Re: [PATCH] xen/x86: Remap text/data/bss with appropriate permissions

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 17/03/16 15:32, Jan Beulich wrote:
>>>> On 17.03.16 at 15:44, <andrew.cooper3@citrix.com> wrote:
>> On 17/03/16 14:31, Jan Beulich wrote:
>>> Also - do we really want to make this code dependent on
>>> map_pages_to_xen() not intermediately zapping the mappings
>>> being changed?
>> Do you mean "immediately"?
> No.
>
>> As far as I can tell, it is guaranteed to be safe, even when remapping
>> the code section.  Updates to the live pagetables are using atomic
>> writes, and I didn't spot a point which would end up with a transient
>> non-present mapping.
> But we may, at some point and for whatever reason, come to make
> the function zap the mapping (i.e. clear the present bit), flush, and
> only the re-establish the new mapping.

This change is temporary until I can fix the legacy boot issue and
reintroduce the proper 2M functionality.

If someone in the future wants to change the behaviour of
map_pages_to_xen() then we can reconsider.  However, I think it is
unlikely that this will actually happen at all, and if it ever does, I
hope to have already fixed the 2M alignment and deleted this change.

This change is a big security improvement, and absolutely should be
taken, especially as the current implementation of map_pages_to_xen() is
safe.

~Andrew

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