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 | 33 +++++++++++++++++++--------------
kernel/bpf/verifier.c | 25 ++++++++++++++++++++++---
3 files changed, 43 insertions(+), 18 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 38b24ee8d8c2..c1fc4af47f69 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_func);
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..162807e3b4a5 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5753,6 +5753,10 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
return -EINVAL;
}
+ /* Only kfunc can be release func */
+ 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 +5781,7 @@ 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 +5813,11 @@ 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",
@@ -5891,18 +5899,15 @@ 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) {
- 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",
- func_name);
- return -EINVAL;
- }
- /* Allow (!rel && ref_obj_id), so that passing such referenced PTR_TO_BTF_ID to
- * other kfuncs works
- */
+ /* We already made sure ref_obj_id is set only for one argument. We do
+ * allow (!rel && ref_obj_id), so that passing such referenced
+ * PTR_TO_BTF_ID to other kfuncs works. Note that rel is only true when
+ * is_kfunc is true.
+ */
+ if (rel && !ref_obj_id) {
+ bpf_log(log, "release kernel function %s expects refcounted PTR_TO_BTF_ID\n",
+ func_name);
+ return -EINVAL;
}
/* returns argument register number > 0 in case of reference release kfunc */
return rel ? ref_regno : 0;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 455b4ab69e47..fe9a513e2314 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5367,10 +5367,11 @@ 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)
{
+ bool fixed_off_ok = false, release_reg;
enum bpf_reg_type type = reg->type;
- bool fixed_off_ok = false;
switch ((u32)type) {
case SCALAR_VALUE:
@@ -5395,6 +5396,21 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
* fixed offset.
*/
case PTR_TO_BTF_ID:
+ /* 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. In the other cases, fixed offset can be non-zero.
+ */
+ release_reg = is_release_func && reg->ref_obj_id;
+ if (release_reg && reg->off) {
+ verbose(env, "R%d must have zero offset when passed to release func\n",
+ regno);
+ return -EINVAL;
+ }
+ /* For release_reg == true, fixed_off_ok must be false, but we
+ * already checked and rejected reg->off != 0 above, so set to
+ * true to allow fixed offset for all other cases.
+ */
fixed_off_ok = true;
break;
default:
@@ -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
