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SubjectRe: [PATCH 1/3] x86, pkeys: do not special case protection key 0
On Sat, 17 Mar 2018, Dave Hansen wrote:
> On 03/17/2018 02:12 AM, Thomas Gleixner wrote:
> >> 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.
> > What's the consequence of that? Application crashing and burning itself or
> > something more subtle?
> You would have to:
> pkey_free(0)
> ... later
> new_key = pkey_alloc();
> // now new_key=0
> pkey_deny_access(new_key); // or whatever
> At which point most apps would probably croak because its stack is
> inaccessible. The free itself does not make the key inaccessible, *but*
> we could also do that within the existing ABI if we want. I think I
> called out that behavior as undefined in the manpage.

As long as its documented and the change only allows people to shoot them
more in the foot, we're all good.



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