Re: [PATCH] xen/gntdev: fix gntdev_mmap() error exit path

From: Luca Fancellu
Date: Fri Apr 23 2021 - 02:56:03 EST




> On 23 Apr 2021, at 06:40, Juergen Gross <jgross@xxxxxxxx> wrote:
>
> Commit d3eeb1d77c5d0af ("xen/gntdev: use mmu_interval_notifier_insert")
> introduced an error in gntdev_mmap(): in case the call of
> mmu_interval_notifier_insert_locked() fails the exit path should not
> call mmu_interval_notifier_remove(), as this might result in NULL
> dereferences.
>
> One reason for failure is e.g. a signal pending for the running
> process.
>
> Fixes: d3eeb1d77c5d0af ("xen/gntdev: use mmu_interval_notifier_insert")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> drivers/xen/gntdev.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index f01d58c7a042..a3e7be96527d 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -1017,8 +1017,10 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
> err = mmu_interval_notifier_insert_locked(
> &map->notifier, vma->vm_mm, vma->vm_start,
> vma->vm_end - vma->vm_start, &gntdev_mmu_ops);
> - if (err)
> + if (err) {
> + map->vma = NULL;
> goto out_unlock_put;
> + }
> }
> mutex_unlock(&priv->lock);
>
> --
> 2.26.2
>
>

Hi Juergen,

I can see from the code that there is another path to out_unlock_put label some lines before:

[…]
vma->vm_private_data = map;
if (map->flags) {
if ((vma->vm_flags & VM_WRITE) &&
(map->flags & GNTMAP_readonly))
goto out_unlock_put;
} else {
map->flags = GNTMAP_host_map;
if (!(vma->vm_flags & VM_WRITE))
map->flags |= GNTMAP_readonly;
}
[…]

Can be the case that use_ptemod is != 0 also for that path?

Cheers,
Luca