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

[Andrii Nakryiko]

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