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SubjectRe: hv_hypercall_pg page permissios
On Tue, Apr 07, 2020 at 06:10:42PM +0000, Dexuan Cui wrote:
> > From: linux-hyperv-owner@vger.kernel.org
> > <linux-hyperv-owner@vger.kernel.org> On Behalf Of Vitaly Kuznetsov
> > Sent: Tuesday, April 7, 2020 12:28 AM
> > 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
> >
> > 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.

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