linux-kernel - Re: [PATCH 1/9] x86, pkeys: do not special case protection key 0

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Message-Id: <20180327022718.GD5743@ram.oc3035372033.ibm.com>
Date:   Mon, 26 Mar 2018 19:27:18 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        tglx@...utronix.de, dave.hansen@...el.com, mpe@...erman.id.au,
        mingo@...nel.org, akpm@...ux-foundation.org, shuah@...nel.org
Subject: Re: [PATCH 1/9] x86, pkeys: do not special case protection key 0

On Fri, Mar 23, 2018 at 11:09:05AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> mm_pkey_is_allocated() treats pkey 0 as unallocated.  That is
> inconsistent with the manpages, and also inconsistent with
> mm->context.pkey_allocation_map.  Stop special casing it and only
> disallow values that are actually bad (< 0).
> 
> The end-user visible effect of this is that you can now use
> mprotect_pkey() to set pkey=0.
> 
> This is a bit nicer than what Ram proposed because it is simpler
> and removes special-casing for pkey 0.  On the other hand, it does
> allow applciations to pkey_free() pkey-0, but that's just a silly
> thing to do, so we are not going to protect against it.

The more I think about this, the more I feel we are opening up a can
of worms.  I am ok with a bad application, shooting itself in its feet.
But I am worried about all the bug reports and support requests we
will encounter when applications inadvertently shoot themselves 
and blame it on the kernel.

a warning in dmesg logs indicating a free-of-pkey-0 can help deflect
the blame from the kernel.

RP