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

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Date:   Fri, 9 Mar 2018 11:54:35 -0800
From:   Ram Pai <linuxram@...ibm.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Ingo Molnar <mingo@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>, linux-mm <linux-mm@...ck.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>,
        Haren Myneni/Beaverton/IBM <hbabu@...ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jonathan Corbet <corbet@....net>,
        Arnd Bergmann <arnd@...db.de>, fweimer@...hat.com,
        msuchanek@...e.com, Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

On Fri, Mar 09, 2018 at 07:37:04PM +1100, Balbir Singh wrote:
> On Fri, Mar 9, 2018 at 7:12 PM, Ram Pai <linuxram@...ibm.com> 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.  On
> > the contrary applications need the ability to do so.
> >
> > The patch relaxes the restriction.
> 
> I looked at the code and my observation was going to be that we need
> to change mm_pkey_is_allocated. I still fail to understand what
> happens if pkey 0 is reserved? What is the default key is it the first
> available key? Assuming 0 is the default key may work and seems to
> work, but I am sure its mostly by accident. It would be nice, if we
> could have  a notion of the default key. I don't like the special
> meaning given to key 0 here. Remember on powerpc if 0 is reserved and
> UAMOR/AMOR does not allow modification because it's reserved, setting
> 0 will still fail

The linux pkey API, assumes pkey-0 is the default key. If no key is
explicitly associated with a page, the default key gets associated.
When a default key gets associated with a page, the permissions on the
page are not dictated by the permissions of the default key, but by the
permission of other bits in the pte; i.e _PAGE_RWX.

On powerpc, and AFAICT on x86, neither the hardware nor the hypervisor
reserves key-0. Hence the OS is free to use the key value, the
way it chooses. On Linux we choose to associate key-0 the special status
called default-key.

However I see your point. If some cpu architecture takes away key-0 from
Linux, than implementing the special status for key-0 on that
architecture can become challenging, though not impossible. That
architecture implementation can internally map key-0 value to some other
available key, and associate that key to the page. And offcourse make
sure that the hardware/MMU uses the pte's RWX bits to enforce
permissions, for that key.

-- 
Ram Pai