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

1 files changed, 3 insertions, 13 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/rcu_read_lock.c b/tools/testing/selftests/bpf/prog_tests/rcu_read_lock.c
index 447d8560ecb6..3f1f58d3a729 100644
--- a/tools/testing/selftests/bpf/prog_tests/rcu_read_lock.c
+++ b/tools/testing/selftests/bpf/prog_tests/rcu_read_lock.c
@@ -25,10 +25,10 @@ static void test_success(void)
 
 	bpf_program__set_autoload(skel->progs.get_cgroup_id, true);
 	bpf_program__set_autoload(skel->progs.task_succ, true);
-	bpf_program__set_autoload(skel->progs.no_lock, true);
 	bpf_program__set_autoload(skel->progs.two_regions, true);
 	bpf_program__set_autoload(skel->progs.non_sleepable_1, true);
 	bpf_program__set_autoload(skel->progs.non_sleepable_2, true);
+	bpf_program__set_autoload(skel->progs.task_trusted_non_rcuptr, true);
 	err = rcu_read_lock__load(skel);
 	if (!ASSERT_OK(err, "skel_load"))
 		goto out;
@@ -69,6 +69,7 @@ out:
 
 static const char * const inproper_region_tests[] = {
 	"miss_lock",
+	"no_lock",
 	"miss_unlock",
 	"non_sleepable_rcu_mismatch",
 	"inproper_sleepable_helper",
@@ -99,7 +100,6 @@ out:
 }
 
 static const char * const rcuptr_misuse_tests[] = {
-	"task_untrusted_non_rcuptr",
 	"task_untrusted_rcuptr",
 	"cross_rcu_region",
 };
@@ -128,17 +128,8 @@ out:
 
 void test_rcu_read_lock(void)
 {
-	struct btf *vmlinux_btf;
 	int cgroup_fd;
 
-	vmlinux_btf = btf__load_vmlinux_btf();
-	if (!ASSERT_OK_PTR(vmlinux_btf, "could not load vmlinux BTF"))
-		return;
-	if (btf__find_by_name_kind(vmlinux_btf, "rcu", BTF_KIND_TYPE_TAG) < 0) {
-		test__skip();
-		goto out;
-	}
-
 	cgroup_fd = test__join_cgroup("/rcu_read_lock");
 	if (!ASSERT_GE(cgroup_fd, 0, "join_cgroup /rcu_read_lock"))
 		goto out;
@@ -153,6 +144,5 @@ void test_rcu_read_lock(void)
 	if (test__start_subtest("negative_tests_rcuptr_misuse"))
 		test_rcuptr_misuse();
 	close(cgroup_fd);
-out:
-	btf__free(vmlinux_btf);
+out:;
 }