Add a selftest that reproduces the arena VMA use-after-free fixed in
the previous commit. The test creates an arena, mmaps it, allocates
pages via BPF, forks, has the parent munmap the arena, then has the
child call bpf_arena_free_pages. Without the fix this triggers a
KASAN slab-use-after-free in zap_page_range_single.

Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 .../selftests/bpf/prog_tests/arena_fork.c     | 86 +++++++++++++++++++
 .../testing/selftests/bpf/progs/arena_fork.c  | 41 +++++++++
 2 files changed, 127 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_fork.c
 create mode 100644 tools/testing/selftests/bpf/progs/arena_fork.c

diff --git a/tools/testing/selftests/bpf/prog_tests/arena_fork.c b/tools/testing/selftests/bpf/prog_tests/arena_fork.c
new file mode 100644
index 000000000000..445574827891
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/arena_fork.c
@@ -0,0 +1,86 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 */
+
+/*
+ * Test that forking a process with an arena mmap does not cause a
+ * use-after-free when the parent unmaps and the child frees arena pages.
+ *
+ * The bug: arena_vm_open() only incremented a refcount but never registered
+ * the child's VMA. After parent munmap, vml->vma pointed to a freed
+ * vm_area_struct. bpf_arena_free_pages -> zap_pages would then UAF.
+ */
+#include <test_progs.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <sys/user.h>
+#ifndef PAGE_SIZE
+#define PAGE_SIZE getpagesize()
+#endif
+
+#include "arena_fork.skel.h"
+
+void test_arena_fork(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct bpf_map_info info = {};
+	__u32 info_len = sizeof(info);
+	struct arena_fork *skel;
+	size_t arena_sz;
+	void *arena_addr;
+	int arena_fd, ret, status;
+	pid_t pid;
+
+	skel = arena_fork__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	arena_fd = bpf_map__fd(skel->maps.arena);
+
+	/* libbpf mmaps the arena via initial_value */
+	arena_addr = bpf_map__initial_value(skel->maps.arena, &arena_sz);
+	if (!ASSERT_OK_PTR(arena_addr, "arena_mmap"))
+		goto out;
+
+	/* Get real arena byte size for munmap */
+	bpf_map_get_info_by_fd(arena_fd, &info, &info_len);
+	arena_sz = (size_t)info.max_entries * PAGE_SIZE;
+
+	/* Allocate 4 pages in the arena via BPF */
+	ret = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.arena_alloc),
+				     &opts);
+	if (!ASSERT_OK(ret, "alloc_run") ||
+	    !ASSERT_OK(opts.retval, "alloc_ret"))
+		goto out;
+
+	/* Fault in a page so zap_pages has work to do */
+	((char *)arena_addr)[0] = 'A';
+
+	/* Fork: child inherits the arena VMA */
+	pid = fork();
+	if (!ASSERT_GE(pid, 0, "fork"))
+		goto out;
+
+	if (pid == 0) {
+		/* Child: parent will unmap first, then we free pages.
+		 * Without the fix, this triggers UAF in zap_pages.
+		 */
+		LIBBPF_OPTS(bpf_test_run_opts, child_opts);
+		int free_fd = bpf_program__fd(skel->progs.arena_free);
+
+		usleep(200000); /* let parent munmap first */
+
+		ret = bpf_prog_test_run_opts(free_fd, &child_opts);
+		_exit(ret || child_opts.retval);
+	}
+
+	/* Parent: unmap the arena, making vml->vma stale */
+	munmap(arena_addr, arena_sz);
+
+	/* Wait for child -- if kernel UAFs, child will crash/hang */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status), "child_exited");
+	ASSERT_EQ(WEXITSTATUS(status), 0, "child_exit_code");
+out:
+	arena_fork__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/arena_fork.c b/tools/testing/selftests/bpf/progs/arena_fork.c
new file mode 100644
index 000000000000..b1f8435f1834
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/arena_fork.c
@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_arena_common.h"
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARENA);
+	__uint(map_flags, BPF_F_MMAPABLE);
+	__uint(max_entries, 16); /* number of pages */
+#ifdef __TARGET_ARCH_arm64
+	__ulong(map_extra, 0x1ull << 32); /* start of mmap() region */
+#else
+	__ulong(map_extra, 0x1ull << 44); /* start of mmap() region */
+#endif
+} arena SEC(".maps");
+
+void __arena *alloc_addr;
+
+SEC("syscall")
+int arena_alloc(void *ctx)
+{
+	void __arena *p;
+
+	p = bpf_arena_alloc_pages(&arena, NULL, 4, NUMA_NO_NODE, 0);
+	if (!p)
+		return 1;
+	alloc_addr = p;
+	return 0;
+}
+
+SEC("syscall")
+int arena_free(void *ctx)
+{
+	if (!alloc_addr)
+		return 1;
+	bpf_arena_free_pages(&arena, alloc_addr, 4);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.43.0