bpf: relax zero fixed offset constraint on KF_TRUSTED_ARGS/KF_RCU

Currently, BPF kfuncs which accept trusted pointer arguments
i.e. those flagged as KF_TRUSTED_ARGS, KF_RCU, or KF_RELEASE, all
require an original/unmodified trusted pointer argument to be supplied
to them. By original/unmodified, it means that the backing register
holding the trusted pointer argument that is to be supplied to the BPF
kfunc must have its fixed offset set to zero, or else the BPF verifier
will outright reject the BPF program load. However, this zero fixed
offset constraint that is currently enforced by the BPF verifier onto
BPF kfuncs specifically flagged to accept KF_TRUSTED_ARGS or KF_RCU
trusted pointer arguments is rather unnecessary, and can limit their
usability in practice. Specifically, it completely eliminates the
possibility of constructing a derived trusted pointer from an original
trusted pointer. To put it simply, a derived pointer is a pointer
which points to one of the nested member fields of the object being
pointed to by the original trusted pointer.

This patch relaxes the zero fixed offset constraint that is enforced
upon BPF kfuncs which specifically accept KF_TRUSTED_ARGS, or KF_RCU
arguments. Although, the zero fixed offset constraint technically also
applies to BPF kfuncs accepting KF_RELEASE arguments, relaxing this
constraint for such BPF kfuncs has subtle and unwanted
side-effects. This was discovered by experimenting a little further
with an initial version of this patch series [0]. The primary issue
with relaxing the zero fixed offset constraint on BPF kfuncs accepting
KF_RELEASE arguments is that it'd would open up the opportunity for
BPF programs to supply both trusted pointers and derived trusted
pointers to them. For KF_RELEASE BPF kfuncs specifically, this could
be problematic as resources associated with the backing pointer could
be released by the backing BPF kfunc and cause instabilities for the
rest of the kernel.

With this new fixed offset semantic in-place for BPF kfuncs accepting
KF_TRUSTED_ARGS and KF_RCU arguments, we now have more flexibility
when it comes to the BPF kfuncs that we're able to introduce moving
forward.

Early discussions covering the possibility of relaxing the zero fixed
offset constraint can be found using the link below. This will provide
more context on where all this has stemmed from [1].

Notably, pre-existing tests have been updated such that they provide
coverage for the updated zero fixed offset
functionality. Specifically, the nested offset test was converted from
a negative to positive test as it was already designed to assert zero
fixed offset semantics of a KF_TRUSTED_ARGS BPF kfunc.

[0] https://lore.kernel.org/bpf/ZnA9ndnXKtHOuYMe@google.com/
[1] https://lore.kernel.org/bpf/ZhkbrM55MKQ0KeIV@google.com/Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20240709210939.1544011-1-mattbobrowski@google.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -11335,7 +11335,9 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
 	    btf_type_ids_nocast_alias(&env->log, reg_btf, reg_ref_id, meta->btf, ref_id))
 		strict_type_match = true;
 
-	WARN_ON_ONCE(is_kfunc_trusted_args(meta) && reg->off);
+	WARN_ON_ONCE(is_kfunc_release(meta) &&
+		     (reg->off || !tnum_is_const(reg->var_off) ||
+		      reg->var_off.value));
 
 	reg_ref_t = btf_type_skip_modifiers(reg_btf, reg_ref_id, &reg_ref_id);
 	reg_ref_tname = btf_name_by_offset(reg_btf, reg_ref_t->name_off);
@@ -11917,12 +11919,8 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 					return -EINVAL;
 				}
 			}
-
 			fallthrough;
 		case KF_ARG_PTR_TO_CTX:
-			/* Trusted arguments have the same offset checks as release arguments */
-			arg_type |= OBJ_RELEASE;
-			break;
 		case KF_ARG_PTR_TO_DYNPTR:
 		case KF_ARG_PTR_TO_ITER:
 		case KF_ARG_PTR_TO_LIST_HEAD:
@@ -11935,7 +11933,6 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		case KF_ARG_PTR_TO_REFCOUNTED_KPTR:
 		case KF_ARG_PTR_TO_CONST_STR:
 		case KF_ARG_PTR_TO_WORKQUEUE:
-			/* Trusted by default */
 			break;
 		default:
 			WARN_ON_ONCE(1);

tools/testing/selftests/bpf/progs/nested_trust_failure.c

@@ -31,14 +31,6 @@ int BPF_PROG(test_invalid_nested_user_cpus, struct task_struct *task, u64 clone_
 	return 0;
 }
 
-SEC("tp_btf/task_newtask")
-__failure __msg("R1 must have zero offset when passed to release func or trusted arg to kfunc")
-int BPF_PROG(test_invalid_nested_offset, struct task_struct *task, u64 clone_flags)
-{
-	bpf_cpumask_first_zero(&task->cpus_mask);
-	return 0;
-}
-
 /* Although R2 is of type sk_buff but sock_common is expected, we will hit untrusted ptr first. */
 SEC("tp_btf/tcp_probe")
 __failure __msg("R2 type=untrusted_ptr_ expected=ptr_, trusted_ptr_, rcu_ptr_")

tools/testing/selftests/bpf/progs/nested_trust_success.c

@@ -32,3 +32,11 @@ int BPF_PROG(test_skb_field, struct sock *sk, struct sk_buff *skb)
 	bpf_sk_storage_get(&sk_storage_map, skb->sk, 0, 0);
 	return 0;
 }
+
+SEC("tp_btf/task_newtask")
+__success
+int BPF_PROG(test_nested_offset, struct task_struct *task, u64 clone_flags)
+{
+	bpf_cpumask_first_zero(&task->cpus_mask);
+	return 0;
+}

tools/testing/selftests/bpf/verifier/calls.c

@@ -76,7 +76,7 @@
 	},
 	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 	.result = REJECT,
-	.errstr = "R1 must have zero offset when passed to release func or trusted arg to kfunc",
+	.errstr = "arg#0 expected pointer to ctx, but got PTR",
 	.fixup_kfunc_btf_id = {
 		{ "bpf_kfunc_call_test_pass_ctx", 2 },
 	},