Re: [PATCH bpf-next v6] bpf: Fix use-after-free on mm_struct in bpf_find_vma()

[Sanghyun Park]

Sure,

I’ll take a look at that issue once the current patch is sorted out.

2026년 7월 7일 (화) 오후 4:44, Daniel Borkmann <daniel@xxxxxxxxxxxxx>님이 작성:
>
> On 7/6/26 1:28 PM, Puranjay Mohan wrote:
> > On Wed, Jul 1, 2026 at 8:48 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> >>
> >> On Mon, Jun 29, 2026 at 8:25 PM Sanghyun Park
> >> <sanghyun.park.cnu@xxxxxxxxx> wrote:
> >>>
> >>> bpf_find_vma() reads task->mm and calls mmap_read_trylock(mm) without
> >>> holding a reference on the mm. On a foreign task, a concurrent exit_mm()
> >>> can free the mm_struct between the lockless read and the trylock,
> >>> resulting in a use-after-free. mm_struct is not SLAB_TYPESAFE_BY_RCU.
> >>>
> >>> For the current task, task->mm is stable. For a foreign task, pin the mm
> >>> under task->alloc_lock and release it with mmput_async(), mirroring commit
> >>> d8e27d2d22b6 ("bpf: fix mm lifecycle in open-coded task_vma iterator").
> >>> Use spin_trylock() instead of get_task_mm() so BPF context does not block
> >>> on alloc_lock. Reject irqs-disabled contexts and !CONFIG_MMU on the
> >>> foreign-task path because dropping the mm reference is not safe there.
> >>>
> >>> Race:
> >>>
> >>>    CPU0 (BPF program)                  CPU1 (exiting task)
> >>>    ============================        ==========================
> >>>    bpf_find_vma(foreign_task):
> >>>      mm = task->mm
> >>>                                        exit_mm():
> >>>                                          task->mm = NULL
> >>>                                          mmput(mm) -> frees mm_struct
> >>>      mmap_read_trylock(mm)
> >>>          // UAF on mm
> >>>
> >>> Fixes: 7c7e3d31e785 ("bpf: Introduce helper bpf_find_vma")
> >>> Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> >>> Signed-off-by: Sanghyun Park <sanghyun.park.cnu@xxxxxxxxx>
> >>> ---
> >>
> >> Puranjay, PTAL, I think you've been fixing similar issue in
> >> bpf_iter_task_vma_new(), would be nice for you to check this fix as
> >> well. Thanks!
> >>
> >>> v6:
> >>>   - Restore bpf_iter_mmput_async() because bpf_iter_task_vma_destroy()
> >>>     still uses it.
> >>>   - Keep the direct mmput_async() call in bpf_find_vma(), but guard it with
> >>>     CONFIG_MMU because mmput_async() is declared only for CONFIG_MMU builds.
> >>> v5: https://lore.kernel.org/bpf/20260630023443.3026627-2-sanghyun.park.cnu@xxxxxxxxx/
> >>>   - Replace bpf_iter_mmput_async() with mmput_async() directly as suggested
> >>>     by Yonghong.
> >>>   - Add Yonghong's Acked-by.
> >>> v4: https://lore.kernel.org/bpf/20260610024637.343364-1-sanghyun.park.cnu@xxxxxxxxx/
> >>>   - Use [PATCH bpf-next] subject as requested by Alexei.
> >>>   - Add the missing BPF maintainers/reviewers to Cc.
> >>> v3: https://lore.kernel.org/bpf/20260609105216.3536839-1-sanghyun.park.cnu@xxxxxxxxx/
> >>>   - Drop get_task_mm()+mmput(); mirror d8e27d2d22b6 with alloc_lock
> >>>     trylock + mmput_async(). (Yonghong Song)
> >>>   - Reject irqs-disabled contexts on the foreign-task path.
> >>>   - Reject foreign-task path when !CONFIG_MMU: bpf_iter_mmput_async()
> >>>     falls back to mmput() which may sleep, and bpf_find_vma() can run
> >>>     in non-sleepable context.
> >>>   - Shorten the foreign-task rationale comment and trim the changelog body.
> >>>   - Fix the v2's whitespace damage.
> >>> v2: https://lore.kernel.org/bpf/CAOrxSK5_7e4114VyfEU9htGi+UneuNt88fGVKOAa3_ZenPOFkA@xxxxxxxxxxxxxx/
> >>>
> >>>   kernel/bpf/task_iter.c | 36 +++++++++++++++++++++++++++++++++---
> >>>   1 file changed, 33 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> >>> index fc5f463ca529a..1d56193caf5e4 100644
> >>> --- a/kernel/bpf/task_iter.c
> >>> +++ b/kernel/bpf/task_iter.c
> >>> @@ -760,6 +760,7 @@ BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
> >>>          struct mmap_unlock_irq_work *work = NULL;
> >>>          struct vm_area_struct *vma;
> >>>          bool irq_work_busy = false;
> >>> +       bool mmput_needed = false;
> >>>          struct mm_struct *mm;
> >>>          int ret = -ENOENT;
> >>>
> >>> @@ -769,14 +770,38 @@ BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
> >>>          if (!task)
> >>>                  return -ENOENT;
> >>>
> >>> -       mm = task->mm;
> >>> +       if (task == current) {
> >>> +               mm = task->mm;
> >>> +       } else {
> >>> +               /*
> >>> +                * Foreign task: pin task->mm against a concurrent exit_mm().
> >>> +                * Use trylock on alloc_lock instead of get_task_mm()'s
> >>> +                * blocking task_lock() to avoid deadlocking the target task.
> >>> +                */
> >>> +               if (!IS_ENABLED(CONFIG_MMU))
> >>> +                       return -EOPNOTSUPP;
> >>> +               if (irqs_disabled())
> >>> +                       return -EBUSY;
> >>> +               if (!spin_trylock(&task->alloc_lock))
> >>> +                       return -EBUSY;
> >>> +               mm = task->mm;
> >>> +               if (mm && !(task->flags & PF_KTHREAD)) {
> >>> +                       mmget(mm);
> >>> +                       mmput_needed = true;
> >>> +               } else {
> >>> +                       mm = NULL;
> >>> +               }
> >>> +               spin_unlock(&task->alloc_lock);
> >>> +       }
> >>>          if (!mm)
> >>>                  return -ENOENT;
> >>>
> >>>          irq_work_busy = bpf_mmap_unlock_get_irq_work(&work);
> >>>
> >>> -       if (irq_work_busy || !mmap_read_trylock(mm))
> >>> -               return -EBUSY;
> >>> +       if (irq_work_busy || !mmap_read_trylock(mm)) {
> >>> +               ret = -EBUSY;
> >>> +               goto out;
> >>> +       }
> >>>
> >>>          vma = find_vma(mm, start);
> >>>
> >>> @@ -786,6 +811,11 @@ BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
> >>>                  ret = 0;
> >>>          }
> >>>          bpf_mmap_unlock_mm(work, mm);
> >>> +out:
> >>> +#ifdef CONFIG_MMU
> >>> +       if (mmput_needed)
> >>> +               mmput_async(mm);
> >>> +#endif
> >>>          return ret;
> >>>   }
> >>>
> >>> --
> >>> 2.48.1
> >
> > Thanks for this, it looks good to me.
> >
> > Reviewed-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
>
> Applied, thanks! Could either you or Sanghyun look into the pre-existing
> sashiko-reported issue as well?