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Sat, 11 Apr 2026 04:21:25 -0700 (PDT) From: Weiming Shi To: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko Cc: Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Barret Rhoden , bpf@vger.kernel.org, linux-kernel@vger.kernel.org, Xiang Mei , Weiming Shi Subject: [PATCH bpf 2/2] selftests/bpf: Add test for arena VMA use-after-free on fork Date: Sat, 11 Apr 2026 04:20:51 -0700 Message-ID: <20260411112050.1454548-4-bestswngs@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260411112050.1454548-2-bestswngs@gmail.com> References: <20260411112050.1454548-2-bestswngs@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 --- .../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 +#include +#include +#include +#include +#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 +#include +#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