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From: =?windows-1252?Q?Roger_Pau_Monn=E9?= <roger.pau@citrix.com>
Subject: Re: [xen-unstable test] 56456: regressions - FAIL
Date: Tue, 19 May 2015 17:07:00 +0200
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To: Jan Beulich <JBeulich@suse.com>, Tim Deegan <tim@xen.org>
Cc: AndrewCooper <andrew.cooper3@citrix.com>, ian.jackson@eu.citrix.com, xen-devel <xen-devel@lists.xenproject.org>
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El 19/05/15 a les 12.29, Jan Beulich ha escrit:
>>>> On 19.05.15 at 12:20, <tim@xen.org> wrote:
>> At 12:19 +0100 on 18 May (1431951570), Jan Beulich wrote:
>>>>>> On 18.05.15 at 12:50, <roger.pau@citrix.com> wrote:
>>>> El 18/05/15 a les 12.17, Tim Deegan ha escrit:
>>>>>  - have map_dirty_bitmap() DTRT, with something like access_ok() +
>>>>>    a linear-pagetable lookup to find the frame.
>>>>
>>>> That was my first intention, but AFAICT we have no function in tree to
>>>> resolve a PV guest VA into a GFN/MFN. The closest thing I could find was
>>>> using guest_walk_tables + guest_walk_to_gfn in order to obtain the gfn.
>>>> Should I send a patch to introduce a pv_gva_to_gfn function based on that?
>>>
>>> Isn't that what we have the linear page table and guest_map_l1e()
>>> for?
>>
>> Yes, or in this case guest_get_eff_l1e().  We'd want to make sure we
>> get_page() the underlying page as well to guard against it being freed
>> and reused while we have a mapping.
>>
>> That won't check user/supervisor or write permissions in the upper
>> levels of the tree.  OTOH, __copy_to_user() doesn't either, so maybe
>> we don't care.
> 
> Hmm, permissions are being checked by __copy_to_user() afaict
> (due to us using the actual page tables), so that being bypassed
> here would seem wrong then.

The only way I see to check for permissions of all levels is to use
guest_walk_tables instead of guest_get_eff_l1e, but that's going to make
this quite slow (as compared to the previous implementation).

Roger.