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

[Alexei Starovoitov]

On Sun, Apr 12, 2026 at 10:50 AM Emil Tsalapatis <emil@xxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxx>
> > Signed-off-by: Weiming Shi <bestswngs@xxxxxxxxx>
> > ---
> >  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?

+1

Weiming,

you should stop trusting AI so blindly.
First, analyze the root cause (the first paragraph of the commit log).
Is this really the case?

Second, I copy pasted it to claude and got the same "fix" back,
but implemented without your bug:
+       vml = kmalloc_obj(*vml);
+       if (!vml) {
+               vma->vm_private_data = NULL;
+               return;
+       }
+       vml->vma = vma;
+       vma->vm_private_data = vml;
+       guard(mutex)(&arena->lock);
+       list_add(&vml->head, &arena->vma_list);

at least this part is kinda makes sense...

and, of course, this part too:

-       if (!refcount_dec_and_test(&vml->mmap_count))
+       if (!vml)
                return;

when you look at it you MUST ask AI back:
"Is this buggy?"

and it will reply:
"
Right — silently dropping the VMA from the list means zap_pages()
won't unmap pages from it, which is a correctness problem, not just
degraded behavior. Since vm_open can't fail, the allocation should use
__GFP_NOFAIL. The struct is tiny so that's fine.
"

and it proceeded adding __GFP_NOFAIL.

which is wrong too.

So please don't just throw broken patches at maintainers.
Do your homework. Fixing one maybe-bug and introducing
more real bugs is not a step forward.

pw-bot: cr