Re: hv_hypercall_pg page permissios

[Wei Liu]

On Tue, Apr 07, 2020 at 06:10:42PM +0000, Dexuan Cui wrote:
> > From: linux-hyperv-owner@xxxxxxxxxxxxxxx
> > <linux-hyperv-owner@xxxxxxxxxxxxxxx> On Behalf Of Vitaly Kuznetsov
> > Sent: Tuesday, April 7, 2020 12:28 AM
> > Christoph Hellwig <hch@xxxxxx> 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
> > 
> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > index 7581cab74acb..17845db67fe2 100644
> > --- a/arch/x86/hyperv/hv_init.c
> > +++ b/arch/x86/hyperv/hv_init.c
> > @@ -382,7 +382,7 @@ void __init hyperv_init(void)
> >         guest_id = generate_guest_id(0, LINUX_VERSION_CODE, 0);
> >         wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id);
> > 
> > -       hv_hypercall_pg  = __vmalloc(PAGE_SIZE, GFP_KERNEL,
> > PAGE_KERNEL_RX);
> > +       hv_hypercall_pg  = vmalloc_exec(PAGE_SIZE);
> 
> If we try to write into the page, Hyper-V will kill the guest immediately
> by a virtual double-fault (or triple fault?), IIRC.
> 

The guest would get injected a #GP fault in that case FWIW.  Perhaps
that leads to further double-fault or triple-fault.

Wei.