Reading guest page flags

Reading guest page flags

[Spencer Michaels]

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Hello,

I'm writing an application that can read/write guest memory, and as part of
that I need to check whether the page being manipulated is
readable/writable by the guest itself. Does Xen have an API that would
allow me to read the flags of an arbitrary page on a guest, given either
its address or its PFN? I've only been able to find functions that update
the existing memory mappings, but none to read them.

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Re: Reading guest page flags

[Razvan Cojocaru]

On 9/13/18 8:29 PM, Spencer Michaels wrote:
> I'm writing an application that can read/write guest memory, and as part
> of that I need to check whether the page being manipulated is
> readable/writable by the guest itself. Does Xen have an API that would
> allow me to read the flags of an arbitrary page on a guest, given either
> its address or its PFN? I've only been able to find functions that
> update the existing memory mappings, but none to read them.

If you mean EPT, yes: look at xc_get_mem_access() in xenctrl.h / libxc.
If you mean "regular" PTEs, no - you'd have to write your own code to
retrieve that.

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Re: Reading guest page flags

[Tamas K Lengyel]

On Thu, Sep 13, 2018 at 11:48 AM Razvan Cojocaru
<rcojocaru@bitdefender.com> wrote:
>
> On 9/13/18 8:29 PM, Spencer Michaels wrote:
> > I'm writing an application that can read/write guest memory, and as part
> > of that I need to check whether the page being manipulated is
> > readable/writable by the guest itself. Does Xen have an API that would
> > allow me to read the flags of an arbitrary page on a guest, given either
> > its address or its PFN? I've only been able to find functions that
> > update the existing memory mappings, but none to read them.
>
> If you mean EPT, yes: look at xc_get_mem_access() in xenctrl.h / libxc.
> If you mean "regular" PTEs, no - you'd have to write your own code to
> retrieve that.

LibVMI does that for guest PTEs:
https://github.com/libvmi/libvmi/blob/master/libvmi/libvmi.h#L936

Tamas

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Re: Reading guest page flags

[Spencer Michaels]

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Thanks! Sorry, I should have been more specific - I do indeed mean the
regular guest PTEs (and I'm using PV guests which as I understand don't use
EPT anyway). So, it seems I'll have to write my own code for this. I'll
take a look at libvmi's implementation; if anyone knows any other libraries
/ programs that read guest PTEs, that would be helpful to know as well.

On Thu, Sep 13, 2018, 10:47 AM Razvan Cojocaru <rcojocaru@bitdefender.com>
wrote:

> On 9/13/18 8:29 PM, Spencer Michaels wrote:
> > I'm writing an application that can read/write guest memory, and as part
> > of that I need to check whether the page being manipulated is
> > readable/writable by the guest itself. Does Xen have an API that would
> > allow me to read the flags of an arbitrary page on a guest, given either
> > its address or its PFN? I've only been able to find functions that
> > update the existing memory mappings, but none to read them.
>
> If you mean EPT, yes: look at xc_get_mem_access() in xenctrl.h / libxc.
> If you mean "regular" PTEs, no - you'd have to write your own code to
> retrieve that.
>

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