bpf: Make KF_TRUSTED_ARGS the default for all kfuncs

Change the verifier to make trusted args the default requirement for
all kfuncs by removing is_kfunc_trusted_args() assuming it be to always
return true.

This works because:
1. Context pointers (xdp_md, __sk_buff, etc.) are handled through their
   own KF_ARG_PTR_TO_CTX case label and bypass the trusted check
2. Struct_ops callback arguments are already marked as PTR_TRUSTED during
   initialization and pass is_trusted_reg()
3. KF_RCU kfuncs are handled separately via is_kfunc_rcu() checks at
   call sites (always checked with || alongside is_kfunc_trusted_args)

This simple change makes all kfuncs require trusted args by default
while maintaining correct behavior for all existing special cases.

Note: This change means kfuncs that previously accepted NULL pointers
without KF_TRUSTED_ARGS will now reject NULL at verification time.
Several netfilter kfuncs are affected: bpf_xdp_ct_lookup(),
bpf_skb_ct_lookup(), bpf_xdp_ct_alloc(), and bpf_skb_ct_alloc() all
accept NULL for their bpf_tuple and opts parameters internally (checked
in __bpf_nf_ct_lookup), but after this change the verifier rejects NULL
before the kfunc is even called. This is acceptable because these kfuncs
don't work with NULL parameters in their proper usage. Now they will be
rejected rather than returning an error, which shouldn't make a
difference to BPF programs that were using these kfuncs properly.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260102180038.2708325-2-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 3 insertions, 11 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3d44c5d06623..359a962d69a1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12040,11 +12040,6 @@ static bool is_kfunc_release(struct bpf_kfunc_call_arg_meta *meta)
 	return meta->kfunc_flags & KF_RELEASE;
 }
 
-static bool is_kfunc_trusted_args(struct bpf_kfunc_call_arg_meta *meta)
-{
-	return (meta->kfunc_flags & KF_TRUSTED_ARGS) || is_kfunc_release(meta);
-}
-
 static bool is_kfunc_sleepable(struct bpf_kfunc_call_arg_meta *meta)
 {
 	return meta->kfunc_flags & KF_SLEEPABLE;
@@ -13253,9 +13248,9 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 			return -EINVAL;
 		}
 
-		if ((is_kfunc_trusted_args(meta) || is_kfunc_rcu(meta)) &&
-		    (register_is_null(reg) || type_may_be_null(reg->type)) &&
-			!is_kfunc_arg_nullable(meta->btf, &args[i])) {
+		if ((register_is_null(reg) || type_may_be_null(reg->type)) &&
+		    !is_kfunc_arg_nullable(meta->btf, &args[i]) &&
+		    !is_kfunc_arg_optional(meta->btf, &args[i])) {
 			verbose(env, "Possibly NULL pointer passed to trusted arg%d\n", i);
 			return -EACCES;
 		}
@@ -13320,9 +13315,6 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 			fallthrough;
 		case KF_ARG_PTR_TO_ALLOC_BTF_ID:
 		case KF_ARG_PTR_TO_BTF_ID:
-			if (!is_kfunc_trusted_args(meta) && !is_kfunc_rcu(meta))
-				break;
-
 			if (!is_trusted_reg(reg)) {
 				if (!is_kfunc_rcu(meta)) {
 					verbose(env, "R%d must be referenced or trusted\n", regno);