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

From: Dave Hansen <dave.hansen@linux.intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Andy Lutomirski <luto@amacapital.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>, 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>
Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?
Date: Fri, 18 Dec 2015 13:12:44 -0800	[thread overview]
Message-ID: <567476CC.8080805@linux.intel.com> (raw)
In-Reply-To: <CA+55aFz9XdUP0BQXuq7UuZT1ke-+JVThs-bssMi2sV2xLubHVg@mail.gmail.com>

On 12/18/2015 01:04 PM, Linus Torvalds wrote:
> On Fri, Dec 18, 2015 at 12:49 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> IOW, I like my idea in which signal delivery always sets PKRU to the
>> application-requested-by-syscall values and sigreturn restores it.
> 
> So I don't mind that, as long as the whole "sigreturn restores it" is
> part of things.
> 
> Your original email with the suggestion to *not* resture PKRU I didn't
> like. Setting it and restoring it is fine.
> 
> I do wonder if you need an explicit value, though. I think it's
> reasonable to say that PKRU value 0 is special. It's what we'd start
> processes with, and why not just say that it's what we run signal
> handlers in?
> 
> Would any other value ever make sense, really?

Having a PKRU with the execute-only permissions set is the only one I
can think of.  For a system with a _dedicated_ PKEY for execute-only,
this is easy and could even be made a part of init_fpstate with no other
code changes.

But, if we are picking out an execute-only pkey more dynamically, we've
got to keep the default value for the entire process somewhere.