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Sat, 11 Apr 2026 04:21:24 -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 , stable@vger.kernel.org Subject: [PATCH bpf 1/2] bpf: Fix use-after-free of arena VMA on fork Date: Sat, 11 Apr 2026 04:20:50 -0700 Message-ID: <20260411112050.1454548-3-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: quoted-printable Content-Type: text/plain; charset="utf-8" arena_vm_open() only increments a refcount on the shared vma_list entry but never registers the new VMA or updates the stored vma pointer. When the original VMA is unmapped while a forked/split copy still exists, arena_vm_close() drops the refcount without freeing the vma_list entry. The entry's vma pointer now refers to a freed vm_area_struct. A subsequent bpf_arena_free_pages() call iterates vma_list and passes the dangling pointer to zap_page_range_single(), causing a use-after-free. The bug is reachable by any process with CAP_BPF and CAP_PERFMON that can create a BPF_MAP_TYPE_ARENA, mmap it, and fork. It triggers deterministically -- no race condition is involved. BUG: KASAN: slab-use-after-free in zap_page_range_single (mm/memory.c:2234) Call Trace: zap_page_range_single+0x101/0x110 mm/memory.c:2234 zap_pages+0x80/0xf0 kernel/bpf/arena.c:658 arena_free_pages+0x67a/0x860 kernel/bpf/arena.c:712 bpf_prog_test_run_syscall+0x3da net/bpf/test_run.c:1640 __sys_bpf+0x1662/0x50b0 kernel/bpf/syscall.c:6267 __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:6360 do_syscall_64+0xf1/0x530 arch/x86/entry/syscall_64.c:63 entry_SYSCALL_64_after_hwframe+0x77 arch/x86/entry/entry_64.S:130 Fix this by giving each VMA its own vma_list entry, following the HugeTLB vma_lock pattern (hugetlb_vm_op_open). arena_vm_open() now detects an inherited vm_private_data pointer via the vma_lock->vma !=3D vma check, clears it, and allocates a fresh entry for the new VMA. arena_vm_close() unconditionally removes and frees the entry. The shared refcount is no longer needed and is removed. Fixes: b90d77e5fd78 ("bpf: Fix remap of arena.") Cc: stable@vger.kernel.org Signed-off-by: Weiming Shi --- kernel/bpf/arena.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-) diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c index f355cf1c1a16..3a156ec473a8 100644 --- a/kernel/bpf/arena.c +++ b/kernel/bpf/arena.c @@ -317,7 +317,6 @@ static u64 arena_map_mem_usage(const struct bpf_map *ma= p) struct vma_list { struct vm_area_struct *vma; struct list_head head; - refcount_t mmap_count; }; =20 static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vm= a) @@ -327,7 +326,6 @@ static int remember_vma(struct bpf_arena *arena, struct= vm_area_struct *vma) vml =3D kmalloc_obj(*vml); if (!vml) return -ENOMEM; - refcount_set(&vml->mmap_count, 1); vma->vm_private_data =3D vml; vml->vma =3D vma; list_add(&vml->head, &arena->vma_list); @@ -336,9 +334,28 @@ static int remember_vma(struct bpf_arena *arena, struc= t vm_area_struct *vma) =20 static void arena_vm_open(struct vm_area_struct *vma) { + struct bpf_map *map =3D vma->vm_file->private_data; + struct bpf_arena *arena =3D container_of(map, struct bpf_arena, map); struct vma_list *vml =3D vma->vm_private_data; =20 - refcount_inc(&vml->mmap_count); + /* + * If vm_private_data points to a vma_list for a different VMA, it was + * inherited via vm_area_dup (fork or split). Clear it and allocate a + * fresh entry for this VMA, following the HugeTLB vma_lock pattern. + */ + if (vml && vml->vma !=3D vma) + vma->vm_private_data =3D NULL; + + if (vma->vm_private_data) + return; + + vml =3D kmalloc_obj(*vml); + if (!vml) + return; + vml->vma =3D vma; + vma->vm_private_data =3D vml; + guard(mutex)(&arena->lock); + list_add(&vml->head, &arena->vma_list); } =20 static void arena_vm_close(struct vm_area_struct *vma) @@ -347,10 +364,9 @@ static void arena_vm_close(struct vm_area_struct *vma) struct bpf_arena *arena =3D container_of(map, struct bpf_arena, map); struct vma_list *vml =3D vma->vm_private_data; =20 - if (!refcount_dec_and_test(&vml->mmap_count)) + if (!vml) return; guard(mutex)(&arena->lock); - /* update link list under lock */ list_del(&vml->head); vma->vm_private_data =3D NULL; kfree(vml); --=20 2.43.0 From nobody Mon Jun 15 15:06:17 2026 Received: from mail-dy1-f175.google.com (mail-dy1-f175.google.com [74.125.82.175]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8944333F587 for ; 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charset="utf-8" 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/te= sting/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 register= ed + * 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 =3D {}; + __u32 info_len =3D sizeof(info); + struct arena_fork *skel; + size_t arena_sz; + void *arena_addr; + int arena_fd, ret, status; + pid_t pid; + + skel =3D arena_fork__open_and_load(); + if (!ASSERT_OK_PTR(skel, "open_and_load")) + return; + + arena_fd =3D bpf_map__fd(skel->maps.arena); + + /* libbpf mmaps the arena via initial_value */ + arena_addr =3D 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 =3D (size_t)info.max_entries * PAGE_SIZE; + + /* Allocate 4 pages in the arena via BPF */ + ret =3D 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] =3D 'A'; + + /* Fork: child inherits the arena VMA */ + pid =3D fork(); + if (!ASSERT_GE(pid, 0, "fork")) + goto out; + + if (pid =3D=3D 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 =3D bpf_program__fd(skel->progs.arena_free); + + usleep(200000); /* let parent munmap first */ + + ret =3D 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 =3D bpf_arena_alloc_pages(&arena, NULL, 4, NUMA_NO_NODE, 0); + if (!p) + return 1; + alloc_addr =3D 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") =3D "GPL"; --=20 2.43.0