linux-kernel - Re: [PATCH v2] vduse: Validate vq_num in vduse_validate

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Date:   Mon, 28 Nov 2022 12:13:10 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Dan Carpenter <error27@...il.com>,
        Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
Cc:     harshit.m.mogalapalli@...il.com,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Xie Yongji <xieyongji@...edance.com>,
        Gautam Dawar <gautam.dawar@...inx.com>,
        Parav Pandit <parav@...dia.com>, Eli Cohen <elic@...dia.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vduse: Validate vq_num in vduse_validate_config()

On Mon, Nov 28, 2022 at 01:58:00PM +0300, Dan Carpenter wrote:
>On Mon, Nov 28, 2022 at 11:53:12AM +0100, Stefano Garzarella wrote:
>> On Mon, Nov 28, 2022 at 12:36:26AM -0800, Harshit Mogalapalli wrote:
>> > Add a limit to 'config->vq_num' which is user controlled data which
>> > comes from an vduse_ioctl to prevent large memory allocations.
>> >
>> > This is found using static analysis with smatch.
>> >
>> > Suggested-by: Michael S. Tsirkin <mst@...hat.com>
>> > Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
>> > ---
>> > v1->v2: Change title of the commit and description, add a limit to
>> > 	vq_num.
>> >
>> > Note: I think here 0xffff is the max size of vring =  no: of vqueues.
>> > Only compile and boot tested.
>> > ---
>> > drivers/vdpa/vdpa_user/vduse_dev.c | 3 +++
>> > 1 file changed, 3 insertions(+)
>> >
>> > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
>> > index 35dceee3ed56..31017ebc4d7c 100644
>> > --- a/drivers/vdpa/vdpa_user/vduse_dev.c
>> > +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
>> > @@ -1440,6 +1440,9 @@ static bool vduse_validate_config(struct vduse_dev_config *config)
>> > 	if (config->config_size > PAGE_SIZE)
>> > 		return false;
>> >
>> > +	if (config->vq_num > 0xffff)
>>
>> What about using U16_MAX here?
>
>Where is the ->vq_num stored in a u16?  I looked for this but didn't
>see it.

I thought vq_num referred to the number of elements in the vq (like 
.get_vq_num_max), since this patch wants to limit to 0xffff.

But it actually refers to the number of virtqueue, so @Harshit why do we 
limit it to 0xffff?

Maybe we should explain it in the commit message or in a comment.

Thanks,
Stefano