[PATCH bpf-next v2 10/25] bpf: Introduce local kptrs

Kumar Kartikeya Dwivedi
2022-10-13T06:23:57Z

Introduce the idea of local kptrs, i.e. PTR_TO_BTF_ID that point to a
type in program BTF. This is indicated by the presence of MEM_TYPE_LOCAL
type tag in reg->type to avoid having to check btf_is_kernel when trying
to match argument types in helpers.

For now, these local kptrs will always be referenced in verifier
context, hence ref_obj_id == 0 for them is a bug. It is allowed to write
to such objects, as long fields that are special are not touched
(support for which will be added in subsequent patches).

No PROBE_MEM handling is hence done since they can never be in an
undefined state, and their lifetime will always be valid.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
include/linux/bpf.h | 14 +++++++++++---
include/linux/filter.h | 4 +++-
kernel/bpf/btf.c | 9 ++++++++-
kernel/bpf/verifier.c | 15 ++++++++++-----
net/bpf/bpf_dummy_struct_ops.c | 3 ++-
net/core/filter.c | 13 ++++++++-----
net/ipv4/bpf_tcp_ca.c | 3 ++-
net/netfilter/nf_conntrack_bpf.c | 1 +
8 files changed, 45 insertions(+), 17 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 46330d871d4e..a2f4d3356cc8 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -526,6 +526,11 @@ enum bpf_type_flag {
/* Size is known at compile time. */
MEM_FIXED_SIZE = BIT(10 + BPF_BASE_TYPE_BITS),

+ /* MEM is of a type from program BTF, not kernel BTF. This is used to
+ * tag PTR_TO_BTF_ID allocated using bpf_kptr_alloc.
+ */
+ MEM_TYPE_LOCAL = BIT(11 + BPF_BASE_TYPE_BITS),
+
__BPF_TYPE_FLAG_MAX,
__BPF_TYPE_LAST_FLAG = __BPF_TYPE_FLAG_MAX - 1,
};
@@ -774,6 +779,7 @@ struct bpf_prog_ops {
union bpf_attr __user *uattr);
};

+struct bpf_reg_state;
struct bpf_verifier_ops {
/* return eBPF function prototype for verification */
const struct bpf_func_proto *
@@ -795,6 +801,7 @@ struct bpf_verifier_ops {
struct bpf_insn *dst,
struct bpf_prog *prog, u32 *target_size);
int (*btf_struct_access)(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg,
const struct btf *btf,
const struct btf_type *t, int off, int size,
enum bpf_access_type atype,
@@ -2076,10 +2083,11 @@ static inline bool bpf_tracing_btf_ctx_access(int off, int size,
return btf_ctx_access(off, size, type, prog, info);
}

-int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
+int btf_struct_access(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg, const struct btf *btf,
const struct btf_type *t, int off, int size,
- enum bpf_access_type atype,
- u32 *next_btf_id, enum bpf_type_flag *flag);
+ enum bpf_access_type atype, u32 *next_btf_id,
+ enum bpf_type_flag *flag);
bool btf_struct_ids_match(struct bpf_verifier_log *log,
const struct btf *btf, u32 id, int off,
const struct btf *need_btf, u32 need_type_id,
diff --git a/include/linux/filter.h b/include/linux/filter.h
index efc42a6e3aed..9b94e24f90b9 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -568,7 +568,9 @@ struct sk_filter {
DECLARE_STATIC_KEY_FALSE(bpf_stats_enabled_key);

extern struct mutex nf_conn_btf_access_lock;
-extern int (*nfct_btf_struct_access)(struct bpf_verifier_log *log, const struct btf *btf,
+extern int (*nfct_btf_struct_access)(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg,
+ const struct btf *btf,
const struct btf_type *t, int off, int size,
enum bpf_access_type atype, u32 *next_btf_id,
enum bpf_type_flag *flag);
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 066984d73a8b..65f444405d9c 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6019,11 +6019,13 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
return -EINVAL;
}

-int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,
+int btf_struct_access(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg, const struct btf *btf,
const struct btf_type *t, int off, int size,
enum bpf_access_type atype __maybe_unused,
u32 *next_btf_id, enum bpf_type_flag *flag)
{
+ bool local_type = reg && (type_flag(reg->type) & MEM_TYPE_LOCAL);
enum bpf_type_flag tmp_flag = 0;
int err;
u32 id;
@@ -6033,6 +6035,11 @@ int btf_struct_access(struct bpf_verifier_log *log, const struct btf *btf,

switch (err) {
case WALK_PTR:
+ /* For local types, the destination register cannot
+ * become a pointer again.
+ */
+ if (local_type)
+ return SCALAR_VALUE;
/* If we found the pointer or scalar on t+off,
* we're done.
*/
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3c47cecda302..6ee8c06c2080 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4522,16 +4522,20 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
return -EACCES;
}

- if (env->ops->btf_struct_access) {
- ret = env->ops->btf_struct_access(&env->log, reg->btf, t,
+ if (env->ops->btf_struct_access && !(type_flag(reg->type) & MEM_TYPE_LOCAL)) {
+ WARN_ON_ONCE(!btf_is_kernel(reg->btf));
+ ret = env->ops->btf_struct_access(&env->log, reg, reg->btf, t,
off, size, atype, &btf_id, &flag);
} else {
- if (atype != BPF_READ) {
+ if (atype != BPF_READ && !(type_flag(reg->type) & MEM_TYPE_LOCAL)) {
verbose(env, "only read is supported\n");
return -EACCES;
}

- ret = btf_struct_access(&env->log, reg->btf, t, off, size,
+ if (reg->type & MEM_TYPE_LOCAL)
+ WARN_ON_ONCE(!reg->ref_obj_id);
+
+ ret = btf_struct_access(&env->log, reg, reg->btf, t, off, size,
atype, &btf_id, &flag);
}

@@ -4596,7 +4600,7 @@ static int check_ptr_to_map_access(struct bpf_verifier_env *env,
return -EACCES;
}

- ret = btf_struct_access(&env->log, btf_vmlinux, t, off, size, atype, &btf_id, &flag);
+ ret = btf_struct_access(&env->log, NULL, btf_vmlinux, t, off, size, atype, &btf_id, &flag);
if (ret < 0)
return ret;

@@ -5816,6 +5820,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
* fixed offset.
*/
case PTR_TO_BTF_ID:
+ case PTR_TO_BTF_ID | MEM_TYPE_LOCAL:
/* When referenced PTR_TO_BTF_ID is passed to release function,
* it's fixed offset must be 0. In the other cases, fixed offset
* can be non-zero.
diff --git a/net/bpf/bpf_dummy_struct_ops.c b/net/bpf/bpf_dummy_struct_ops.c
index e78dadfc5829..d7aa636d90ce 100644
--- a/net/bpf/bpf_dummy_struct_ops.c
+++ b/net/bpf/bpf_dummy_struct_ops.c
@@ -156,6 +156,7 @@ static bool bpf_dummy_ops_is_valid_access(int off, int size,
}

static int bpf_dummy_ops_btf_struct_access(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg,
const struct btf *btf,
const struct btf_type *t, int off,
int size, enum bpf_access_type atype,
@@ -177,7 +178,7 @@ static int bpf_dummy_ops_btf_struct_access(struct bpf_verifier_log *log,
return -EACCES;
}

- err = btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+ err = btf_struct_access(log, reg, btf, t, off, size, atype, next_btf_id,
flag);
if (err < 0)
return err;
diff --git a/net/core/filter.c b/net/core/filter.c
index bb0136e7a8e4..cc7af7be91d9 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8647,13 +8647,15 @@ static bool tc_cls_act_is_valid_access(int off, int size,
DEFINE_MUTEX(nf_conn_btf_access_lock);
EXPORT_SYMBOL_GPL(nf_conn_btf_access_lock);

-int (*nfct_btf_struct_access)(struct bpf_verifier_log *log, const struct btf *btf,
+int (*nfct_btf_struct_access)(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg, const struct btf *btf,
const struct btf_type *t, int off, int size,
enum bpf_access_type atype, u32 *next_btf_id,
enum bpf_type_flag *flag);
EXPORT_SYMBOL_GPL(nfct_btf_struct_access);

static int tc_cls_act_btf_struct_access(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg,
const struct btf *btf,
const struct btf_type *t, int off,
int size, enum bpf_access_type atype,
@@ -8663,12 +8665,12 @@ static int tc_cls_act_btf_struct_access(struct bpf_verifier_log *log,
int ret = -EACCES;

if (atype == BPF_READ)
- return btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+ return btf_struct_access(log, reg, btf, t, off, size, atype, next_btf_id,
flag);

mutex_lock(&nf_conn_btf_access_lock);
if (nfct_btf_struct_access)
- ret = nfct_btf_struct_access(log, btf, t, off, size, atype, next_btf_id, flag);
+ ret = nfct_btf_struct_access(log, reg, btf, t, off, size, atype, next_btf_id, flag);
mutex_unlock(&nf_conn_btf_access_lock);

return ret;
@@ -8734,6 +8736,7 @@ void bpf_warn_invalid_xdp_action(struct net_device *dev, struct bpf_prog *prog,
EXPORT_SYMBOL_GPL(bpf_warn_invalid_xdp_action);

static int xdp_btf_struct_access(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg,
const struct btf *btf,
const struct btf_type *t, int off,
int size, enum bpf_access_type atype,
@@ -8743,12 +8746,12 @@ static int xdp_btf_struct_access(struct bpf_verifier_log *log,
int ret = -EACCES;

if (atype == BPF_READ)
- return btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+ return btf_struct_access(log, reg, btf, t, off, size, atype, next_btf_id,
flag);

mutex_lock(&nf_conn_btf_access_lock);
if (nfct_btf_struct_access)
- ret = nfct_btf_struct_access(log, btf, t, off, size, atype, next_btf_id, flag);
+ ret = nfct_btf_struct_access(log, reg, btf, t, off, size, atype, next_btf_id, flag);
mutex_unlock(&nf_conn_btf_access_lock);

return ret;
diff --git a/net/ipv4/bpf_tcp_ca.c b/net/ipv4/bpf_tcp_ca.c
index 6da16ae6a962..1fe3935c4260 100644
--- a/net/ipv4/bpf_tcp_ca.c
+++ b/net/ipv4/bpf_tcp_ca.c
@@ -69,6 +69,7 @@ static bool bpf_tcp_ca_is_valid_access(int off, int size,
}

static int bpf_tcp_ca_btf_struct_access(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg,
const struct btf *btf,
const struct btf_type *t, int off,
int size, enum bpf_access_type atype,
@@ -78,7 +79,7 @@ static int bpf_tcp_ca_btf_struct_access(struct bpf_verifier_log *log,
size_t end;

if (atype == BPF_READ)
- return btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+ return btf_struct_access(log, reg, btf, t, off, size, atype, next_btf_id,
flag);

if (t != tcp_sock_type) {
diff --git a/net/netfilter/nf_conntrack_bpf.c b/net/netfilter/nf_conntrack_bpf.c
index 8639e7efd0e2..f6036a84484b 100644
--- a/net/netfilter/nf_conntrack_bpf.c
+++ b/net/netfilter/nf_conntrack_bpf.c
@@ -191,6 +191,7 @@ BTF_ID(struct, nf_conn___init)

/* Check writes into `struct nf_conn` */
static int _nf_conntrack_btf_struct_access(struct bpf_verifier_log *log,
+ const struct bpf_reg_state *reg,
const struct btf *btf,
const struct btf_type *t, int off,
int size, enum bpf_access_type atype,
--
2.38.0