495a 100644
> > --- a/include/linux/bpf_verifier.h
> > +++ b/include/linux/bpf_verifier.h
> > @@ -523,7 +523,8 @@ int check_ptr_off_reg(struct bpf_verifier_env *env,
> > const struct bpf_reg_state *reg, int regno);
> > int check_func_arg_reg_off(struct bpf_verifier_env *env,
> > const struct bpf_reg_state *reg, int regno,
> > - enum bpf_arg_type arg_type);
> > + enum bpf_arg_type arg_type,
> > + bool is_release_function);
> > int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
> > u32 regno);
> > int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 7f6a0ae5028b..c9a1019dc60d 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -5753,6 +5753,9 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> > return -EINVAL;
> > }
> >
> > + if (is_kfunc)
> > + rel = btf_kfunc_id_set_contains(btf, resolve_prog_type(env->prog),
> > + BTF_KFUNC_TYPE_RELEASE, func_id);
> > /* check that BTF function arguments match actual types that the
> > * verifier sees.
> > */
> > @@ -5777,7 +5780,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> > ref_t = btf_type_skip_modifiers(btf, t->type, &ref_id);
> > ref_tname = btf_name_by_offset(btf, ref_t->name_off);
> >
> > - ret = check_func_arg_reg_off(env, reg, regno, ARG_DONTCARE);
> > + ret = check_func_arg_reg_off(env, reg, regno, ARG_DONTCARE, rel);
> > if (ret < 0)
> > return ret;
> >
> > @@ -5809,7 +5812,11 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> > if (reg->type == PTR_TO_BTF_ID) {
> > reg_btf = reg->btf;
> > reg_ref_id = reg->btf_id;
> > - /* Ensure only one argument is referenced PTR_TO_BTF_ID */
> > + /* Ensure only one argument is referenced
> > + * PTR_TO_BTF_ID, check_func_arg_reg_off relies
> > + * on only one referenced register being allowed
> > + * for kfuncs.
> > + */
> > if (reg->ref_obj_id) {
> > if (ref_obj_id) {
> > bpf_log(log, "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",
> > @@ -5892,8 +5899,6 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> > /* Either both are set, or neither */
> > WARN_ON_ONCE((ref_obj_id && !ref_regno) || (!ref_obj_id && ref_regno));
> > if (is_kfunc) {
> This test is no longer needed?
>
If you mean the rel && !ref_obj_id below (which is guarded by this check), I do
think it is needed, why do you think so? Because of the check in
check_func_arg_reg_off? That only checks reg->off when it sees that both
release_func and ref_obj_id are true, but ref_obj_id may not be set for any
argument(s) passed to a release function, so we need to reject when we don't get
atleast one referenced register for release function.
Or were you referring to the WARN_ON_ONCE above it?
> > - rel = btf_kfunc_id_set_contains(btf, resolve_prog_type(env->prog),
> > - BTF_KFUNC_TYPE_RELEASE, func_id);
> > /* We already made sure ref_obj_id is set only for one argument */
> > if (rel && !ref_obj_id) {
> > bpf_log(log, "release kernel function %s expects refcounted PTR_TO_BTF_ID\n",
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index e55bfd23e81b..c31407d156e7 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5367,11 +5367,28 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
> >
> > int check_func_arg_reg_off(struct bpf_verifier_env *env,
> > const struct bpf_reg_state *reg, int regno,
> > - enum bpf_arg_type arg_type)
> > + enum bpf_arg_type arg_type,
> > + bool is_release_func)
> > {
> > enum bpf_reg_type type = reg->type;
> > + bool fixed_off_ok = false;
> > int err;
> >
> > + /* When referenced PTR_TO_BTF_ID is passed to release function, it's
> > + * fixed offset must be 0. We rely on the property that only one
> > + * referenced register can be passed to BPF helpers and kfuncs.
> > + */
> > + if (type == PTR_TO_BTF_ID) {
> > + bool release_reg = is_release_func && reg->ref_obj_id;
> > +
> > + if (release_reg && reg->off) {
> iiuc, the reason for not going through __check_ptr_off_reg() is
> because it prefers a different verifier log message for release_reg
> case for fixed off. How about var_off?
>
If reg->off is zero, we still call __check_ptr_off_reg with fixed_off_ok =
false, which should handle non-zero var_off.
> > + verbose(env, "R%d must have zero offset when passed to release func\n",
> > + regno);
> > + return -EINVAL;
> > + }
> > + fixed_off_ok = release_reg ? false : true;
> nit.
> fixed_off_ok = !release_reg;
>
> but this is a bit moot here considering the reg->off
> check has already been done for the release_reg case.
>
Yes, it would be a redundant check inside __check_ptr_off_reg, but we still need
to call it for checking bad var_off.
> > + }
> > +
> > switch ((u32)type) {
> > case SCALAR_VALUE:
> > /* Pointer types where reg offset is explicitly allowed: */
> > @@ -5394,8 +5411,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
> > /* All the rest must be rejected: */
> > default:
> > force_off_check:
> > - err = __check_ptr_off_reg(env, reg, regno,
> > - type == PTR_TO_BTF_ID);
> > + err = __check_ptr_off_reg(env, reg, regno, fixed_off_ok);
> > if (err < 0)
> > return err;
> > break;
> > @@ -5452,11 +5468,14 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> > if (err)
> > return err;
> >
> > - err = check_func_arg_reg_off(env, reg, regno, arg_type);
> > + err = check_func_arg_reg_off(env, reg, regno, arg_type, is_release_function(meta->func_id));
> > if (err)
> > return err;
> >
> > skip_type_check:
> > + /* check_func_arg_reg_off relies on only one referenced register being
> > + * allowed for BPF helpers.
> > + */
> > if (reg->ref_obj_id) {
> > if (meta->ref_obj_id) {
> > verbose(env, "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",
> > --
> > 2.35.1
> >
--
Kartikeya
