RE: hv_hypercall_pg page permissios

[Dexuan Cui]

> 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.

> on a Hyper-V 2016 guest and nothing broke, feel free to go ahead and
> kill PAGE_KERNEL_RX.
> --
> Vitaly

This should be OK. Just remember never try to write into the page, unless
you're trying to use this as a means of forcing a guest reboot. :-)

Thanks,
-- Dexuan