Re: [PATCH bpf-next v4] bpf: Fix use-after-free on mm_struct in bpf_find_vma()

[Yonghong Song <yonghong.song@linux.dev>]

On 6/9/26 7:46 PM, Sanghyun Park 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")
> Signed-off-by: Sanghyun Park <sanghyun.park.cnu@gmail.com>

LGTM with a nit below.

Acked-by: Yonghong Song <yonghong.song@linux.dev>

> ---
> v4:
>   - 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@gmail.com/
>   - 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@mail.gmail.com/
>
>   kernel/bpf/task_iter.c | 50 +++++++++++++++++++++++++++++++++++-------
>   1 file changed, 42 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index fc5f463ca5..baee813290 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -754,12 +754,22 @@ static struct bpf_iter_reg task_vma_reg_info = {
>   	.show_fdinfo		= bpf_iter_task_show_fdinfo,
>   };
>   
> +static inline void bpf_iter_mmput_async(struct mm_struct *mm)
> +{
> +#ifdef CONFIG_MMU
> +	mmput_async(mm);
> +#else
> +	mmput(mm);
> +#endif
> +}
> +
>   BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
>   	   bpf_callback_t, callback_fn, void *, callback_ctx, u64, flags)
>   {
>   	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 +779,38 @@ 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 +820,9 @@ BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
>   		ret = 0;
>   	}
>   	bpf_mmap_unlock_mm(work, mm);
> +out:
> +	if (mmput_needed)
> +		bpf_iter_mmput_async(mm);

mmput_needed is true requiring CONFIG_MMU enabled, so the above
bpf_iter_mmput_async(mm) can be replaced with mmput_async(mm).
This will make code easier to understand.

>   	return ret;
>   }
>   
> @@ -800,15 +837,6 @@ const struct bpf_func_proto bpf_find_vma_proto = {
>   	.arg5_type	= ARG_ANYTHING,
>   };
>   
> -static inline void bpf_iter_mmput_async(struct mm_struct *mm)
> -{
> -#ifdef CONFIG_MMU
> -	mmput_async(mm);
> -#else
> -	mmput(mm);
> -#endif
> -}
> -
>   struct bpf_iter_task_vma_kern_data {
>   	struct task_struct *task;
>   	struct mm_struct *mm;