Merge branch 'bpf-kptr-rcu'

Alexei Starovoitov says:

====================
v4->v5:
fix typos, add acks.

v3->v4:
- patch 3 got much cleaner after BPF_KPTR_RCU was removed as suggested by David.

- make KF_RCU stronger and require that bpf program checks for NULL
before passing such pointers into kfunc. The prog has to do that anyway
to access fields and it aligns with BTF_TYPE_SAFE_RCU allowlist.

- New patch 6: refactor RCU enforcement in the verifier.
The patches 2,3,6 are part of one feature.
The 2 and 3 alone are incomplete, since RCU pointers are barely useful
without bpf_rcu_read_lock/unlock in GCC compiled kernel.
Even if GCC lands support for btf_type_tag today it will take time
to mandate that version for kernel builds. Hence go with allow list
approach. See patch 6 for details.
This allows to start strict enforcement of TRUSTED | UNTRUSTED
in one part of PTR_TO_BTF_ID accesses.
One step closer to KF_TRUSTED_ARGS by default.

v2->v3:
- Instead of requiring bpf progs to tag fields with __kptr_rcu
teach the verifier to infer RCU properties based on the type.
BPF_KPTR_RCU becomes kernel internal type of struct btf_field.
- Add patch 2 to tag cgroups and dfl_cgrp as trusted.
That bug was spotted by BPF CI on clang compiler kernels,
since patch 3 is doing:
static bool in_rcu_cs(struct bpf_verifier_env *env)
{
        return env->cur_state->active_rcu_lock || !env->prog->aux->sleepable;
}
which makes all non-sleepable programs behave like they have implicit
rcu_read_lock around them. Which is the case in practice.
It was fine on gcc compiled kernels where task->cgroup deference was producing
PTR_TO_BTF_ID, but on clang compiled kernels task->cgroup deference was
producing PTR_TO_BTF_ID | MEM_RCU | MAYBE_NULL, which is more correct,
but selftests were failing. Patch 2 fixes this discrepancy.
With few more patches like patch 2 we can make KF_TRUSTED_ARGS default
for kfuncs and helpers.
- Add comment in selftest patch 5 that it's verifier only check.

v1->v2:
Instead of agressively allow dereferenced kptr_rcu pointers into KF_TRUSTED_ARGS
kfuncs only allow them into KF_RCU funcs.
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.
No changes in patch 1.
Patches 2,3,4 adjusted with above behavior.

v1:
The __kptr_ref turned out to be too limited, since any "trusted" pointer access
requires bpf_kptr_xchg() which is impractical when the same pointer needs
to be dereferenced by multiple cpus.
The __kptr "untrusted" only access isn't very useful in practice.
Rename __kptr to __kptr_untrusted with eventual goal to deprecate it,
and rename __kptr_ref to __kptr, since that looks to be more common use of kptrs.
Introduce __kptr_rcu that can be directly dereferenced and used similar
to native kernel C code.
Once bpf_cpumask and task_struct kfuncs are converted to observe RCU GP
when refcnt goes to zero, both __kptr and __kptr_untrusted can be deprecated
and __kptr_rcu can become the only __kptr tag.
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

4 files changed, 199 insertions, 80 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index ef2d8969ed1f..a8cb09e5973b 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3288,9 +3288,9 @@ static int btf_find_kptr(const struct btf *btf, const struct btf_type *t,
 	/* Reject extra tags */
 	if (btf_type_is_type_tag(btf_type_by_id(btf, t->type)))
 		return -EINVAL;
-	if (!strcmp("kptr", __btf_name_by_offset(btf, t->name_off)))
+	if (!strcmp("kptr_untrusted", __btf_name_by_offset(btf, t->name_off)))
 		type = BPF_KPTR_UNREF;
-	else if (!strcmp("kptr_ref", __btf_name_by_offset(btf, t->name_off)))
+	else if (!strcmp("kptr", __btf_name_by_offset(btf, t->name_off)))
 		type = BPF_KPTR_REF;
 	else
 		return -EINVAL;
@@ -6163,6 +6163,7 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 	const char *tname, *mname, *tag_value;
 	u32 vlen, elem_id, mid;
 
+	*flag = 0;
 again:
 	tname = __btf_name_by_offset(btf, t->name_off);
 	if (!btf_type_is_struct(t)) {
@@ -6329,6 +6330,15 @@ error:
 		 * of this field or inside of this struct
 		 */
 		if (btf_type_is_struct(mtype)) {
+			if (BTF_INFO_KIND(mtype->info) == BTF_KIND_UNION &&
+			    btf_type_vlen(mtype) != 1)
+				/*
+				 * walking unions yields untrusted pointers
+				 * with exception of __bpf_md_ptr and other
+				 * unions with a single member
+				 */
+				*flag |= PTR_UNTRUSTED;
+
 			/* our field must be inside that union or struct */
 			t = mtype;
 
@@ -6373,7 +6383,7 @@ error:
 			stype = btf_type_skip_modifiers(btf, mtype->type, &id);
 			if (btf_type_is_struct(stype)) {
 				*next_btf_id = id;
-				*flag = tmp_flag;
+				*flag |= tmp_flag;
 				return WALK_PTR;
 			}
 		}
@@ -8357,7 +8367,7 @@ out:
 
 bool btf_nested_type_is_trusted(struct bpf_verifier_log *log,
 				const struct bpf_reg_state *reg,
-				int off)
+				int off, const char *suffix)
 {
 	struct btf *btf = reg->btf;
 	const struct btf_type *walk_type, *safe_type;
@@ -8374,7 +8384,7 @@ bool btf_nested_type_is_trusted(struct bpf_verifier_log *log,
 
 	tname = btf_name_by_offset(btf, walk_type->name_off);
 
-	ret = snprintf(safe_tname, sizeof(safe_tname), "%s__safe_fields", tname);
+	ret = snprintf(safe_tname, sizeof(safe_tname), "%s%s", tname, suffix);
 	if (ret < 0)
 		return false;
 
diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
index 2b3fbbfebdc5..b6587ec40f1b 100644
--- a/kernel/bpf/cpumask.c
+++ b/kernel/bpf/cpumask.c
@@ -427,26 +427,26 @@ BTF_ID_FLAGS(func, bpf_cpumask_create, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cpumask_release, KF_RELEASE | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_cpumask_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_cpumask_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
-BTF_ID_FLAGS(func, bpf_cpumask_first, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_first_zero, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_set_cpu, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_clear_cpu, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_test_cpu, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_test_and_set_cpu, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_test_and_clear_cpu, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_setall, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_clear, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_and, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_or, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_xor, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_equal, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_intersects, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_subset, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_empty, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_full, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_copy, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_any, KF_TRUSTED_ARGS)
-BTF_ID_FLAGS(func, bpf_cpumask_any_and, KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_cpumask_first, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_first_zero, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_set_cpu, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_clear_cpu, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_test_cpu, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_test_and_set_cpu, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_test_and_clear_cpu, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_setall, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_clear, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_and, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_or, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_xor, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_equal, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_intersects, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_subset, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_empty, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_full, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_copy, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_any, KF_RCU)
+BTF_ID_FLAGS(func, bpf_cpumask_any_and, KF_RCU)
 BTF_SET8_END(cpumask_kfunc_btf_ids)
 
 static const struct btf_kfunc_id_set cpumask_kfunc_set = {
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 12f12e879bcf..637ac4e92e75 100644
--- a/kernel/bpf/helpers.c
+++ b/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)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index bf580f246a01..c2adf3c24c64 100644
--- a/kernel/bpf/verifier.c
+++ b/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 @@ bad_type:
 	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) {
@@ -5042,23 +5073,76 @@ static int bpf_map_direct_read(struct bpf_map *map, int off, int size, u64 *val)
 	return 0;
 }
 
-#define BTF_TYPE_SAFE_NESTED(__type)  __PASTE(__type, __safe_fields)
+#define BTF_TYPE_SAFE_RCU(__type)  __PASTE(__type, __safe_rcu)
+#define BTF_TYPE_SAFE_TRUSTED(__type)  __PASTE(__type, __safe_trusted)
+
+/*
+ * Allow list few fields as RCU trusted or full trusted.
+ * This logic doesn't allow mix tagging and will be removed once GCC supports
+ * btf_type_tag.
+ */
 
-BTF_TYPE_SAFE_NESTED(struct task_struct) {
+/* RCU trusted: these fields are trusted in RCU CS and never NULL */
+BTF_TYPE_SAFE_RCU(struct task_struct) {
 	const cpumask_t *cpus_ptr;
+	struct css_set __rcu *cgroups;
+	struct task_struct __rcu *real_parent;
+	struct task_struct *group_leader;
 };
 
-static bool nested_ptr_is_trusted(struct bpf_verifier_env *env,
-				  struct bpf_reg_state *reg,
-				  int off)
+BTF_TYPE_SAFE_RCU(struct css_set) {
+	struct cgroup *dfl_cgrp;
+};
+
+/* full trusted: these fields are trusted even outside of RCU CS and never NULL */
+BTF_TYPE_SAFE_TRUSTED(struct bpf_iter_meta) {
+	__bpf_md_ptr(struct seq_file *, seq);
+};
+
+BTF_TYPE_SAFE_TRUSTED(struct bpf_iter__task) {
+	__bpf_md_ptr(struct bpf_iter_meta *, meta);
+	__bpf_md_ptr(struct task_struct *, task);
+};
+
+BTF_TYPE_SAFE_TRUSTED(struct linux_binprm) {
+	struct file *file;
+};
+
+BTF_TYPE_SAFE_TRUSTED(struct file) {
+	struct inode *f_inode;
+};
+
+BTF_TYPE_SAFE_TRUSTED(struct dentry) {
+	/* no negative dentry-s in places where bpf can see it */
+	struct inode *d_inode;
+};
+
+BTF_TYPE_SAFE_TRUSTED(struct socket) {
+	struct sock *sk;
+};
+
+static bool type_is_rcu(struct bpf_verifier_env *env,
+			struct bpf_reg_state *reg,
+			int off)
 {
-	/* If its parent is not trusted, it can't regain its trusted status. */
-	if (!is_trusted_reg(reg))
-		return false;
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_RCU(struct task_struct));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_RCU(struct css_set));
+
+	return btf_nested_type_is_trusted(&env->log, reg, off, "__safe_rcu");
+}
 
-	BTF_TYPE_EMIT(BTF_TYPE_SAFE_NESTED(struct task_struct));
+static bool type_is_trusted(struct bpf_verifier_env *env,
+			    struct bpf_reg_state *reg,
+			    int off)
+{
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct bpf_iter_meta));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct bpf_iter__task));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct linux_binprm));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct file));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct dentry));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct socket));
 
-	return btf_nested_type_is_trusted(&env->log, reg, off);
+	return btf_nested_type_is_trusted(&env->log, reg, off, "__safe_trusted");
 }
 
 static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
@@ -5144,41 +5228,56 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 	if (ret < 0)
 		return ret;
 
-	/* If this is an untrusted pointer, all pointers formed by walking it
-	 * also inherit the untrusted flag.
-	 */
-	if (type_flag(reg->type) & PTR_UNTRUSTED)
-		flag |= PTR_UNTRUSTED;
+	if (ret != PTR_TO_BTF_ID) {
+		/* just mark; */
 
-	/* By default any pointer obtained from walking a trusted pointer is no
-	 * longer trusted, unless the field being accessed has explicitly been
-	 * marked as inheriting its parent's state of trust.
-	 *
-	 * 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))
-		flag |= PTR_TRUSTED;
-	else
-		flag &= ~PTR_TRUSTED;
-
-	if (flag & MEM_RCU) {
-		/* Mark value register as MEM_RCU only if it is protected by
-		 * bpf_rcu_read_lock() and the ptr reg is rcu or trusted. MEM_RCU
-		 * itself can already indicate trustedness inside the rcu
-		 * read lock region. Also mark rcu pointer as PTR_MAYBE_NULL since
-		 * it could be null in some cases.
+	} else if (type_flag(reg->type) & PTR_UNTRUSTED) {
+		/* If this is an untrusted pointer, all pointers formed by walking it
+		 * also inherit the untrusted flag.
 		 */
-		if (!env->cur_state->active_rcu_lock ||
-		    !(is_trusted_reg(reg) || is_rcu_reg(reg)))
-			flag &= ~MEM_RCU;
-		else
-			flag |= PTR_MAYBE_NULL;
-	} 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.
+		flag = PTR_UNTRUSTED;
+
+	} else if (is_trusted_reg(reg) || is_rcu_reg(reg)) {
+		/* By default any pointer obtained from walking a trusted pointer is no
+		 * longer trusted, unless the field being accessed has explicitly been
+		 * marked as inheriting its parent's state of trust (either full or RCU).
+		 * For example:
+		 * 'cgroups' pointer is untrusted if task->cgroups dereference
+		 * happened in a sleepable program outside of bpf_rcu_read_lock()
+		 * section. In a non-sleepable program it's trusted while in RCU CS (aka MEM_RCU).
+		 * Note bpf_rcu_read_unlock() converts MEM_RCU pointers to PTR_UNTRUSTED.
+		 *
+		 * A regular RCU-protected pointer with __rcu tag can also be deemed
+		 * trusted if we are in an RCU CS. Such pointer can be NULL.
 		 */
-		flag |= PTR_UNTRUSTED;
+		if (type_is_trusted(env, reg, off)) {
+			flag |= PTR_TRUSTED;
+		} else if (in_rcu_cs(env) && !type_may_be_null(reg->type)) {
+			if (type_is_rcu(env, reg, off)) {
+				/* ignore __rcu tag and mark it MEM_RCU */
+				flag |= MEM_RCU;
+			} else if (flag & MEM_RCU) {
+				/* __rcu tagged pointers can be NULL */
+				flag |= PTR_MAYBE_NULL;
+			} else if (flag & (MEM_PERCPU | MEM_USER)) {
+				/* keep as-is */
+			} else {
+				/* walking unknown pointers yields untrusted pointer */
+				flag = PTR_UNTRUSTED;
+			}
+		} else {
+			/*
+			 * If not in RCU CS or MEM_RCU pointer can be NULL then
+			 * aggressively mark as untrusted otherwise such
+			 * pointers will be plain PTR_TO_BTF_ID without flags
+			 * and will be allowed to be passed into helpers for
+			 * compat reasons.
+			 */
+			flag = PTR_UNTRUSTED;
+		}
+	} else {
+		/* Old compat. Deprecated */
+		flag &= ~PTR_TRUSTED;
 	}
 
 	if (atype == BPF_READ && value_regno >= 0)
@@ -9670,7 +9769,7 @@ 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;
@@ -10006,10 +10105,6 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 
 	rcu_lock = is_kfunc_bpf_rcu_read_lock(&meta);
 	rcu_unlock = is_kfunc_bpf_rcu_read_unlock(&meta);
-	if ((rcu_lock || rcu_unlock) && !env->rcu_tag_supported) {
-		verbose(env, "no vmlinux btf rcu tag support for kfunc %s\n", func_name);
-		return -EACCES;
-	}
 
 	if (env->cur_state->active_rcu_lock) {
 		struct bpf_func_state *state;
@@ -14868,8 +14963,22 @@ static int do_check(struct bpf_verifier_env *env)
 				 * src_reg == stack|map in some other branch.
 				 * Reject it.
 				 */
-				verbose(env, "same insn cannot be used with different pointers\n");
-				return -EINVAL;
+				if (base_type(src_reg_type) == PTR_TO_BTF_ID &&
+				    base_type(*prev_src_type) == PTR_TO_BTF_ID) {
+					/*
+					 * Have to support a use case when one path through
+					 * the program yields TRUSTED pointer while another
+					 * is UNTRUSTED. Fallback to UNTRUSTED to generate
+					 * BPF_PROBE_MEM.
+					 */
+					*prev_src_type = PTR_TO_BTF_ID | PTR_UNTRUSTED;
+				} else {
+					verbose(env,
+						"The same insn cannot be used with different pointers: %s",
+						reg_type_str(env, src_reg_type));
+					verbose(env, " != %s\n", reg_type_str(env, *prev_src_type));
+					return -EINVAL;
+				}
 			}
 
 		} else if (class == BPF_STX) {
@@ -17941,8 +18050,6 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr)
 	env->bypass_spec_v1 = bpf_bypass_spec_v1();
 	env->bypass_spec_v4 = bpf_bypass_spec_v4();
 	env->bpf_capable = bpf_capable();
-	env->rcu_tag_supported = btf_vmlinux &&
-		btf_find_by_name_kind(btf_vmlinux, "rcu", BTF_KIND_TYPE_TAG) > 0;
 
 	if (is_priv)
 		env->test_state_freq = attr->prog_flags & BPF_F_TEST_STATE_FREQ;