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

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Date:	Fri, 18 Dec 2015 13:08:08 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>
Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

On 12/18/2015 01:02 PM, Andy Lutomirski wrote:
> On Fri, Dec 18, 2015 at 12:58 PM, H. Peter Anvin <hpa@...or.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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