* [PATCH bpf 1/2] bpf: Fix use-after-free of arena VMA on fork
2026-04-11 11:20 [PATCH bpf 0/2] bpf: Fix arena VMA use-after-free on fork Weiming Shi
@ 2026-04-11 11:20 ` Weiming Shi
2026-04-11 11:20 ` [PATCH bpf 2/2] selftests/bpf: Add test for arena VMA use-after-free " Weiming Shi
1 sibling, 0 replies; 3+ messages in thread
From: Weiming Shi @ 2026-04-11 11:20 UTC (permalink / raw)
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, linux-kernel, Xiang Mei, Weiming Shi, stable
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:
<TASK>
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
</TASK>
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 !=
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 <bestswngs@gmail.com>
---
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 *map)
struct vma_list {
struct vm_area_struct *vma;
struct list_head head;
- refcount_t mmap_count;
};
static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
@@ -327,7 +326,6 @@ static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
vml = kmalloc_obj(*vml);
if (!vml)
return -ENOMEM;
- refcount_set(&vml->mmap_count, 1);
vma->vm_private_data = vml;
vml->vma = vma;
list_add(&vml->head, &arena->vma_list);
@@ -336,9 +334,28 @@ static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma)
static void arena_vm_open(struct vm_area_struct *vma)
{
+ struct bpf_map *map = vma->vm_file->private_data;
+ struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
struct vma_list *vml = vma->vm_private_data;
- 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 != vma)
+ vma->vm_private_data = NULL;
+
+ if (vma->vm_private_data)
+ return;
+
+ vml = kmalloc_obj(*vml);
+ if (!vml)
+ return;
+ vml->vma = vma;
+ vma->vm_private_data = vml;
+ guard(mutex)(&arena->lock);
+ list_add(&vml->head, &arena->vma_list);
}
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 = container_of(map, struct bpf_arena, map);
struct vma_list *vml = vma->vm_private_data;
- 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 = NULL;
kfree(vml);
--
2.43.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* [PATCH bpf 2/2] selftests/bpf: Add test for arena VMA use-after-free on fork
2026-04-11 11:20 [PATCH bpf 0/2] bpf: Fix arena VMA use-after-free on fork Weiming Shi
2026-04-11 11:20 ` [PATCH bpf 1/2] bpf: Fix use-after-free of arena VMA " Weiming Shi
@ 2026-04-11 11:20 ` Weiming Shi
1 sibling, 0 replies; 3+ messages in thread
From: Weiming Shi @ 2026-04-11 11:20 UTC (permalink / raw)
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, linux-kernel, Xiang Mei, Weiming Shi
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
^ permalink raw reply related [flat|nested] 3+ messages in thread