Re: [Xen-devel] [PATCH 1/2] xen/gntdev: replace global limit of mapped pages by limit per call

[Juergen Gross]

On 18.07.19 19:36, Andrew Cooper wrote:
> On 18/07/2019 07:52, Juergen Gross wrote:
> > Today there is a global limit of pages mapped via /dev/xen/gntdev set
> > to 1 million pages per default.
>
> The Xen default limit even for dom0 is 1024 pages * 16 entries per page,
> which is far lower than this limit.

Actually its 256 entries per page, but this is still lower than the
current limit.

> > There is no reason why that limit is
> > existing, as total number of foreign mappings is limited by the
>
> s/foreign/grant/ ?

Can do.

> > hypervisor anyway and preferring kernel mappings over userspace ones
> > doesn't make sense.
>
> Its probably also worth stating that this a root-only device, which
> further brings in to question the user/kernel split.

Yes.

> > Additionally checking of that limit is fragile, as the number of pages
> > to map via one call is specified in a 32-bit unsigned variable which
> > isn't tested to stay within reasonable limits (the only test is the
> > value to be <= zero, which basically excludes only calls without any
> > mapping requested). So trying to map e.g. 0xffff0000 pages while
> > already nearly 1000000 pages are mapped will effectively lower the
> > global number of mapped pages such that a parallel call mapping a
> > reasonable amount of pages can succeed in spite of the global limit
> > being violated.
> >
> > So drop the global limit and introduce per call limit instead.
>
> Its probably worth talking about this new limit. What is it trying to
> protect?

Out-of-bounds allocations.


Juergen