On 8/21/23 7:37 PM, Yonghong Song wrote:
On 8/21/23 12:33 PM, Dave Marchevsky wrote:
An earlier patch in the series ensures that the underlying memory of
nodes with bpf_refcount - which can have multiple owners - is not reused
until RCU 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.
Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
---
include/linux/bpf.h | 3 ++-
kernel/bpf/verifier.c | 13 ++++++++++++-
2 files changed, 14 insertions(+), 2 deletions(-)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index eced6400f778..12596af59c00 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 8db0afa5985c..55607ab30522 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8013,6 +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
@@ -10479,6 +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");
@@ -10491,6 +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;
Should the above MEM_RCU marking be done unless reg access is in
rcu critical section?
I think we still have issues for state resetting
with bpf_spin_unlock() and bpf_rcu_read_unlock(), both of which
will try to convert the reg state to PTR_UNTRUSTED.
Let us say reg state is
PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU
(1). If hitting bpf_spin_unlock(), since MEM_RCU is in
the reg state, the state should become
PTR_TO_BTF_ID | MEM_ALLOC | MEM_RCU
some additional code might be needed so we wont have
verifier complaints about ref_obj_id == 0.
(2). If hitting bpf_rcu_read_unlock(), the state should become
PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF
since register access still in bpf_spin_lock() region.
Does this make sense?
Okay, seems scenario (2) is not possible as bpf_rcu_read_unlock()
is not allowed in bpf spin lock region.
+
return 0;
}
@@ -11328,6 +11333,11 @@ 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, "Calling bpf_rcu_read_{lock,unlock} in
unnecessary rbtree callback\n");
+ return -EACCES;
+ }
+
if (rcu_lock) {
verbose(env, "nested rcu read lock (kernel function
%s)\n", func_name);
return -EINVAL;
@@ -16689,7 +16699,8 @@ 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;
}
