[PATCH bpf] bpf: Reject writes through untrusted allocated pointers

[Ning Ding <dingning04@gmail.com>]

A kptr loaded under an RCU read lock has MEM_ALLOC set. After
bpf_rcu_read_unlock(), the verifier marks it PTR_UNTRUSTED, but
check_ptr_to_btf_access() still allows writes because
type_is_ptr_alloc_obj() ignores that flag.

Reject writes through MEM_ALLOC pointers that are PTR_UNTRUSTED. Reads
remain allowed and are converted to probe-memory accesses.

Also skip the owning-reference check for these pointers, since they no
longer have a live reference after leaving the RCU critical section.

Add verifier tests for the rejected write and permitted read cases.

Fixes: 503e4def5414 ("bpf: Replace open code with for allocated object check")
Closes: https://lore.kernel.org/all/20260726021304.97ED91F000E9@smtp.kernel.org/
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Ning Ding <dingning04@gmail.com>
---
 kernel/bpf/verifier.c                         |  6 +-
 .../bpf/progs/local_kptr_stash_fail.c         | 58 +++++++++++++++++++
 2 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7aa47342dc65..24c151ad2e62 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5804,13 +5804,15 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 		 * program allocated objects (which always have id > 0),
 		 * but not for untrusted PTR_TO_BTF_ID | MEM_ALLOC.
 		 */
-		if (atype != BPF_READ && !type_is_ptr_alloc_obj(reg->type)) {
+		if (atype != BPF_READ &&
+		    (!type_is_ptr_alloc_obj(reg->type) || reg->type & PTR_UNTRUSTED)) {
 			verbose(env, "only read is supported\n");
 			return -EACCES;
 		}
 
 		if (type_is_alloc(reg->type) && !type_is_non_owning_ref(reg->type) &&
-		    !(reg->type & MEM_RCU) && !reg_is_referenced(env, reg)) {
+		    !(reg->type & (MEM_RCU | PTR_UNTRUSTED)) &&
+		    !reg_is_referenced(env, reg)) {
 			verifier_bug(env, "allocated object must have a referenced id");
 			return -EFAULT;
 		}
diff --git a/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c b/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c
index fcf7a7567da2..d7b8cd6458bb 100644
--- a/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c
+++ b/tools/testing/selftests/bpf/progs/local_kptr_stash_fail.c
@@ -8,14 +8,22 @@
 #include "../bpf_experimental.h"
 #include "bpf_misc.h"
 
+extern void bpf_rcu_read_lock(void) __ksym;
+extern void bpf_rcu_read_unlock(void) __ksym;
+
 struct node_data {
 	long key;
 	long data;
 	struct bpf_rb_node node;
 };
 
+struct plain_data {
+	long key;
+};
+
 struct map_value {
 	struct node_data __kptr *node;
+	struct plain_data __kptr *plain;
 };
 
 struct node_data2 {
@@ -31,6 +39,7 @@ struct node_data2 {
  * [35] TYPE_TAG 'kptr_ref' type_id=34
  */
 struct node_data *just_here_because_btf_bug;
+struct plain_data *just_here_because_btf_bug2;
 
 struct {
 	__uint(type, BPF_MAP_TYPE_ARRAY);
@@ -82,4 +91,53 @@ long drop_rb_node_off(void *ctx)
 	return 0;
 }
 
+SEC("?tc")
+__failure __msg("only read is supported")
+long write_untrusted_alloc_obj(void *ctx)
+{
+	struct map_value *mapval;
+	struct node_data *res;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&some_nodes, &idx);
+	if (!mapval)
+		return 1;
+
+	bpf_rcu_read_lock();
+	res = mapval->node;
+	if (!res) {
+		bpf_rcu_read_unlock();
+		return 2;
+	}
+	barrier_var(res);
+	bpf_rcu_read_unlock();
+
+	res->key = 42;
+	return 0;
+}
+
+SEC("?tc")
+__success
+long read_untrusted_alloc_obj(void *ctx)
+{
+	struct map_value *mapval;
+	struct plain_data *res;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&some_nodes, &idx);
+	if (!mapval)
+		return 1;
+
+	bpf_rcu_read_lock();
+	res = mapval->plain;
+	if (!res) {
+		bpf_rcu_read_unlock();
+		return 2;
+	}
+	barrier_var(res);
+	bpf_rcu_read_unlock();
+
+	return res->key;
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0