linux-kernel - Re: hv_hypercall

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Message-ID: <20200407073830.GA29279@lst.de>
Date:   Tue, 7 Apr 2020 09:38:30 +0200
From:   Christoph Hellwig <hch@....de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, x86@...nel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: hv_hypercall_pg page permissios

On Tue, Apr 07, 2020 at 09:28:01AM +0200, Vitaly Kuznetsov wrote:
> Christoph Hellwig <hch@....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.