Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

From: Jason Wang
Date: Wed Jun 03 2020 - 02:36:58 EST

On 2020/6/3 äå2:30, Michael S. Tsirkin wrote:
On Wed, Jun 03, 2020 at 02:23:08PM +0800, Jason Wang wrote:
BTW now I re-read it I don't understand __vhost_get_user_slow:

static void __user *__vhost_get_user_slow(struct vhost_virtqueue *vq,
void __user *addr, unsigned int size,
int type)
int ret;

ret = translate_desc(vq, (u64)(uintptr_t)addr, size, vq->iotlb_iov,


how does this work? how can we cast a pointer to guest address without
adding any offsets?

I'm not sure I get you here. What kind of offset did you mean?

OK so points:

1. type argument seems unused. Right?

Yes, we can remove that.

2. Second argument to translate_desc is a GPA, isn't it?

No, it's IOVA, this function will be called only when IOTLB is enabled.


Here we cast a userspace address to this type. What if it
matches a valid GPA by mistake?