[PATCH v2 bpf-next 09/13] bpf: Special verifier handling for bpf_rbtree_{remove, first}

Dave Marchevsky
2022-12-17T08:25:41Z

Newly-added bpf_rbtree_{remove,first} kfuncs have some special properties
that require handling in the verifier:

* both bpf_rbtree_remove and bpf_rbtree_first return the type containing
the bpf_rb_node field, with the offset set to that field's offset,
instead of a struct bpf_rb_node *
* mark_reg_graph_node helper added in previous patch generalizes
this logic, use it

* bpf_rbtree_remove's node input is a node that's been inserted
in the tree - a non-owning reference.

* bpf_rbtree_remove must invalidate non-owning references in order to
avoid aliasing issue. Add KF_INVALIDATE_NON_OWN flag, which
indicates that the marked kfunc is a non-owning ref invalidation
point, and associated verifier logic using previously-added
invalidate_non_owning_refs helper.

* Unlike other functions, which convert one of their input arg regs to
non-owning reference, bpf_rbtree_first takes no arguments and just
returns a non-owning reference (possibly null)
* For now verifier logic for this is special-cased instead of
adding new kfunc flag.

This patch, along with the previous one, complete special verifier
handling for all rbtree API functions added in this series.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
---
include/linux/btf.h | 1 +
kernel/bpf/helpers.c | 2 +-
kernel/bpf/verifier.c | 34 ++++++++++++++++++++++++++++------
3 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/include/linux/btf.h b/include/linux/btf.h
index 8aee3f7f4248..3663911bb7c0 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -72,6 +72,7 @@
#define KF_DESTRUCTIVE (1 << 6) /* kfunc performs destructive actions */
#define KF_RCU (1 << 7) /* kfunc only takes rcu pointer arguments */
#define KF_RELEASE_NON_OWN (1 << 8) /* kfunc converts its referenced arg into non-owning ref */
+#define KF_INVALIDATE_NON_OWN (1 << 9) /* kfunc invalidates non-owning refs after return */

/*
* Return the name of the passed struct, if exists, or halt the build if for
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index de4523c777b7..0e6d010e6423 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2121,7 +2121,7 @@ BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
BTF_ID_FLAGS(func, bpf_task_acquire_not_zero, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
BTF_ID_FLAGS(func, bpf_task_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE)
-BTF_ID_FLAGS(func, bpf_rbtree_remove, KF_ACQUIRE)
+BTF_ID_FLAGS(func, bpf_rbtree_remove, KF_ACQUIRE | KF_INVALIDATE_NON_OWN)
BTF_ID_FLAGS(func, bpf_rbtree_add, KF_RELEASE | KF_RELEASE_NON_OWN)
BTF_ID_FLAGS(func, bpf_rbtree_first, KF_RET_NULL)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 75979f78399d..b4bf3701de7f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8393,6 +8393,11 @@ static bool is_kfunc_release_non_own(struct bpf_kfunc_call_arg_meta *meta)
return meta->kfunc_flags & KF_RELEASE_NON_OWN;
}

+static bool is_kfunc_invalidate_non_own(struct bpf_kfunc_call_arg_meta *meta)
+{
+ return meta->kfunc_flags & KF_INVALIDATE_NON_OWN;
+}
+
static bool is_kfunc_trusted_args(struct bpf_kfunc_call_arg_meta *meta)
{
return meta->kfunc_flags & KF_TRUSTED_ARGS;
@@ -9425,10 +9430,20 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
verbose(env, "arg#%d expected pointer to allocated object\n", i);
return -EINVAL;
}
- if (!reg->ref_obj_id) {
+ if (meta->func_id == special_kfunc_list[KF_bpf_rbtree_remove]) {
+ if (reg->ref_obj_id) {
+ verbose(env, "rbtree_remove node input must be non-owning ref\n");
+ return -EINVAL;
+ }
+ if (in_rbtree_lock_required_cb(env)) {
+ verbose(env, "rbtree_remove not allowed in rbtree cb\n");
+ return -EINVAL;
+ }
+ } else if (!reg->ref_obj_id) {
verbose(env, "allocated object must be referenced\n");
return -EINVAL;
}
+
ret = process_kf_arg_ptr_to_rbtree_node(env, reg, regno, meta);
if (ret < 0)
return ret;
@@ -9665,11 +9680,12 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
meta.func_id == special_kfunc_list[KF_bpf_list_pop_back]) {
struct btf_field *field = meta.arg_list_head.field;

- mark_reg_known_zero(env, regs, BPF_REG_0);
- regs[BPF_REG_0].type = PTR_TO_BTF_ID | MEM_ALLOC;
- regs[BPF_REG_0].btf = field->graph_root.btf;
- regs[BPF_REG_0].btf_id = field->graph_root.value_btf_id;
- regs[BPF_REG_0].off = field->graph_root.node_offset;
+ mark_reg_graph_node(regs, BPF_REG_0, &field->graph_root);
+ } else if (meta.func_id == special_kfunc_list[KF_bpf_rbtree_remove] ||
+ meta.func_id == special_kfunc_list[KF_bpf_rbtree_first]) {
+ struct btf_field *field = meta.arg_rbtree_root.field;
+
+ mark_reg_graph_node(regs, BPF_REG_0, &field->graph_root);
} else if (meta.func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) {
mark_reg_known_zero(env, regs, BPF_REG_0);
regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_TRUSTED;
@@ -9735,7 +9751,13 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
if (is_kfunc_ret_null(&meta))
regs[BPF_REG_0].id = id;
regs[BPF_REG_0].ref_obj_id = id;
+ } else if (meta.func_id == special_kfunc_list[KF_bpf_rbtree_first]) {
+ ref_set_non_owning_lock(env, &regs[BPF_REG_0]);
}
+
+ if (is_kfunc_invalidate_non_own(&meta))
+ invalidate_non_owning_refs(env, &env->cur_state->active_lock);
+
if (reg_may_point_to_spin_lock(&regs[BPF_REG_0]) && !regs[BPF_REG_0].id)
regs[BPF_REG_0].id = ++env->id_gen;
} /* else { add_kfunc_call() ensures it is btf_type_is_void(t) } */
--
2.30.2