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

[Juergen Gross]

On 23.04.21 08:55, Luca Fancellu wrote:
> > 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?

map->vma will be NULL in this case, so there will be no problem
resulting from that path.


Juergen

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