bpf: Introduce kptr_rcu.

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

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

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230303041446.3630-4-alexei.starovoitov@gmail.com

Documentation/bpf/kfuncs.rst

@@ -249,11 +249,13 @@ 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. The pointers are not
+NULL, but the object's refcount could have reached zero. The kfuncs need to
+consider doing refcnt != 0 check, especially when returning a KF_ACQUIRE
+pointer. Note as well that a KF_ACQUIRE kfunc that is KF_RCU should very likely
+also be KF_RET_NULL.
 
 .. _KF_deprecated_flag:
 

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

kernel/bpf/helpers.c

@@ -2163,8 +2163,10 @@ __bpf_kfunc struct cgroup *bpf_cgroup_ancestor(struct cgroup *cgrp, int level)
 	if (level > cgrp->level || level < 0)
 		return NULL;
 
+	/* cgrp's refcnt could be 0 here, but ancestors can still be accessed */
 	ancestor = cgrp->ancestors[level];
-	cgroup_get(ancestor);
+	if (!cgroup_tryget(ancestor))
+		return NULL;
 	return ancestor;
 }
 
@@ -2382,7 +2384,7 @@ 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)

kernel/bpf/verifier.c

@@ -4218,7 +4218,7 @@ 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 */
@@ -4285,6 +4285,34 @@ 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;
+}
+
+/* Once GCC supports btf_type_tag the following mechanism will be replaced with tag check */
+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 bool rcu_safe_kptr(const struct btf_field *field)
+{
+	const struct btf_field_kptr *kptr = &field->kptr;
+
+	return field->type == BPF_KPTR_REF && rcu_protected_object(kptr->btf, kptr->btf_id);
+}
+
 static int check_map_kptr_access(struct bpf_verifier_env *env, u32 regno,
 				 int value_regno, int insn_idx,
 				 struct btf_field *kptr_field)
@@ -4319,7 +4347,10 @@ 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,
+				rcu_safe_kptr(kptr_field) && 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) {
@@ -5163,10 +5194,17 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 	 * An RCU-protected pointer can also be deemed trusted if we are in an
 	 * RCU read region. This case is handled below.
 	 */
-	if (nested_ptr_is_trusted(env, reg, off))
+	if (nested_ptr_is_trusted(env, reg, off)) {
 		flag |= PTR_TRUSTED;
-	else
+		/*
+		 * task->cgroups is trusted. It provides a stronger guarantee
+		 * than __rcu tag on 'cgroups' field in 'struct task_struct'.
+		 * Clear MEM_RCU in such case.
+		 */
+		flag &= ~MEM_RCU;
+	} else {
 		flag &= ~PTR_TRUSTED;
+	}
 
 	if (flag & MEM_RCU) {
 		/* Mark value register as MEM_RCU only if it is protected by
@@ -5175,11 +5213,10 @@ 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.
@@ -9676,7 +9713,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 			return -EINVAL;
 		}
 
-		if (is_kfunc_trusted_args(meta) &&
+		if ((is_kfunc_trusted_args(meta) || is_kfunc_rcu(meta)) &&
 		    (register_is_null(reg) || type_may_be_null(reg->type))) {
 			verbose(env, "Possibly NULL pointer passed to trusted arg%d\n", i);
 			return -EACCES;

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 != NULL, but 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)

tools/testing/selftests/bpf/progs/cgrp_kfunc_failure.c

@@ -205,7 +205,7 @@ int BPF_PROG(cgrp_kfunc_get_unreleased, struct cgroup *cgrp, const char *path)
 }
 
 SEC("tp_btf/cgroup_mkdir")
-__failure __msg("arg#0 is untrusted_ptr_or_null_ expected ptr_ or socket")
+__failure __msg("expects refcounted")
 int BPF_PROG(cgrp_kfunc_release_untrusted, struct cgroup *cgrp, const char *path)
 {
 	struct __cgrps_kfunc_map_value *v;

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;

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",

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",