Re: [Xen-devel] [PATCH] Paging and memory sharing for HVM guests

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 04.01.10 16:32 >>>
>On 04/01/2010 14:14, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>
>> Isn't this undermining the purpose of privcmd_enforce_singleshot_mapping()?
>> You don't check that the eventual single page re-mapping attempt is
>> really due to an earlier -ENOENT failure, and hence the whole single
>> shot mapping checks are now pointless (though other than possibly to
>> enforce some minimal security I don't really know what its purpose
>> is/was - Keir?).
>
>It was just to avoid the BUG_ON(!pte_none(*pte)) in
>direct_remap_area_pte_fn(). We trust privcmd users enough we could just get
>rid of privcmd_enforce_singleshot_mapping(). We could instead forcibly clear
>ptes before calling direct_remap_area(); or just do nothing.

I'm of the opinion that even privileged users shouldn't be able to oops
the kernel, and hence hitting the BUG_ON() you're mentioning should
be avoided. Clearing the old mappings seems like a reasonable thing
to do.

Jan


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