[PATCH v3 bpf-next 3/5] bpf: Introduce kptr_rcu.

Alexei Starovoitov
2023-02-28T04:02:34Z

From: Alexei Starovoitov <ast@kernel.org>

The life time of certain kernel structures like 'struct cgroup' is protected by RCU.
Hence it's safe to dereference them directly from __kptr tagged pointers in bpf maps.
The resulting pointer is MEM_RCU and can be passed to kfuncs that expect KF_RCU.
Derefrence of other kptr-s returns PTR_UNTRUSTED.

For example:
struct map_value {
struct cgroup __kptr *cgrp;
};

SEC("tp_btf/cgroup_mkdir")
int BPF_PROG(test_cgrp_get_ancestors, struct cgroup *cgrp_arg, const char *path)
{
struct cgroup *cg, *cg2;

cg = bpf_cgroup_acquire(cgrp_arg); // cg is PTR_TRUSTED and ref_obj_id > 0
bpf_kptr_xchg(&v->cgrp, cg);

cg2 = v->cgrp; // This is new feature introduced by this patch.
// cg2 is PTR_MAYBE_NULL | MEM_RCU.
// When cg2 != NULL, it's a valid cgroup, but its percpu_ref could be zero

bpf_cgroup_ancestor(cg2, level); // safe to do.
}

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
Documentation/bpf/kfuncs.rst | 11 ++++---
include/linux/bpf.h | 15 ++++++---
include/linux/btf.h | 2 +-
kernel/bpf/btf.c | 16 +++++++++
kernel/bpf/helpers.c | 7 ++--
kernel/bpf/syscall.c | 4 +++
kernel/bpf/verifier.c | 33 ++++++++++++-------
net/bpf/test_run.c | 3 +-
.../selftests/bpf/progs/map_kptr_fail.c | 4 +--
tools/testing/selftests/bpf/verifier/calls.c | 2 +-
.../testing/selftests/bpf/verifier/map_kptr.c | 2 +-
11 files changed, 69 insertions(+), 30 deletions(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 7d7c1144372a..49c5cb6f46e7 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -232,11 +232,12 @@ added later.
2.4.8 KF_RCU flag
-----------------

-The KF_RCU flag is used for kfuncs which have a rcu ptr as its argument.
-When used together with KF_ACQUIRE, it indicates the kfunc should have a
-single argument which must be a trusted argument or a MEM_RCU pointer.
-The argument may have reference count of 0 and the kfunc must take this
-into consideration.
+The KF_RCU flag is a weaker version of KF_TRUSTED_ARGS. The kfuncs marked with
+KF_RCU expect either PTR_TRUSTED or MEM_RCU arguments. The verifier guarantees
+that the objects are valid and there is no use-after-free, but the pointers
+maybe NULL and pointee object's reference count could have reached zero, hence
+kfuncs must do != NULL check and consider refcnt==0 case when accessing such
+arguments.

.. _KF_deprecated_flag:

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 520b238abd5a..d4b5faa0a777 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -178,11 +178,12 @@ enum btf_field_type {
BPF_TIMER = (1 << 1),
BPF_KPTR_UNREF = (1 << 2),
BPF_KPTR_REF = (1 << 3),
- BPF_KPTR = BPF_KPTR_UNREF | BPF_KPTR_REF,
- BPF_LIST_HEAD = (1 << 4),
- BPF_LIST_NODE = (1 << 5),
- BPF_RB_ROOT = (1 << 6),
- BPF_RB_NODE = (1 << 7),
+ BPF_KPTR_RCU = (1 << 4), /* kernel internal. not exposed to bpf prog */
+ BPF_KPTR = BPF_KPTR_UNREF | BPF_KPTR_REF | BPF_KPTR_RCU,
+ BPF_LIST_HEAD = (1 << 5),
+ BPF_LIST_NODE = (1 << 6),
+ BPF_RB_ROOT = (1 << 7),
+ BPF_RB_NODE = (1 << 8),
BPF_GRAPH_NODE_OR_ROOT = BPF_LIST_NODE | BPF_LIST_HEAD |
BPF_RB_NODE | BPF_RB_ROOT,
};
@@ -284,6 +285,8 @@ static inline const char *btf_field_type_name(enum btf_field_type type)
case BPF_KPTR_UNREF:
case BPF_KPTR_REF:
return "kptr";
+ case BPF_KPTR_RCU:
+ return "kptr_rcu";
case BPF_LIST_HEAD:
return "bpf_list_head";
case BPF_LIST_NODE:
@@ -307,6 +310,7 @@ static inline u32 btf_field_type_size(enum btf_field_type type)
return sizeof(struct bpf_timer);
case BPF_KPTR_UNREF:
case BPF_KPTR_REF:
+ case BPF_KPTR_RCU:
return sizeof(u64);
case BPF_LIST_HEAD:
return sizeof(struct bpf_list_head);
@@ -331,6 +335,7 @@ static inline u32 btf_field_type_align(enum btf_field_type type)
return __alignof__(struct bpf_timer);
case BPF_KPTR_UNREF:
case BPF_KPTR_REF:
+ case BPF_KPTR_RCU:
return __alignof__(u64);
case BPF_LIST_HEAD:
return __alignof__(struct bpf_list_head);
diff --git a/include/linux/btf.h b/include/linux/btf.h
index 49e0fe6d8274..556b3e2e7471 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -70,7 +70,7 @@
#define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer arguments */
#define KF_SLEEPABLE (1 << 5) /* kfunc may sleep */
#define KF_DESTRUCTIVE (1 << 6) /* kfunc performs destructive actions */
-#define KF_RCU (1 << 7) /* kfunc only takes rcu pointer arguments */
+#define KF_RCU (1 << 7) /* kfunc takes either rcu or trusted pointer arguments */

/*
* Tag marking a kernel function as a kfunc. This is meant to minimize the
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 01dee7d48e6d..a44ea1f6164b 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3552,6 +3552,18 @@ static int btf_find_field(const struct btf *btf, const struct btf_type *t,
return -EINVAL;
}

+BTF_SET_START(rcu_protected_types)
+BTF_ID(struct, prog_test_ref_kfunc)
+BTF_ID(struct, cgroup)
+BTF_SET_END(rcu_protected_types)
+
+static bool rcu_protected_object(const struct btf *btf, u32 btf_id)
+{
+ if (!btf_is_kernel(btf))
+ return false;
+ return btf_id_set_contains(&rcu_protected_types, btf_id);
+}
+
static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
struct btf_field_info *info)
{
@@ -3615,6 +3627,10 @@ static int btf_parse_kptr(const struct btf *btf, struct btf_field *field,
field->kptr.dtor = (void *)addr;
}

+ if (info->type == BPF_KPTR_REF && rcu_protected_object(kernel_btf, id))
+ /* rcu dereference of this field will return MEM_RCU instead of PTR_UNTRUSTED */
+ field->type = BPF_KPTR_RCU;
+
field->kptr.btf_id = id;
field->kptr.btf = kernel_btf;
field->kptr.module = mod;
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a784be6f8bac..fed74afd45d1 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2094,11 +2094,12 @@ __bpf_kfunc struct cgroup *bpf_cgroup_ancestor(struct cgroup *cgrp, int level)
{
struct cgroup *ancestor;

- if (level > cgrp->level || level < 0)
+ if (!cgrp || level > cgrp->level || level < 0)
return NULL;

ancestor = cgrp->ancestors[level];
- cgroup_get(ancestor);
+ if (!cgroup_tryget(ancestor))
+ return NULL;
return ancestor;
}

@@ -2183,7 +2184,7 @@ BTF_ID_FLAGS(func, bpf_rbtree_first, KF_RET_NULL)
BTF_ID_FLAGS(func, bpf_cgroup_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
BTF_ID_FLAGS(func, bpf_cgroup_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
BTF_ID_FLAGS(func, bpf_cgroup_release, KF_RELEASE)
-BTF_ID_FLAGS(func, bpf_cgroup_ancestor, KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_cgroup_ancestor, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
BTF_ID_FLAGS(func, bpf_cgroup_from_id, KF_ACQUIRE | KF_RET_NULL)
#endif
BTF_ID_FLAGS(func, bpf_task_from_pid, KF_ACQUIRE | KF_RET_NULL)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index e3fcdc9836a6..2e730918911c 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -539,6 +539,7 @@ void btf_record_free(struct btf_record *rec)
switch (rec->fields[i].type) {
case BPF_KPTR_UNREF:
case BPF_KPTR_REF:
+ case BPF_KPTR_RCU:
if (rec->fields[i].kptr.module)
module_put(rec->fields[i].kptr.module);
btf_put(rec->fields[i].kptr.btf);
@@ -584,6 +585,7 @@ struct btf_record *btf_record_dup(const struct btf_record *rec)
switch (fields[i].type) {
case BPF_KPTR_UNREF:
case BPF_KPTR_REF:
+ case BPF_KPTR_RCU:
btf_get(fields[i].kptr.btf);
if (fields[i].kptr.module && !try_module_get(fields[i].kptr.module)) {
ret = -ENXIO;
@@ -669,6 +671,7 @@ void bpf_obj_free_fields(const struct btf_record *rec, void *obj)
WRITE_ONCE(*(u64 *)field_ptr, 0);
break;
case BPF_KPTR_REF:
+ case BPF_KPTR_RCU:
field->kptr.dtor((void *)xchg((unsigned long *)field_ptr, 0));
break;
case BPF_LIST_HEAD:
@@ -1058,6 +1061,7 @@ static int map_check_btf(struct bpf_map *map, const struct btf *btf,
break;
case BPF_KPTR_UNREF:
case BPF_KPTR_REF:
+ case BPF_KPTR_RCU:
if (map->map_type != BPF_MAP_TYPE_HASH &&
map->map_type != BPF_MAP_TYPE_LRU_HASH &&
map->map_type != BPF_MAP_TYPE_ARRAY &&
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e4234266e76d..0b728ce0dde9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4183,7 +4183,7 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
struct bpf_reg_state *reg, u32 regno)
{
const char *targ_name = kernel_type_name(kptr_field->kptr.btf, kptr_field->kptr.btf_id);
- int perm_flags = PTR_MAYBE_NULL | PTR_TRUSTED;
+ int perm_flags = PTR_MAYBE_NULL | PTR_TRUSTED | MEM_RCU;
const char *reg_name = "";

/* Only unreferenced case accepts untrusted pointers */
@@ -4230,12 +4230,12 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
* In the kptr_ref case, check_func_arg_reg_off already ensures reg->off
* is zero. We must also ensure that btf_struct_ids_match does not walk
* the struct to match type against first member of struct, i.e. reject
- * second case from above. Hence, when type is BPF_KPTR_REF, we set
+ * second case from above. Hence, when type is BPF_KPTR_REF | BPF_KPTR_RCU, we set
* strict mode to true for type match.
*/
if (!btf_struct_ids_match(&env->log, reg->btf, reg->btf_id, reg->off,
kptr_field->kptr.btf, kptr_field->kptr.btf_id,
- kptr_field->type == BPF_KPTR_REF))
+ kptr_field->type == BPF_KPTR_REF || kptr_field->type == BPF_KPTR_RCU))
goto bad_type;
return 0;
bad_type:
@@ -4250,6 +4250,14 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
return -EINVAL;
}

+/* The non-sleepable programs and sleepable programs with explicit bpf_rcu_read_lock()
+ * can dereference RCU protected pointers and result is PTR_TRUSTED.
+ */
+static bool in_rcu_cs(struct bpf_verifier_env *env)
+{
+ return env->cur_state->active_rcu_lock || !env->prog->aux->sleepable;
+}
+
static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,
int value_regno, int insn_idx,
struct btf_field *kptr_field)
@@ -4273,7 +4281,7 @@ static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,
/* We only allow loading referenced kptr, since it will be marked as
* untrusted, similar to unreferenced kptr.
*/
- if (class != BPF_LDX && kptr_field->type == BPF_KPTR_REF) {
+ if (class != BPF_LDX && kptr_field->type != BPF_KPTR_UNREF) {
verbose(env, "store to referenced kptr disallowed\n");
return -EACCES;
}
@@ -4284,7 +4292,10 @@ static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,
* value from map as PTR_TO_BTF_ID, with the correct type.
*/
mark_btf_ld_reg(env, cur_regs(env), value_regno, PTR_TO_BTF_ID, kptr_field->kptr.btf,
- kptr_field->kptr.btf_id, PTR_MAYBE_NULL | PTR_UNTRUSTED);
+ kptr_field->kptr.btf_id,
+ kptr_field->type == BPF_KPTR_RCU && in_rcu_cs(env) ?
+ PTR_MAYBE_NULL | MEM_RCU :
+ PTR_MAYBE_NULL | PTR_UNTRUSTED);
/* For mark_ptr_or_null_reg */
val_reg->id = ++env->id_gen;
} else if (class == BPF_STX) {
@@ -4338,6 +4349,7 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno,
switch (field->type) {
case BPF_KPTR_UNREF:
case BPF_KPTR_REF:
+ case BPF_KPTR_RCU:
if (src != ACCESS_DIRECT) {
verbose(env, "kptr cannot be accessed indirectly by helper\n");
return -EACCES;
@@ -5139,11 +5151,10 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
* read lock region. Also mark rcu pointer as PTR_MAYBE_NULL since
* it could be null in some cases.
*/
- if (!env->cur_state->active_rcu_lock ||
- !(is_trusted_reg(reg) || is_rcu_reg(reg)))
- flag &= ~MEM_RCU;
- else
+ if (in_rcu_cs(env) && (is_trusted_reg(reg) || is_rcu_reg(reg)))
flag |= PTR_MAYBE_NULL;
+ else
+ flag &= ~MEM_RCU;
} else if (reg->type & MEM_RCU) {
/* ptr (reg) is marked as MEM_RCU, but the struct field is not tagged
* with __rcu. Mark the flag as PTR_UNTRUSTED conservatively.
@@ -6187,7 +6198,7 @@ static int process_kptr_func(struct bpf_verifier_env *env, int regno,
verbose(env, "off=%d doesn't point to kptr\n", kptr_off);
return -EACCES;
}
- if (kptr_field->type != BPF_KPTR_REF) {
+ if (kptr_field->type != BPF_KPTR_REF && kptr_field->type != BPF_KPTR_RCU) {
verbose(env, "off=%d kptr isn't referenced kptr\n", kptr_off);
return -EACCES;
}
@@ -9111,7 +9122,7 @@ static int process_kf_arg_ptr_to_kptr(struct bpf_verifier_env *env,
}

kptr_field = btf_record_find(reg->map_ptr->record, reg->off + reg->var_off.value, BPF_KPTR);
- if (!kptr_field || kptr_field->type != BPF_KPTR_REF) {
+ if (!kptr_field || (kptr_field->type != BPF_KPTR_REF && kptr_field->type != BPF_KPTR_RCU)) {
verbose(env, "arg#0 no referenced kptr at map value offset=%llu\n",
reg->off + reg->var_off.value);
return -EINVAL;
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 6f3d654b3339..73e5029ab5c9 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -737,6 +737,7 @@ __bpf_kfunc void bpf_kfunc_call_test_mem_len_fail2(u64 *mem, int len)

__bpf_kfunc void bpf_kfunc_call_test_ref(struct prog_test_ref_kfunc *p)
{
+ /* p could be NULL and p->cnt could be 0 */
}

__bpf_kfunc void bpf_kfunc_call_test_destructive(void)
@@ -784,7 +785,7 @@ BTF_ID_FLAGS(func, bpf_kfunc_call_test_fail3)
BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_pass1)
BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_fail1)
BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_fail2)
-BTF_ID_FLAGS(func, bpf_kfunc_call_test_ref, KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test_ref, KF_TRUSTED_ARGS | KF_RCU)
BTF_ID_FLAGS(func, bpf_kfunc_call_test_destructive, KF_DESTRUCTIVE)
BTF_ID_FLAGS(func, bpf_kfunc_call_test_static_unused_arg)
BTF_SET8_END(test_sk_check_kfunc_ids)
diff --git a/tools/testing/selftests/bpf/progs/map_kptr_fail.c b/tools/testing/selftests/bpf/progs/map_kptr_fail.c
index e19e2a5f38cf..08f9ec18c345 100644
--- a/tools/testing/selftests/bpf/progs/map_kptr_fail.c
+++ b/tools/testing/selftests/bpf/progs/map_kptr_fail.c
@@ -281,7 +281,7 @@ int reject_kptr_get_bad_type_match(struct __sk_buff *ctx)
}

SEC("?tc")
-__failure __msg("R1 type=untrusted_ptr_or_null_ expected=percpu_ptr_")
+__failure __msg("R1 type=rcu_ptr_or_null_ expected=percpu_ptr_")
int mark_ref_as_untrusted_or_null(struct __sk_buff *ctx)
{
struct map_value *v;
@@ -316,7 +316,7 @@ int reject_untrusted_store_to_ref(struct __sk_buff *ctx)
}

SEC("?tc")
-__failure __msg("R2 type=untrusted_ptr_ expected=ptr_")
+__failure __msg("R2 must be referenced")
int reject_untrusted_xchg(struct __sk_buff *ctx)
{
struct prog_test_ref_kfunc *p;
diff --git a/tools/testing/selftests/bpf/verifier/calls.c b/tools/testing/selftests/bpf/verifier/calls.c
index 289ed202ec66..9a326a800e5c 100644
--- a/tools/testing/selftests/bpf/verifier/calls.c
+++ b/tools/testing/selftests/bpf/verifier/calls.c
@@ -243,7 +243,7 @@
},
.result_unpriv = REJECT,
.result = REJECT,
- .errstr = "R1 must be referenced",
+ .errstr = "R1 must be",
},
{
"calls: valid kfunc call: referenced arg needs refcounted PTR_TO_BTF_ID",
diff --git a/tools/testing/selftests/bpf/verifier/map_kptr.c b/tools/testing/selftests/bpf/verifier/map_kptr.c
index 6914904344c0..d775ccb01989 100644
--- a/tools/testing/selftests/bpf/verifier/map_kptr.c
+++ b/tools/testing/selftests/bpf/verifier/map_kptr.c
@@ -336,7 +336,7 @@
.prog_type = BPF_PROG_TYPE_SCHED_CLS,
.fixup_map_kptr = { 1 },
.result = REJECT,
- .errstr = "R1 type=untrusted_ptr_or_null_ expected=percpu_ptr_",
+ .errstr = "R1 type=rcu_ptr_or_null_ expected=percpu_ptr_",
},
{
"map_kptr: ref: reject off != 0",
--
2.30.2