Re: [PATCH] dma-buf: udmabuf: avoid list copy size overflow

[Christian König]

On 6/26/26 13:31, David Laight wrote:
> On Wed, 24 Jun 2026 14:58:58 +0200
> Christian König <christian.koenig@xxxxxxx> wrote:
> 
>> On 6/24/26 14:52, Yousef Alhouseen wrote:
>>> UDMABUF_CREATE_LIST copies an array whose element count comes from
>>> userspace. The count is compared against list_limit, but list_limit is a
>>> signed module parameter while the count is u32.  
>>
>> We should probably just drop the sign from the module parameter instead.
> 
> Does anything sanity-check the module parameter?

Do we need to? I mean shooting into your own foot is supposed to hurt.

Christian.

> 
> 	David
> 
>>
>> I don't see an use case for negative values here.
>>
>> Regards,
>> Christian.
>>
>>>
>>> If the limit is raised too far or made negative, that comparison no
>>> longer bounds the count to a range where sizeof(*list) * count fits in
>>> the u32 temporary used for the copy length. A wrapped copy length lets
>>> memdup_user() copy fewer entries than udmabuf_create() subsequently
>>> walks, leading to out-of-bounds reads from the copied list.
>>>
>>> Take a positive snapshot of the module limit and use memdup_array_user()
>>> so the multiplication is checked before copying.
>>>
>>> Signed-off-by: Yousef Alhouseen <alhouseenyousef@xxxxxxxxx>
>>> ---
>>>  drivers/dma-buf/udmabuf.c | 9 +++++----
>>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
>>> index bced421c0..b4078ec84 100644
>>> --- a/drivers/dma-buf/udmabuf.c
>>> +++ b/drivers/dma-buf/udmabuf.c
>>> @@ -469,14 +469,15 @@ static long udmabuf_ioctl_create_list(struct file *filp, unsigned long arg)
>>>         struct udmabuf_create_list head;
>>>         struct udmabuf_create_item *list;
>>>         int ret = -EINVAL;
>>> -       u32 lsize;
>>> +       int limit;
>>>
>>>         if (copy_from_user(&head, (void __user *)arg, sizeof(head)))
>>>                 return -EFAULT;
>>> -       if (head.count > list_limit)
>>> +       limit = READ_ONCE(list_limit);
>>> +       if (!head.count || limit <= 0 || head.count > limit)
>>>                 return -EINVAL;
>>> -       lsize = sizeof(struct udmabuf_create_item) * head.count;
>>> -       list = memdup_user((void __user *)(arg + sizeof(head)), lsize);
>>> +       list = memdup_array_user((void __user *)(arg + sizeof(head)),
>>> +                                head.count, sizeof(*list));
>>>         if (IS_ERR(list))
>>>                 return PTR_ERR(list);
>>>
>>> --
>>> 2.54.0
>>>   
>>
>>
>