Re: [patch 05/16] mm: Allow special mappings with user access cleared

[Aneesh Kumar K.V]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Wed, Dec 13, 2017 at 7:54 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>
>> Which is why get_user_pages() _should_ enforce this.
>>
>> What use are protection keys if you can trivially circumvent them?
>
> No, we will *not* worry about protection keys in get_user_pages().
>
> They are not "security". They are a debug aid and safety against random mis-use.
>
> In particular, they are very much *NOT* about "trivially circumvent
> them". The user could just change their mapping thing, for chrissake!
>
> We already allow access to PROT_NONE for gdb and friends, very much on purpose.
>

Can you clarify this? We recently did fix read access on PROT_NONE via
gup here for ppc64 https://lkml.kernel.org/r/20171204021912.25974-2-aneesh.kumar@xxxxxxxxxxxxxxxxxx

What is the expected behaviour against gup and get_user_pages for
PROT_NONE. 

Another issue is we end up behaving differently with PROT_NONE mapping
based on whether autonuma is enabled or not. For a PROT_NONE mapping we
return true with pte_protnone().

-aneesh