Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?
To: Andy Lutomirski <luto@amacapital.net>, "H. Peter Anvin" <hpa@zytor.com>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <567475B8.8020800@linux.intel.com>
Date: Fri, 18 Dec 2015 13:08:08 -0800
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On 12/18/2015 01:02 PM, Andy Lutomirski wrote:
> On Fri, Dec 18, 2015 at 12:58 PM, H. Peter Anvin <hpa@zytor.com> 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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