Discussion around a recently-submitted patch provided historical
context for check_refcount_ok [0]. Specifically, the function and its
helpers - may_be_acquire_function and arg_type_may_be_refcounted -
predate the OBJ_RELEASE type flag and the addition of many more helpers
with acquire/release semantics.
The purpose of check_refcount_ok is to ensure:
1) Helper doesn't have multiple uses of return reg's ref_obj_id
2) Helper with release semantics only has one arg needing to be
released, since that's tracked using meta->ref_obj_id
With current verifier, it's safe to remove check_refcount_ok and its
helpers. Since addition of OBJ_RELEASE type flag, case 2) has been
handled by the arg_type_is_release check in check_func_arg. To ensure
case 1) won't result in verifier silently prioritizing one use of
ref_obj_id, this patch adds a helper_multiple_ref_obj_use check which
fails loudly if a helper passes > 1 test for use of ref_obj_id.
[0]: lore.kernel.org/bpf/20220713234529.4154673-1-davemarchevsky@xxxxxx
Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
---
No extant helpers fail the helper_multiple_ref_obj_use check (as
expected). I validated this by adding BPF_FUNC_dynptr_data to
is_acquire_function check and observing that dynptr selftests failed
with expected error, then doing the same for is_ptr_cast_function.
kernel/bpf/verifier.c | 72 +++++++++++++++++--------------------------
1 file changed, 28 insertions(+), 44 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c59c3df0fea6..b3e057a9384d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -467,25 +467,11 @@ static bool type_is_rdonly_mem(u32 type)
return type & MEM_RDONLY;
}
-static bool arg_type_may_be_refcounted(enum bpf_arg_type type)
-{
- return type == ARG_PTR_TO_SOCK_COMMON;
-}
-
static bool type_may_be_null(u32 type)
{
return type & PTR_MAYBE_NULL;
}
-static bool may_be_acquire_function(enum bpf_func_id func_id)
-{
- return func_id == BPF_FUNC_sk_lookup_tcp ||
- func_id == BPF_FUNC_sk_lookup_udp ||
- func_id == BPF_FUNC_skc_lookup_tcp ||
- func_id == BPF_FUNC_map_lookup_elem ||
- func_id == BPF_FUNC_ringbuf_reserve;
-}
-
static bool is_acquire_function(enum bpf_func_id func_id,
const struct bpf_map *map)
{
@@ -518,6 +504,26 @@ static bool is_ptr_cast_function(enum bpf_func_id func_id)
func_id == BPF_FUNC_skc_to_tcp_request_sock;
}
+static bool is_dynptr_acquire_function(enum bpf_func_id func_id)
+{
+ return func_id == BPF_FUNC_dynptr_data;
+}
+
+static bool helper_multiple_ref_obj_use(enum bpf_func_id func_id,
+ const struct bpf_map *map)
+{
+ int ref_obj_uses = 0;
+
+ if (is_ptr_cast_function(func_id))
+ ref_obj_uses++;
+ if (is_acquire_function(func_id, map))
+ ref_obj_uses++;
+ if (is_dynptr_acquire_function(func_id))
+ ref_obj_uses++;
+
+ return ref_obj_uses > 1;
+}
+
static bool is_cmpxchg_insn(const struct bpf_insn *insn)
{
return BPF_CLASS(insn->code) == BPF_STX &&
@@ -6453,33 +6459,6 @@ static bool check_arg_pair_ok(const struct bpf_func_proto *fn)
return true;
}
-static bool check_refcount_ok(const struct bpf_func_proto *fn, int func_id)
-{
- int count = 0;
-
- if (arg_type_may_be_refcounted(fn->arg1_type))
- count++;
- if (arg_type_may_be_refcounted(fn->arg2_type))
- count++;
- if (arg_type_may_be_refcounted(fn->arg3_type))
- count++;
- if (arg_type_may_be_refcounted(fn->arg4_type))
- count++;
- if (arg_type_may_be_refcounted(fn->arg5_type))
- count++;
-
- /* A reference acquiring function cannot acquire
- * another refcounted ptr.
- */
- if (may_be_acquire_function(func_id) && count)
- return false;
-
- /* We only support one arg being unreferenced at the moment,
- * which is sufficient for the helper functions we have right now.
- */
- return count <= 1;
-}
-
static bool check_btf_id_ok(const struct bpf_func_proto *fn)
{
int i;
@@ -6503,8 +6482,7 @@ static int check_func_proto(const struct bpf_func_proto *fn, int func_id,
{
return check_raw_mode_ok(fn) &&
check_arg_pair_ok(fn) &&
- check_btf_id_ok(fn) &&
- check_refcount_ok(fn, func_id) ? 0 : -EINVAL;
+ check_btf_id_ok(fn) ? 0 : -EINVAL;
}
/* Packet data might have moved, any old PTR_TO_PACKET[_META,_END]
@@ -7457,6 +7435,12 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
if (type_may_be_null(regs[BPF_REG_0].type))
regs[BPF_REG_0].id = ++env->id_gen;
+ if (helper_multiple_ref_obj_use(func_id, meta.map_ptr)) {
+ verbose(env, "verifier internal error: func %s#%d sets ref_obj_id more than once\n",
+ func_id_name(func_id), func_id);
+ return -EINVAL;
+ }
+
if (is_ptr_cast_function(func_id)) {
/* For release_reference() */
regs[BPF_REG_0].ref_obj_id = meta.ref_obj_id;
@@ -7469,7 +7453,7 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
regs[BPF_REG_0].id = id;
/* For release_reference() */
regs[BPF_REG_0].ref_obj_id = id;
- } else if (func_id == BPF_FUNC_dynptr_data) {
+ } else if (is_dynptr_acquire_function(func_id)) {
int dynptr_id = 0, i;
/* Find the id of the dynptr we're acquiring a reference to */
--
2.30.2
