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Sat, 11 Apr 2026 13:08:59 -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 , Emil Tsalapatis , bpf@vger.kernel.org, Xiang Mei , Weiming Shi Subject: [PATCH bpf v3 1/2] bpf: Fix use-after-free of arena VMA on fork Date: Sat, 11 Apr 2026 13:08:40 -0700 Message-ID: <20260411200840.1793567-3-bestswngs@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260411200840.1793567-2-bestswngs@gmail.com> References: <20260411200840.1793567-2-bestswngs@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 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 tracking each child VMA separately. arena_vm_open() now clears the inherited vm_private_data and calls remember_vma() to register a fresh vma_list entry for the new VMA. arena_vm_close() unconditionally removes and frees the entry. The shared refcount is no longer needed and is removed. Also add arena_vm_may_split() returning -EINVAL to prevent VMA splitting, which would break the pgoff arithmetic in arena_vm_fault(). Fixes: b90d77e5fd78 ("bpf: Fix remap of arena.") Reported-by: Xiang Mei Signed-off-by: Weiming Shi --- kernel/bpf/arena.c | 23 +++++++++++++++++------ 1 file changed, 17 insertions(+), 6 deletions(-) diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c index f355cf1c1a16..3462c4463617 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 +334,17 @@ static int remember_vma(struct bpf_arena *arena, struct vm_area_struct *vma) static void arena_vm_open(struct vm_area_struct *vma) { - struct vma_list *vml = vma->vm_private_data; + struct bpf_map *map = vma->vm_file->private_data; + struct bpf_arena *arena = container_of(map, struct bpf_arena, map); - refcount_inc(&vml->mmap_count); + /* + * vm_private_data points to the parent's vma_list entry after fork. + * Clear it and register this VMA separately. + */ + vma->vm_private_data = NULL; + guard(mutex)(&arena->lock); + /* OOM is silently ignored; arena_vm_close() handles NULL. */ + remember_vma(arena, vma); } static void arena_vm_close(struct vm_area_struct *vma) @@ -347,10 +353,9 @@ 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); @@ -415,9 +420,15 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf) return VM_FAULT_SIGSEGV; } +static int arena_vm_may_split(struct vm_area_struct *vma, unsigned long addr) +{ + return -EINVAL; +} + static const struct vm_operations_struct arena_vm_ops = { .open = arena_vm_open, .close = arena_vm_close, + .may_split = arena_vm_may_split, .fault = arena_vm_fault, }; -- 2.43.0