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

[Luca Fancellu <luca.fancellu@xxxxxxx>]

> 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