Merge branch 'remove-use-of-current-cgns-in-bpf_cgroup_from_id'

Kumar Kartikeya Dwivedi says:

====================
Remove use of current->cgns in bpf_cgroup_from_id

bpf_cgroup_from_id currently ends up doing a check on whether the cgroup
being looked up is a descendant of the root cgroup of the current task's
cgroup namespace. This leads to unreliable results since this kfunc can
be invoked from any arbitrary context, for any arbitrary value of
current. Fix this by removing namespace-awarness in the kfunc, and
include a test that detects such a case and fails without the fix.

Changelog:
----------
v2 -> v3
v2: https://lore.kernel.org/bpf/20250811195901.1651800-1-memxor@gmail.com

 * Refactor cgroup_get_from_id into non-ns version. (Andrii)
 * Address nits from Eduard.

v1 -> v2
v1: https://lore.kernel.org/bpf/20250811175045.1055202-1-memxor@gmail.com

 * Add Ack from Tejun.
 * Fix selftest to perform namespace migration and cgroup setup in a
   child process to avoid changing test_progs namespace.
====================

Link: https://patch.msgid.link/20250915032618.1551762-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

7 files changed, 126 insertions, 5 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b18fb5fcb38e..b08c8e62881c 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -650,6 +650,7 @@ static inline void cgroup_kthread_ready(void)
 }
 
 void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen);
+struct cgroup *__cgroup_get_from_id(u64 id);
 struct cgroup *cgroup_get_from_id(u64 id);
 #else /* !CONFIG_CGROUPS */
 
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index c0c0764a2025..51229aba5318 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2546,7 +2546,7 @@ __bpf_kfunc struct cgroup *bpf_cgroup_from_id(u64 cgid)
 {
 	struct cgroup *cgrp;
 
-	cgrp = cgroup_get_from_id(cgid);
+	cgrp = __cgroup_get_from_id(cgid);
 	if (IS_ERR(cgrp))
 		return NULL;
 	return cgrp;
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 312c6a8b55bb..6c2d20ac697c 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6343,15 +6343,15 @@ void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen)
 }
 
 /*
- * cgroup_get_from_id : get the cgroup associated with cgroup id
+ * __cgroup_get_from_id : get the cgroup associated with cgroup id
  * @id: cgroup id
  * On success return the cgrp or ERR_PTR on failure
- * Only cgroups within current task's cgroup NS are valid.
+ * There are no cgroup NS restrictions.
  */
-struct cgroup *cgroup_get_from_id(u64 id)
+struct cgroup *__cgroup_get_from_id(u64 id)
 {
 	struct kernfs_node *kn;
-	struct cgroup *cgrp, *root_cgrp;
+	struct cgroup *cgrp;
 
 	kn = kernfs_find_and_get_node_by_id(cgrp_dfl_root.kf_root, id);
 	if (!kn)
@@ -6373,6 +6373,22 @@ struct cgroup *cgroup_get_from_id(u64 id)
 
 	if (!cgrp)
 		return ERR_PTR(-ENOENT);
+	return cgrp;
+}
+
+/*
+ * cgroup_get_from_id : get the cgroup associated with cgroup id
+ * @id: cgroup id
+ * On success return the cgrp or ERR_PTR on failure
+ * Only cgroups within current task's cgroup NS are valid.
+ */
+struct cgroup *cgroup_get_from_id(u64 id)
+{
+	struct cgroup *cgrp, *root_cgrp;
+
+	cgrp = __cgroup_get_from_id(id);
+	if (IS_ERR(cgrp))
+		return cgrp;
 
 	root_cgrp = current_cgns_cgroup_dfl();
 	if (!cgroup_is_descendant(cgrp, root_cgrp)) {
diff --git a/tools/testing/selftests/bpf/cgroup_helpers.c b/tools/testing/selftests/bpf/cgroup_helpers.c
index 15f626014872..20cede4db3ce 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.c
+++ b/tools/testing/selftests/bpf/cgroup_helpers.c
@@ -412,6 +412,26 @@ void remove_cgroup(const char *relative_path)
 		log_err("rmdiring cgroup %s .. %s", relative_path, cgroup_path);
 }
 
+/*
+ * remove_cgroup_pid() - Remove a cgroup setup by process identified by PID
+ * @relative_path: The cgroup path, relative to the workdir, to remove
+ * @pid: PID to be used to find cgroup_path
+ *
+ * This function expects a cgroup to already be created, relative to the cgroup
+ * work dir. It also expects the cgroup doesn't have any children or live
+ * processes and it removes the cgroup.
+ *
+ * On failure, it will print an error to stderr.
+ */
+void remove_cgroup_pid(const char *relative_path, int pid)
+{
+	char cgroup_path[PATH_MAX + 1];
+
+	format_cgroup_path_pid(cgroup_path, relative_path, pid);
+	if (rmdir(cgroup_path))
+		log_err("rmdiring cgroup %s .. %s", relative_path, cgroup_path);
+}
+
 /**
  * create_and_get_cgroup() - Create a cgroup, relative to workdir, and get the FD
  * @relative_path: The cgroup path, relative to the workdir, to join
diff --git a/tools/testing/selftests/bpf/cgroup_helpers.h b/tools/testing/selftests/bpf/cgroup_helpers.h
index 182e1ac36c95..3857304be874 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.h
+++ b/tools/testing/selftests/bpf/cgroup_helpers.h
@@ -19,6 +19,7 @@ int cgroup_setup_and_join(const char *relative_path);
 int get_root_cgroup(void);
 int create_and_get_cgroup(const char *relative_path);
 void remove_cgroup(const char *relative_path);
+void remove_cgroup_pid(const char *relative_path, int pid);
 unsigned long long get_cgroup_id(const char *relative_path);
 int get_cgroup1_hierarchy_id(const char *subsys_name);
 
diff --git a/tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c b/tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c
index adda85f97058..4b42fbc96efc 100644
--- a/tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c
+++ b/tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c
@@ -4,6 +4,8 @@
 #define _GNU_SOURCE
 #include <cgroup_helpers.h>
 #include <test_progs.h>
+#include <sched.h>
+#include <sys/wait.h>
 
 #include "cgrp_kfunc_failure.skel.h"
 #include "cgrp_kfunc_success.skel.h"
@@ -87,6 +89,72 @@ static const char * const success_tests[] = {
 	"test_cgrp_from_id",
 };
 
+static void test_cgrp_from_id_ns(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct cgrp_kfunc_success *skel;
+	struct bpf_program *prog;
+	int pid, pipe_fd[2];
+
+	skel = open_load_cgrp_kfunc_skel();
+	if (!ASSERT_OK_PTR(skel, "open_load_skel"))
+		return;
+
+	if (!ASSERT_OK(skel->bss->err, "pre_mkdir_err"))
+		goto cleanup;
+
+	prog = skel->progs.test_cgrp_from_id_ns;
+
+	if (!ASSERT_OK(pipe(pipe_fd), "pipe"))
+		goto cleanup;
+
+	pid = fork();
+	if (!ASSERT_GE(pid, 0, "fork result")) {
+		close(pipe_fd[0]);
+		close(pipe_fd[1]);
+		goto cleanup;
+	}
+
+	if (pid == 0) {
+		int ret = 0;
+
+		close(pipe_fd[0]);
+
+		if (!ASSERT_GE(cgroup_setup_and_join("cgrp_from_id_ns"), 0, "join cgroup"))
+			exit(1);
+
+		if (!ASSERT_OK(unshare(CLONE_NEWCGROUP), "unshare cgns"))
+			exit(1);
+
+		ret = bpf_prog_test_run_opts(bpf_program__fd(prog), &opts);
+		if (!ASSERT_OK(ret, "test run ret"))
+			exit(1);
+
+		if (!ASSERT_OK(opts.retval, "test run retval"))
+			exit(1);
+
+		if (!ASSERT_EQ(write(pipe_fd[1], &ret, sizeof(ret)), sizeof(ret), "write pipe"))
+			exit(1);
+
+		exit(0);
+	} else {
+		int res;
+
+		close(pipe_fd[1]);
+
+		ASSERT_EQ(read(pipe_fd[0], &res, sizeof(res)), sizeof(res), "read res");
+		ASSERT_EQ(waitpid(pid, NULL, 0), pid, "wait on child");
+
+		remove_cgroup_pid("cgrp_from_id_ns", pid);
+
+		ASSERT_OK(res, "result from run");
+	}
+
+	close(pipe_fd[0]);
+cleanup:
+	cgrp_kfunc_success__destroy(skel);
+}
+
 void test_cgrp_kfunc(void)
 {
 	int i, err;
@@ -102,6 +170,9 @@ void test_cgrp_kfunc(void)
 		run_success_test(success_tests[i]);
 	}
 
+	if (test__start_subtest("test_cgrp_from_id_ns"))
+		test_cgrp_from_id_ns();
+
 	RUN_TESTS(cgrp_kfunc_failure);
 
 cleanup:
diff --git a/tools/testing/selftests/bpf/progs/cgrp_kfunc_success.c b/tools/testing/selftests/bpf/progs/cgrp_kfunc_success.c
index 5354455a01be..02d8f160ca0e 100644
--- a/tools/testing/selftests/bpf/progs/cgrp_kfunc_success.c
+++ b/tools/testing/selftests/bpf/progs/cgrp_kfunc_success.c
@@ -221,3 +221,15 @@ int BPF_PROG(test_cgrp_from_id, struct cgroup *cgrp, const char *path)
 
 	return 0;
 }
+
+SEC("syscall")
+int test_cgrp_from_id_ns(void *ctx)
+{
+	struct cgroup *cg;
+
+	cg = bpf_cgroup_from_id(1);
+	if (!cg)
+		return 42;
+	bpf_cgroup_release(cg);
+	return 0;
+}