[PATCH v1 bpf-next 5/7] bpf: Consider non-owning refs to refcounted nodes RCU protected

Dave Marchevsky
2023-08-01T20:36:52Z

The previous patch in the series ensures that the underlying memory of
nodes with bpf_refcount - which can have multiple owners - is not reused
until RCU Tasks Trace grace period has elapsed. This prevents
use-after-free with non-owning references that may point to
recently-freed memory. While RCU read lock is held, it's safe to
dereference such a non-owning ref, as by definition RCU GP couldn't have
elapsed and therefore underlying memory couldn't have been reused.

From the perspective of verifier "trustedness" non-owning refs to
refcounted nodes are now trusted only in RCU CS and therefore should no
longer pass is_trusted_reg, but rather is_rcu_reg. Let's mark them
MEM_RCU in order to reflect this new state.

Similarly to bpf_spin_unlock being a non-owning ref invalidation point,
where non-owning ref reg states are clobbered so that they cannot be
used outside of the critical section, currently all MEM_RCU regs are
marked untrusted after bpf_rcu_read_unlock. This patch makes
bpf_rcu_read_unlock a non-owning ref invalidation point as well,
clobbering the non-owning refs instead of marking untrusted. In the
future we may want to allow untrusted non-owning refs in which case we
can remove this custom logic without breaking BPF programs as it's more
restrictive than the default. That's a big change in semantics, though,
and this series is focused on fixing the use-after-free in most
straightforward way.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
---
include/linux/bpf.h | 3 ++-
kernel/bpf/verifier.c | 17 +++++++++++++++--
2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index ceaa8c23287f..37fba01b061a 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -653,7 +653,8 @@ enum bpf_type_flag {
MEM_RCU = BIT(13 + BPF_BASE_TYPE_BITS),

/* Used to tag PTR_TO_BTF_ID | MEM_ALLOC references which are non-owning.
- * Currently only valid for linked-list and rbtree nodes.
+ * Currently only valid for linked-list and rbtree nodes. If the nodes
+ * have a bpf_refcount_field, they must be tagged MEM_RCU as well.
*/
NON_OWN_REF = BIT(14 + BPF_BASE_TYPE_BITS),

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9014b469dd9d..4bda365000d3 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -469,7 +469,8 @@ static bool type_is_ptr_alloc_obj(u32 type)

static bool type_is_non_owning_ref(u32 type)
{
- return type_is_ptr_alloc_obj(type) && type_flag(type) & NON_OWN_REF;
+ return type_is_ptr_alloc_obj(type) &&
+ type_flag(type) & NON_OWN_REF;
}

static struct btf_record *reg_btf_record(const struct bpf_reg_state *reg)
@@ -8012,6 +8013,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
case PTR_TO_BTF_ID | PTR_TRUSTED:
case PTR_TO_BTF_ID | MEM_RCU:
case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF:
+ case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU:
/* When referenced PTR_TO_BTF_ID is passed to release function,
* its fixed offset must be 0. In the other cases, fixed offset
* can be non-zero. This was already checked above. So pass
@@ -10478,6 +10480,7 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
{
struct bpf_verifier_state *state = env->cur_state;
+ struct btf_record *rec = reg_btf_record(reg);

if (!state->active_lock.ptr) {
verbose(env, "verifier internal error: ref_set_non_owning w/o active lock\n");
@@ -10490,6 +10493,9 @@ static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state
}

reg->type |= NON_OWN_REF;
+ if (rec->refcount_off >= 0)
+ reg->type |= MEM_RCU;
+
return 0;
}

@@ -11327,10 +11333,16 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
struct bpf_func_state *state;
struct bpf_reg_state *reg;

+ if (in_rbtree_lock_required_cb(env) && (rcu_lock || rcu_unlock)) {
+ verbose(env, "can't rcu read {lock,unlock} in rbtree cb\n");
+ return -EACCES;
+ }
+
if (rcu_lock) {
verbose(env, "nested rcu read lock (kernel function %s)\n", func_name);
return -EINVAL;
} else if (rcu_unlock) {
+ invalidate_non_owning_refs(env);
bpf_for_each_reg_in_vstate(env->cur_state, state, reg, ({
if (reg->type & MEM_RCU) {
reg->type &= ~(MEM_RCU | PTR_MAYBE_NULL);
@@ -16679,7 +16691,8 @@ static int do_check(struct bpf_verifier_env *env)
return -EINVAL;
}

- if (env->cur_state->active_rcu_lock) {
+ if (env->cur_state->active_rcu_lock &&
+ !in_rbtree_lock_required_cb(env)) {
verbose(env, "bpf_rcu_read_unlock is missing\n");
return -EINVAL;
}
--
2.34.1