Re: [PATCH bpf v4 1/2] bpf: Fix use-after-free of arena VMA on fork

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Sat Apr 11, 2026 at 10:27 PM EDT, Weiming Shi wrote:
> 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:
>   <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 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. If remember_vma()
> fails due to OOM, vm_private_data stays NULL and arena_vm_close()
> skips the cleanup for that VMA. The shared refcount is no longer
> needed and is removed.
>
> Also add arena_vm_may_split() returning -EINVAL to prevent VMA
> splitting, so that arena_vm_open() only needs to handle fork and the
> vma_list tracking stays simple.
>
> Fixes: b90d77e5fd78 ("bpf: Fix remap of arena.")
> Reported-by: Xiang Mei <xmei5@asu.edu>
> Signed-off-by: Weiming Shi <bestswngs@gmail.com>
> ---
>  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. */

I don't see any way this approach gonna work, and frankly makes no sense 
to me. This patch doesn't take into account how the vma_list is actually
used. It frankly makes no sense, Please think through this: If we could
silently just not allocate the vml, why do we need it in the first place?

> +	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,
>  };
>