linux-kernel - Re: [PATCH 1/1 v2] x86: pkey-mprotect must allow pkey-0

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Date:   Wed, 14 Mar 2018 11:54:52 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     mingo@...hat.com, mpe@...erman.id.au,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, 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,
        fweimer@...hat.com, msuchanek@...e.com
Subject: Re: [PATCH 1/1 v2] x86: pkey-mprotect must allow pkey-0

On Wed, Mar 14, 2018 at 10:51:26AM -0700, Dave Hansen wrote:
> On 03/14/2018 10:14 AM, Ram Pai wrote:
> > I look at key-0 as 'the key'. It has special status. 
> > (a) It always exist.
> 
> Do you mean "is always allocated"?

always allocated and cannot be freed. Hence always exists.

If we let it freed, than yes 'it is always allocated', but
may not 'always exist'.

> 
> > (b) it cannot be freed.
> 
> This is the one I'm questioning.

this is a philosophical question. Should we allow the application 
shoot-its-own-feet or help it from doing so. I tend to
gravitate towards the later.

> 
> > (c) it is assigned by default.
> 
> I agree on this totally. :)

good. we have some common ground :)

> 
> > (d) its permissions cannot be modified.
> 
> Why not?  You could pretty easily get a thread going that had its stack
> covered with another pkey and that was being very careful what it
> accesses.  It could pretty easily set pkey-0's access or write-disable bits.

ok. I see your point. Will not argue against it.

> 
> > (e) it bypasses key-permission checks when assigned.
> 
> I don't think this is necessary.  I think the only rule we *need* is:
> 
> 	pkey-0 is allocated implicitly at execve() time.  You do not
> 	need to call pkey_alloc() to allocate it.

And can be explicitly associated with any address range ?

> 
> > An arch need not necessarily map 'the key-0' to its key-0.  It could
> > internally map it to any of its internal key of its choice, transparent
> > to the application.
> 
> I don't understand what you are saying here.

I was trying to disassociate the notion that "application's key-0 
means hardware's key-0". Nevermind. its not important for this
discussion.

-- 
Ram Pai