Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

[Dave Hansen]

On 12/18/2015 01:02 PM, Andy Lutomirski wrote:
> On Fri, Dec 18, 2015 at 12:58 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> > On 12/18/2015 12:49 PM, Andy Lutomirski wrote:
>>> >>
>>> >> IOW, I like my idea in which signal delivery always sets PKRU to the
>>> >> application-requested-by-syscall values and sigreturn restores it.
>>> >> Kinda like sigaltstack, but applies to all signals and affects PKRU
>>> >> instead of RSP.
>>> >>
>> > I think this is the only sensible option, with the default being all zero.
>> >
> Or not quite all zero if we do Dave's experimental PROT_EXEC thing.
> 
> Actually, I want to introduce a set of per-mm "incompatible" bits.  By
> default, they'd be zero.  We can, as needed, define bits that do
> something nice but break old code.  I want one of the bits to turn
> vsyscalls off entirely.  Another bit could say that the kernel is
> allowed to steal a protection key for PROT_EXEC.

That really only makes sense if we have userspace that expects all the
protection keys to be available.  If we go the route of having
pkey_alloc/free() syscalls, then the kernel can easily tell userspace to
keep its mitts off a particular pkey by not ever returning it from a
pkey_alloc().
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