xen/gntdev: fix gntdev_mmap() error exit path

Commit d3eeb1d ("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: d3eeb1d ("xen/gntdev: use mmu_interval_notifier_insert")
Cc: [email protected]
Reported-by: Marek Marczykowski-Górecki <[email protected]>
Tested-by: Marek Marczykowski-Górecki <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>
Reviewed-by: Luca Fancellu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Juergen Gross <[email protected]>

Author: jgross1

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);