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

[Florian Weimer]

On 03/14/2018 09:00 AM, Florian Weimer wrote:
> 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.

Ugh.  Should have read the code first before replying:

        /* Do we need to assign a pkey for mm's execute-only maps? */
        if (execute_only_pkey == -1) {
                /* Go allocate one to use, which might fail */
                execute_only_pkey = mm_pkey_alloc(mm);
                if (execute_only_pkey < 0)
                        return -1;
                need_to_set_mm_pkey = true;
        }

So we do allocate the PROT_EXEC-only key, and I assume it means that the 
key can be restored using pkey_mprotect.  So the key 0 behavior is a 
true exception after all, and it makes sense to realign the behavior 
with the other keys.

Thanks,
Florian