linux-kernel - Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

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Date:   Wed, 14 Mar 2018 09:00:36 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Ram Pai <linuxram@...ibm.com>
Cc:     mpe@...erman.id.au, mingo@...hat.com, akpm@...ux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        dave.hansen@...el.com, benh@...nel.crashing.org, paulus@...ba.org,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, hbabu@...ibm.com, mhocko@...nel.org,
        bauerman@...ux.vnet.ibm.com, ebiederm@...ssion.com, corbet@....net,
        arnd@...db.de, msuchanek@...e.com, Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

On 03/09/2018 09:00 PM, Ram Pai wrote:
> On Fri, Mar 09, 2018 at 12:04:49PM +0100, Florian Weimer wrote:
>> On 03/09/2018 09:12 AM, Ram Pai wrote:
>>> Once an address range is associated with an allocated pkey, it cannot be
>>> reverted back to key-0. There is no valid reason for the above behavior.
>>
>> mprotect without a key does not necessarily use key 0, e.g. if
>> protection keys are used to emulate page protection flag combination
>> which is not directly supported by the hardware.
>>
>> Therefore, it seems to me that filtering out non-allocated keys is
>> the right thing to do.
> 
> I am not sure, what you mean. Do you agree with the patch or otherwise?

I think it's inconsistent to make key 0 allocated, but not the key which 
is used for PROT_EXEC emulation, which is still reserved.  Even if you 
change the key 0 behavior, it is still not possible to emulate mprotect 
behavior faithfully with an allocated key.

Thanks,
Florian