Re: [PATCH 4/4] iommu/vt-d: Do access checks before calling handle_mm_fault()

Joerg Roedel
2015-11-10T18:06:20Z

On Tue, Nov 10, 2015 at 09:43:03AM -0800, Linus Torvalds wrote:
> On Tue, Nov 10, 2015 at 5:26 AM, Joerg Roedel <joro@8bytes.org> wrote:
> > +
> > +static bool access_error(struct vm_area_struct *vma, struct page_req_dsc *req)
> > +{
> > + return !((req->rd_req && (vma->vm_flags & VM_READ)) ||
> > + (req->wr_req && (vma->vm_flags & VM_WRITE)) ||
> > + (req->exe_req && (vma->vm_flags & VM_EXEC)));
> > +}
>
> This seems odd.
>
> Shouldn't it be
>
> return ((req->rd_req && !(vma->vm_flags & VM_READ)) ||
> (req->wr_req && !(vma->vm_flags & VM_WRITE)) ||
> (req->exe_req && !(vma->vm_flags & VM_EXEC)));
>
> instead?

Yes, thats better, it solves the multiple-bits-set problem too, which
David mentioned.

> Of course, if you just used the VM_xyz flags internally itself, this
> would all be easier, and you'd end up with something like
>
> /* Do we have requested bits that aren't in the allowed set? */
> return (requested & ~vma->vm_flags) != 0;
>
> instead..

But this is probably the best solution, some architecture page-fault
handlers do something similar. I'll update the patch and use this for
the AMD part too.

Thanks,

Joerg