[PATCH bpf-next v2 05/11] bpf: Check global subprog untrusted arguments in the common path

[Amery Hung <ameryhung@gmail.com>]

Global subprogram arguments tagged __arg_untrusted deliberately
accept any caller value. The callee is verified separately with a
read-only untrusted pointer, whose accesses use protected probe-read
instructions.

Keep the untrusted type in the canonical bpf_subprog_info used to
prepare the callee state. Represent it as ARG_IGNORE only in the
temporary bpf_func_proto used for call-site checking, then extend the
guarded check_func_arg() path to cover it.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
---
 kernel/bpf/verifier.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 147473cf14f1..dd51df530e62 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10805,9 +10805,18 @@ static void gen_subprog_arg_proto(const struct bpf_subprog_info *sub, const stru
 
 	memset(proto, 0, sizeof(*proto));
 	for (arg = 0; arg < btf_type_vlen(func_proto); arg++) {
+		enum bpf_arg_type arg_type = sub->args[slot].arg_type;
 		const struct btf_type *t;
 
-		proto->arg_type[arg] = sub->args[slot].arg_type;
+		if (arg_type & PTR_UNTRUSTED) {
+			/*
+			 * An __arg_untrusted argument accepts any caller value. The
+			 * callee treats it as read-only and uses probe-read instructions
+			 * to protect against invalid memory access.
+			 */
+			arg_type = ARG_IGNORE;
+		}
+		proto->arg_type[arg] = arg_type;
 		t = btf_type_skip_modifiers(btf, args[arg].type, NULL);
 		slot += btf_arg_slots(t);
 	}
@@ -10870,16 +10879,10 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env, int subprog,
 		t = btf_type_skip_modifiers(btf, args[arg].type, NULL);
 		nslots = btf_arg_slots(t);
 
-		if (arg_type == ARG_SCALAR) {
+		if (arg_type == ARG_SCALAR || arg_type == ARG_IGNORE) {
 			ret = check_func_arg(env, arg, slot, 0, &meta, env->insn_idx);
 			if (ret)
 				return ret;
-		} else if (arg_type & PTR_UNTRUSTED) {
-			/*
-			 * Anything is allowed for untrusted arguments, as these are
-			 * read-only and probe read instructions would protect against
-			 * invalid memory access.
-			 */
 		} else if (arg_type == ARG_PTR_TO_CTX) {
 			ret = check_func_arg_reg_off(env, reg, argno, ARG_PTR_TO_CTX);
 			if (ret < 0)
-- 
2.52.0