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Re: [Xen-devel] [PATCH 1/4] x86/mm: mod_l<N>_entry() have no need to use __copy_from_user()

On 20.12.2019 15:47, George Dunlap wrote:
> On 12/20/19 2:42 PM, Andrew Cooper wrote:
>> On 20/12/2019 14:19, Jan Beulich wrote:
>>> mod_l1_entry()'s need to do so went away with commit 2d0557c5cb ("x86:
>>> Fold page_info lock into type_info"), and the other three never had such
>>> a need, at least going back as far as 3.2.0.
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> These presumably want ACCESS_ONCE() to avoid introducing repeated read
>> vulnerabilities?  While the mappings might be safe, they still point to
>> live guest data.
> The L1 itself should be mapped read-only by the guest, and locked at
> this point, no?

True, but I think it won't hurt to use ACCESS_ONCE() nevertheless
- it makes the code more obviously safe independent of any locking


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