bpf: Reject writes through untrusted BTF pointers

check_ptr_to_btf_access() lets program-type btf_struct_access callbacks
validate writes before the default BTF access path rejects non-read
accesses. That bypasses the read-only policy for untrusted BTF pointers
created by helpers such as bpf_rdonly_cast().

Reject non-read accesses through PTR_UNTRUSTED BTF pointers at the
common entry point, before the callback branch to handle all cases.

Fixes: 282de14 ("bpf: Introduce allocated objects support")
Signed-off-by: Nicholas Dudar <main.kalliope@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

Author: KalliopeMain

kernel/bpf/verifier.c

@@ -5790,6 +5790,11 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 		return -EACCES;
 	}
 
+	if (atype != BPF_READ && (type_flag(reg->type) & PTR_UNTRUSTED)) {
+		verbose(env, "only read is supported\n");
+		return -EACCES;
+	}
+
 	if (env->ops->btf_struct_access && !type_is_alloc(reg->type) && atype == BPF_WRITE) {
 		if (!btf_is_kernel(reg->btf)) {
 			verifier_bug(env, "reg->btf must be kernel btf");
@@ -5802,8 +5807,7 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 				reg_arg_name(env, argno), tname, off, size);
 	} else {
 		/* Writes are permitted with default btf_struct_access for
-		 * program allocated objects (which always have id > 0),
-		 * but not for untrusted PTR_TO_BTF_ID | MEM_ALLOC.
+		 * program allocated objects (which always have id > 0).
 		 */
 		if (atype != BPF_READ && !type_is_ptr_alloc_obj(reg->type)) {
 			verbose(env, "only read is supported\n");