RE: hv_hypercall_pg page permissios

[Dexuan Cui <decui@microsoft.com>]

> From: Vitaly Kuznetsov <vkuznets@redhat.com>
> Sent: Monday, June 15, 2020 1:35 AM
> Dexuan Cui <decui@microsoft.com> writes:
> 
> >> From: linux-hyperv-owner@vger.kernel.org
> >> <linux-hyperv-owner@vger.kernel.org> On Behalf Of Andy Lutomirski
> >> Sent: Tuesday, April 7, 2020 2:01 PM
> >> > On Apr 7, 2020, at 12:38 AM, Christoph Hellwig <hch@lst.de> wrote:
> >> >
> >> > On Tue, Apr 07, 2020 at 09:28:01AM +0200, Vitaly Kuznetsov wrote:
> >> >> Christoph Hellwig <hch@lst.de> writes:
> >> >>
> >> >>> Hi all,
> >> >>>
> >> >>> The x86 Hyper-V hypercall page (hv_hypercall_pg) is the only allocation
> >> >>> in the kernel using __vmalloc with exectutable persmissions, and the
> >> >>> only user of PAGE_KERNEL_RX.  Is there any good reason it needs to
> >> >>> be readable?  Otherwise we could use vmalloc_exec and kill off
> >> >>> PAGE_KERNEL_RX.  Note that before 372b1e91343e6 ("drivers: hv:
> Turn
> >> off
> >> >>> write permission on the hypercall page") it was even mapped writable..
> >> >>
> >> >> [There is nothing secret in the hypercall page, by reading it you can
> >> >> figure out if you're running on Intel or AMD (VMCALL/VMMCALL) but it's
> >> >> likely not the only possible way :-)]
> >> >>
> >> >> I see no reason for hv_hypercall_pg to remain readable. I just
> >> >> smoke-tested
> >> >
> >> > Thanks, I have the same in my WIP tree, but just wanted to confirm this
> >> > makes sense.
> >>
> >> Just to make sure we’re all on the same page: x86 doesn’t normally have
> an
> >> execute-only mode. Executable memory in the kernel is readable unless you
> >> are using fancy hypervisor-based XO support.
> >
> > Hi hch,
> > The patch is merged into the mainine recently, but unluckily we noticed
> > a warning with CONFIG_DEBUG_WX=y (it looks typically this config is defined
> > by default in Linux distros, at least in Ubuntu 18.04's
> > /boot/config-4.18.0-11-generic).
> >
> > Should we revert this patch, or figure out a way to ask the DEBUG_WX code
> to
> > ignore this page?
> >
> 
> Are you sure it is hv_hypercall_pg? 
Yes, 100% sure. I printed the value of hv_hypercall_pg and and it matched the
address in the warning line " x86/mm: Found insecure W+X mapping at address".

> AFAIU it shouldn't be W+X as we
> are allocating it with vmalloc_exec(). In other words, if you revert
> 78bb17f76edc, does the issue go away?
> 
> Vitaly

Yes, the warning goes away if I revert
78bb17f76edc ("x86/hyperv: use vmalloc_exec for the hypercall page")
88dca4ca5a93 ("mm: remove the pgprot argument to __vmalloc") 
(I have to revert the second as well with some manual adjustments, since
__vmalloc() has 2 parameters now.)

Thanks,
Dexuan