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

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Date:   Wed, 14 Mar 2018 10:51:26 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ram Pai <linuxram@...ibm.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 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"?

> (b) it cannot be freed.

This is the one I'm questioning.

> (c) it is assigned by default.

I agree on this totally. :)

> (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.

> (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.

> 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.