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

[Juergen Gross <jgross@suse.com>]

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On 23.04.21 08:55, Luca Fancellu wrote:
> 
> 
>> On 23 Apr 2021, at 06:40, Juergen Gross <jgross@suse.com> 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@vger.kernel.org
>> Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
>> Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> 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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