Re: [PATCH v3 2/2] kcov: properly handle subsequent mmap calls

[Andrey Konovalov]

On Mon, Jan 17, 2022 at 4:37 PM Aleksandr Nogikh <nogikh@xxxxxxxxxx> wrote:
>
> Allocate the kcov buffer during KCOV_MODE_INIT in order to untie mmapping
> of a kcov instance and the actual coverage collection process. Modify
> kcov_mmap, so that it can be reliably used any number of times once
> KCOV_MODE_INIT has succeeded.
>
> These changes to the user-facing interface of the tool only weaken the
> preconditions, so all existing user space code should remain compatible
> with the new version.
>
> Signed-off-by: Aleksandr Nogikh <nogikh@xxxxxxxxxx>
> ---
>  kernel/kcov.c | 34 +++++++++++++++-------------------
>  1 file changed, 15 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index e1be7301500b..475524bd900a 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -459,37 +459,28 @@ void kcov_task_exit(struct task_struct *t)
>  static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
>  {
>         int res = 0;
> -       void *area;
>         struct kcov *kcov = vma->vm_file->private_data;
>         unsigned long size, off;
>         struct page *page;
>         unsigned long flags;
>
> -       area = vmalloc_user(vma->vm_end - vma->vm_start);
> -       if (!area)
> -               return -ENOMEM;
> -
>         spin_lock_irqsave(&kcov->lock, flags);
>         size = kcov->size * sizeof(unsigned long);
> -       if (kcov->mode != KCOV_MODE_INIT || vma->vm_pgoff != 0 ||
> +       if (kcov->area == NULL || vma->vm_pgoff != 0 ||
>             vma->vm_end - vma->vm_start != size) {
>                 res = -EINVAL;
>                 goto exit;
>         }
> -       if (!kcov->area) {
> -               kcov->area = area;
> -               vma->vm_flags |= VM_DONTEXPAND;
> -               spin_unlock_irqrestore(&kcov->lock, flags);
> -               for (off = 0; off < size; off += PAGE_SIZE) {
> -                       page = vmalloc_to_page(kcov->area + off);
> -                       if (vm_insert_page(vma, vma->vm_start + off, page))
> -                               WARN_ONCE(1, "vm_insert_page() failed");
> -               }
> -               return 0;
> +       spin_unlock_irqrestore(&kcov->lock, flags);
> +       vma->vm_flags |= VM_DONTEXPAND;
> +       for (off = 0; off < size; off += PAGE_SIZE) {
> +               page = vmalloc_to_page(kcov->area + off);

Hm, you're accessing kcov->area without the lock here. Although, the
old code does this as well. This is probably OK, as kcov->area can't
be changed nor freed while this handler is executing.


> +               if (vm_insert_page(vma, vma->vm_start + off, page))
> +                       WARN_ONCE(1, "vm_insert_page() failed");
>         }
> +       return 0;
>  exit:
>         spin_unlock_irqrestore(&kcov->lock, flags);
> -       vfree(area);
>         return res;
>  }
>
> @@ -674,6 +665,7 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
>         unsigned int remote_num_handles;
>         unsigned long remote_arg_size;
>         unsigned long size, flags;
> +       void *area;
>
>         kcov = filep->private_data;
>         switch (cmd) {
> @@ -683,17 +675,21 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
>                  * Must happen before anything else.
>                  *
>                  * First check the size argument - it must be at least 2
> -                * to hold the current position and one PC. Later we allocate
> -                * size * sizeof(unsigned long) memory, that must not overflow.
> +                * to hold the current position and one PC.
>                  */
>                 size = arg;
>                 if (size < 2 || size > INT_MAX / sizeof(unsigned long))
>                         return -EINVAL;
> +               area = vmalloc_user(size * sizeof(unsigned long));
> +               if (area == NULL)
> +                       return -ENOMEM;
>                 spin_lock_irqsave(&kcov->lock, flags);
>                 if (kcov->mode != KCOV_MODE_DISABLED) {
>                         spin_unlock_irqrestore(&kcov->lock, flags);
> +                       vfree(area);
>                         return -EBUSY;
>                 }
> +               kcov->area = area;
>                 kcov->size = size;
>                 kcov->mode = KCOV_MODE_INIT;
>                 spin_unlock_irqrestore(&kcov->lock, flags);
> --
> 2.34.1.703.g22d0c6ccf7-goog
>

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>