bpf: Refactor RCU enforcement in the verifier.

bpf_rcu_read_lock/unlock() are only available in clang compiled kernels. Lack
of such key mechanism makes it impossible for sleepable bpf programs to use RCU
pointers.

Allow bpf_rcu_read_lock/unlock() in GCC compiled kernels (though GCC doesn't
support btf_type_tag yet) and allowlist certain field dereferences in important
data structures like tast_struct, cgroup, socket that are used by sleepable
programs either as RCU pointer or full trusted pointer (which is valid outside
of RCU CS). Use BTF_TYPE_SAFE_RCU and BTF_TYPE_SAFE_TRUSTED macros for such
tagging. They will be removed once GCC supports btf_type_tag.

With that refactor check_ptr_to_btf_access(). Make it strict in enforcing
PTR_TRUSTED and PTR_UNTRUSTED while deprecating old PTR_TO_BTF_ID without
modifier flags. There is a chance that this strict enforcement might break
existing programs (especially on GCC compiled kernels), but this cleanup has to
start sooner than later. Note PTR_TO_CTX access still yields old deprecated
PTR_TO_BTF_ID. Once it's converted to strict PTR_TRUSTED or PTR_UNTRUSTED the
kfuncs and helpers will be able to default to KF_TRUSTED_ARGS. KF_RCU will
remain as a weaker version of KF_TRUSTED_ARGS where obj refcnt could be 0.

Adjust rcu_read_lock selftest to run on gcc and clang compiled kernels.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230303041446.3630-7-alexei.starovoitov@gmail.com

4 files changed, 7 insertions, 7 deletions

diff --git a/tools/testing/selftests/bpf/progs/cgrp_ls_sleepable.c b/tools/testing/selftests/bpf/progs/cgrp_ls_sleepable.c
index 2d11ed528b6f..7615dc23d301 100644
--- a/tools/testing/selftests/bpf/progs/cgrp_ls_sleepable.c
+++ b/tools/testing/selftests/bpf/progs/cgrp_ls_sleepable.c
@@ -49,7 +49,7 @@ int no_rcu_lock(void *ctx)
 	if (task->pid != target_pid)
 		return 0;
 
-	/* ptr_to_btf_id semantics. should work. */
+	/* task->cgroups is untrusted in sleepable prog outside of RCU CS */
 	cgrp = task->cgroups->dfl_cgrp;
 	ptr = bpf_cgrp_storage_get(&map_a, cgrp, 0,
 				   BPF_LOCAL_STORAGE_GET_F_CREATE);
@@ -71,7 +71,7 @@ int yes_rcu_lock(void *ctx)
 
 	bpf_rcu_read_lock();
 	cgrp = task->cgroups->dfl_cgrp;
-	/* cgrp is untrusted and cannot pass to bpf_cgrp_storage_get() helper. */
+	/* cgrp is trusted under RCU CS */
 	ptr = bpf_cgrp_storage_get(&map_a, cgrp, 0, BPF_LOCAL_STORAGE_GET_F_CREATE);
 	if (ptr)
 		cgroup_id = cgrp->kn->id;
diff --git a/tools/testing/selftests/bpf/progs/cpumask_failure.c b/tools/testing/selftests/bpf/progs/cpumask_failure.c
index 33e8e86dd090..c16f7563b84e 100644
--- a/tools/testing/selftests/bpf/progs/cpumask_failure.c
+++ b/tools/testing/selftests/bpf/progs/cpumask_failure.c
@@ -44,7 +44,7 @@ int BPF_PROG(test_alloc_double_release, struct task_struct *task, u64 clone_flag
 }
 
 SEC("tp_btf/task_newtask")
-__failure __msg("bpf_cpumask_acquire args#0 expected pointer to STRUCT bpf_cpumask")
+__failure __msg("must be referenced")
 int BPF_PROG(test_acquire_wrong_cpumask, struct task_struct *task, u64 clone_flags)
 {
 	struct bpf_cpumask *cpumask;
diff --git a/tools/testing/selftests/bpf/progs/nested_trust_failure.c b/tools/testing/selftests/bpf/progs/nested_trust_failure.c
index 14aff7676436..0d1aa6bbace4 100644
--- a/tools/testing/selftests/bpf/progs/nested_trust_failure.c
+++ b/tools/testing/selftests/bpf/progs/nested_trust_failure.c
@@ -17,7 +17,7 @@ char _license[] SEC("license") = "GPL";
  */
 
 SEC("tp_btf/task_newtask")
-__failure __msg("R2 must be referenced or trusted")
+__failure __msg("R2 must be")
 int BPF_PROG(test_invalid_nested_user_cpus, struct task_struct *task, u64 clone_flags)
 {
 	bpf_cpumask_test_cpu(0, task->user_cpus_ptr);
diff --git a/tools/testing/selftests/bpf/progs/rcu_read_lock.c b/tools/testing/selftests/bpf/progs/rcu_read_lock.c
index 5cecbdbbb16e..7250bb76d18a 100644
--- a/tools/testing/selftests/bpf/progs/rcu_read_lock.c
+++ b/tools/testing/selftests/bpf/progs/rcu_read_lock.c
@@ -81,7 +81,7 @@ int no_lock(void *ctx)
 {
 	struct task_struct *task, *real_parent;
 
-	/* no bpf_rcu_read_lock(), old code still works */
+	/* old style ptr_to_btf_id is not allowed in sleepable */
 	task = bpf_get_current_task_btf();
 	real_parent = task->real_parent;
 	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
@@ -286,13 +286,13 @@ out:
 }
 
 SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
-int task_untrusted_non_rcuptr(void *ctx)
+int task_trusted_non_rcuptr(void *ctx)
 {
 	struct task_struct *task, *group_leader;
 
 	task = bpf_get_current_task_btf();
 	bpf_rcu_read_lock();
-	/* the pointer group_leader marked as untrusted */
+	/* the pointer group_leader is explicitly marked as trusted */
 	group_leader = task->real_parent->group_leader;
 	(void)bpf_task_storage_get(&map_a, group_leader, 0, 0);
 	bpf_rcu_read_unlock();